Corrosion and aging of organic aviation coatings:A review

Tianyu ZHANG, Teng ZHANG, Yuting HE, Yuchen WANG, Yaping BI

Air Force Engineering University, Xi’an 710038, China

KEYWORDSAging;Calendar life;Classification;Corrosion;Experimental methods;Modification;Organic aviation coatings

AbstractOrganic anticorrosive aviation coatings are an effective guarantee for aviation structure,since aircraft corrosion can lead to great economic losses.Whether it is during ground parking or air cruises, organic aviation coatings are important barriers to the corrosion of aviation structure.With the vigorous development of the aviation industry,organic aviation coatings continue to meet the challenges of diverse,complex,and harsh service environments.This review analyzes and summarizes the research status of the types and development of organic aviation coatings, influencing factors and mechanisms, experimental methods, calendar life research methods, and modification methods.It also summarizes the research results that have been achieved to date.The current research deficiencies in the equivalence relationship between atmospheric exposure and artificial acceleration, failure criteria and life prediction were pointed out, and nano-modification technology, and future research strategies and directions that need breakthroughs are discussed.?2022 Chinese Society of Aeronautics and Astronautics.Production and hosting by Elsevier Ltd.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

1.Introduction

The base material of aviation structures usually uses composite materials of aluminum alloy,magnesium alloy,titanium alloy,alloy steel,and non-metallic materials.The coating on the base material is for protection, decoration, camouflage, or other functions.1Whether for civilian or military aircraft, and especially for aeronautical structures that perform cruise missions in severe weather areas (such as areas with high temperature,high humidity, high salt, high sand, strong ultraviolet light,etc.),organic coatings have played an important role in protection.2,3According to an IHS Markit report4in 2018,the global scale of the anti-corrosion coatings industry accounted for 10 % of the total coatings market that year, of which highperformance anti-corrosion coatings accounted for only 7 %.Fig.1(a)4shows the distribution of the consumption of anticorrosion coatings for various countries in the world.4This market is expected to grow at an average annual rate of 3.5 %.According to a report released by Zion Market Research in September 2017, the global aerospace coatings market value in 2016 exceeded $1.31 billion and is expected to exceed$1.95 billion by 2022.The compound annual growth rate for 2017–2022 is expected to be 6.85%.5During the forecast period, the aerospace coatings market in the Asia-Pacific region is expected to grow at a faster rate.Due to regions such as China and India,the Asia-Pacific region is one of the largest markets for aerospace coatings.The fast-growing tourism industry in the Asia-Pacific region is expected to drive the growth of the aerospace market over the next few years.Due to the existence of the automation industry in the United States, North America is developing in a mature direction.The high incidence of cargo transportation in Europe and a large number of domestic and international passengers are factors driving the growth of the aviation market,which in turn is expected to help the growth of the aviation coatings market.Latin America,the Middle East,and Africa may see considerable growth in the coming years.This is mainly due to the continuous development of urbanization and industrialization in countries such as Brazil.In 2012, the web of science database was used to search several types of organic aviation coatings as keywords, which showed that the number of related documents reached thousands.

Fig.1 Global consumption of high-performance anti-corrosion coatings and conventional paint system used for aviation structures.2,4

Generally speaking, organic aviation coatings protect the substrate in three ways.6–10First, the coating blocks the diffusion of corrosive media to the interface between the metal and coating.Although small water and oxygen molecules can penetrate the coating, larger corrosive ions find it difficult to penetrate and the coating effectively prevents the movement of ions between the cathode and anode regions.Second, the corrosion inhibitor and pigment particles in the organic coating can be hydrolyzed and passivated,or react with corrosive ions to protect the substrate from corrosion.Third,the coating has good adhesion to the metal, which prevents peeling due to the accumulation of corrosion products.Aircraft that often serve in coastal areas,islands, and reefs, or industrial areas will suffer from harsh environmental corrosion.11–14In addition, due to some special airtight structures in the aircraft structure, its interior will often accumulate water that can arise to corrosion at skin joints, fastener connections, rivet holes, landing gear,battery area, flaps and hinge grooves, areas affected by engine exhaust,and other areas prone to water accumulation.15,16The function of the organic coating on the surface of the radome and various radio radomes is to prevent the reduction of electrical properties due to water absorption of the material, and to prevent damage from being washed by wind and sand.2Therefore,elastic polyurethane with better electrical properties and wear resistance is generally used on the surface.The materials of the internal metal parts of the aircraft include magnesium alloys, aluminum alloys, steels and titanium alloys.Generally, corresponding organic coatings are used according to the specific working environment, and the coatings should be adapted to the surface treatment of the substrate.17The failure of the anti-corrosion coating is one of the important reasons for the corrosion of the substrate.According to the researches, accidents caused by corrosion or corrosion fatigue account for about 20 % of all accidents of aircraft.18In 1981,the coating on the lower fuselage of the Boeing 737–200 aircraft failed,resulting in pitting and cracking,which eventually led to the disintegration of the aircraft in mid-air.18In 2002,the skin of the China Airlines Boeing 747 tail was scratched and the coating peeled off,resulting in cracks caused by corrosion fatigue of the substrate, and finally crashed, killing 225 people.18When the aircraft was in service on Hainan Island,China, the coating on the skin surface began to damage after half a year,and the aluminum alloy substrate began to corrode after one year.2,19,20In summary,the corrosion of the substrate caused by the failure of aviation coatings poses a serious threat to flight safety.

The complete system of an aerospace organic coating includes anodized film, primer, intermediate paint, and topcoat,as shown in Fig.1(b),2but the actual system combination used depends on the specific service environment.Aviation organic coatings are, in the majority of the cases, neither a one coat system nor a standalone system.The right combination of metal-pretreatment and organic coating systems defines the performance.There are much more individual coating systems used today, in relation to the philosophy of each aircraft manufacture.Research on corrosion and cracking of the metal undercoating has gradually attracted the attention of researchers, but when compared to conventional corrosion, it has not been studied in depth.In addition, the coating will gradually undergo physical or chemical changes when exposed to strong ultraviolet light, high temperature, high humidity, and other environments,resulting in the gradual decline in the protective performance of the coating, which will corrode the substrate.Failure of the protective performance of the coating will seriously affect the structural integrity of the aircraft.21Therefore,research on the corrosion aging laws and mechanisms of coatings,as well as the evaluation of the calendar life of these coatings, has both theoretical and practical significance for the corrosion control of aviation structures.6

In the early 1920s, aviation structures were mainly constructed using wooden frames covered with textiles, which have gradually developed into structures made of aluminum alloy, steel, titanium alloy, and composite materials.Anticorrosion aviation coatings have also developed from low-level single-type coatings to high-level coating matching systems,1,2from nitro coatings, alkyd resin coatings to the current synthetic resins, such as acrylic resin, epoxy resin, and polyurethane resin-based aviation coating systems.22–29Organic anticorrosive coatings become one of the most widely used coatings in aviation structures due to the principle of the cathodic protection of the sacrificial anode, which can protect the substrate from corrosion in extremely harsh environments(such as coastal and industrial environments,etc.).30Recently,the application of fluorine-containing coatings in aviation structures has become more and more extensive, which have the advantages of good weather, corrosion, oxidation, hightemperature and low-temperature resistance.31–33As nanotechnology has continuously made new research progress in the field of materials, more and more researchers have begun to study the application of nanotechnology in anti-corrosion organic coatings.25,34–37

Based on the published literature in Science Citation Index(SCI), the Engineering Index (EI), and Chinese Science Citation Database (CSCD), this paper reviews the research progress from four aspects of corrosion aging, test methods,calendar life research, and prediction and modification methods of organic aviation coatings, which have a great impact on the future.The focus of the research has been summarized.At present,research on organic aviation anticorrosive coatings needs to be improved and the depth of this research needs to be strengthened.Therefore, it is necessary to comprehensively analyze the characteristics of the current research on organic aviation anticorrosive coatings.This paper gives a comprehensive and systematic introduction to the corrosion and aging of organic anti-corrosion aviation coatings.

2.Corrosion and aging of organic aviation coatings

2.1.Classification and development of organic aviation coatings

In general, the role of aviation coatings includes anticorrosion, decoration, adjustment of surface temperature,absorption of electromagnetic waves, etc.Internal coatings play an important role in corrosion protection compared to external coatings, which are mainly used for decoration or camouflage (military aviation aircraft).2The classification and development of aviation organic anti-corrosion coatings are mainly reviewed and discussed.

The general classification and development of organic aviation coatings are shown in Fig.2.8,22,24,25,31,38–43Nitrocellulose was used in the early days as the main film-forming substance,which was gradually replaced by synthetic resins due to its flammability, explosiveness, and poor aging resistance.2,22The weather resistance of alkyd resin coatings is at mid-level,especially their general corrosion resistance in hot and humid environments, and it has gradually been replaced.24,44Epoxy resin accounts for about 40 %–60 % of the global market.It is mainly used as a primer and can be roughly divided into zinc yellow epoxy ester primer,epoxy polyamide primer,and epoxy enamel.It has excellent film performance and good performance,including good corrosion,abrasion,and chemical resistance as well as excellent adhesion.1,2Epoxy zinc yellow primer has been banned in some countries because it will produce harmful hexavalent chromium ions when hydrolyzed.And all maintenance actions generate Cr(VI) dust when coatings systems are abraded.When pigment particles dissolved in the solution, dichromate ion Cr2O72-will be dissociated, which is then converted into ZnCrO4and Zn2(OH)2?CrO4.Chromic acid ions reacted with the matrix to form a dense passivation film, in which the resin components were cross-linked to form polymers that were insoluble in water but soluble in acids.45,46However, it is still widely used in aerospace structures that have been in service for a long time such as aircraft skins and aircraft internal structures because of the effective corrosion resistance of epoxy zinc yellow primer.Therefore, staff need to carry out strict protection treatment processes when spraying and repairing.Polyurethane coatings account for about 10 % of the total consumption.It also has good corrosion,film,and abrasion resistance and flexibility, and its color and gloss retention are better than epoxy resins, but its construction is inconvenient and maintenance is more difficult.38,47Fluorine-containing coatings have excellent weather and chemical resistance, but the cost and difficulty of their maintenance are relatively high.31,32At present, technology for modifying coatings with phytic acid, carbon nanotubes,graphene, cesium dioxide, etc.as fillers is still at the research stage.Although significant results have been achieved, it has not been widely used in engineering.8,34,38,48,49See Section 5 for details on coating modification technology.

2.2.Factors influencing corrosion and aging of organic aviation coatings

Generally speaking, aging, corrosion by corrosive media,mechanical damage, and metal corrosion under the coating often lead to the failure of organic aviation coatings.2,15,50,51The aging of the coating generally arises from water diffusion,ultraviolet light, media erosion, etc.24,52–56The aging forms of organic aviation coatings are mainly manifested as loss of gloss, discoloration, blistering, powdering, brittleness, cracking, etc.15,24,50,52–56

2.2.1.Water diffusion

The aging mechanism of organic aviation coatings is very complicated.First of all, in the coating process, the microscopic defects generated by the coating will become an important factor in the aging of the coating.50,57The final coating process has to undergo a curing process.After the solvent evaporates,micropores, voids, cavities, etc.will be formed in the coating.58–60Water molecules and oxygen molecules with lower air pressure and smaller volume will diffuse to the interface between the organic coating and the substrate through these microscopic defects, while corrosive ions with higher air pressure and larger volume are generally believed to not diffuse directly at the beginning to the substrate interface, unless the coating gradually expands during the corrosion and aging process.50,61–64The diffusion of water molecules will accelerate the diffusion of oxygen molecules and corrosive media, leading to blistering and peeling of the coating.65–67In fact, the binding ability with water of the coating can also be used as an index to evaluate its corrosion resistance.15,53Water molecules will form a continuous water film at the interface between the coating and metal,causing the coating and metal to gradually peel off.9,52,68,69

Fig.2 General classification and development of organic aviation coatings.8,22,24,25,31,38-43

Ester, ether, urea, alcohol, amine, and other groups in the organic aviation coating are more prone to water degradation and, generally speaking, the degree of degradation is in the order of ester > urea > ether > alcohol > amino.70Perrin et al.24found that the aging of alkyd resins is not only due to Norrish type II photodegradation, but also because the molecular chain of phthalic acid loses phthalic acid groups due to water degradation, which causes breakage.Bauer71believed that the curing position of the resin in the organic coating is prone to water degradation, which leads to aging.In addition,the aging rate of the coating in the humid Florida climate is greater than that in the drier Arizona environment.Chemical bonds such as—NH—CH2,—CHO—O—C—,and—CH2—O—CH2—in organic coatings are the most easily attacked by water, which generate small molecular products and lead to water degradation.71Ramezanzadeh and Attar72studied the water degradation of epoxy coatings and found that the ether bonds (—COC—) in epoxy coatings are extremely sensitive to humid environments and are prone to rupture during exposure to corrosive electrolytes,as shown in Fig.3.72It can be seen that some areas of the coating surface have a lower crosslinking density than other parts, and the degradation of the coating starts from these areas.

2.2.2.Sulfur oxide pollution

Generally speaking, the service environment of aviation structures, especially in industrial areas and coastal areas, will be polluted by nitrogen oxides or sulfur oxides.Among them,sulfur dioxide will cause greater damage to the bond between the coating and metal.15Y?ld?z and Dehri73evaluated the corrosion behavior of polyester-coated galvanized steel exposed to SO2gas and different relative humidity.The results showed that the corrosion current density of the polyester coating samples increased with an increase in the relative humidity in the SO2medium.The polyester-coated samples exposed to 70 %Relative Humidity (RH) were defect-free, while the samples significantly lost their protective properties at 80 %RH,90 %RH, and 100 %RH.The higher the SO2content in the atmosphere, the lower the hydroxide and oxide content in the corrosion products,and the higher the sulfate content.74,75Therefore,during the atmospheric corrosion process,the common reaction sequence of SO2gas is to initially form zinc hydroxide or zinc oxide,and then these products interact with SO2to initially generate alkaline zinc sulfite or zinc sulfate.When combined with water, it will form soluble hydrated zinc sulfate.76In an environment with SO2, the adhesion of the coating is reduced to 1/3 of the original in just 5 days.15In an environment without SO2, it would take 28 days to drop to the same level, suggesting that SO2accelerated the decline in adhesion.15Due to the interaction of SO2and H2O,the pigments of the coating would bleed out and crystallize on the surface of the coating.77SO2also reacts with the drying components of the curing agent in the coating,prolonging the curing time of the coating.76

2.2.3.Ultraviolet (UV) irradiation

Fig.3 Water degradation process of epoxy coatings exposed to a 3.5wt% NaCl solution.72

Although ultraviolet light accounts for a small proportion of solar radiation, solar ultraviolet light has a greater impact on the surface paint layer of the fuselage, outer wings, flat tail,and other structural parts.It causes the organic coating to lose gloss, to chalk, to crack, and to fall off, which will eventually lead to corrosion of the matrix and cause destructive effects.13,78Most organic polymers possess higher energy covalent bonds,which are generally higher than the energy of ultraviolet light.50,79However, the polymer chain also contains unsaturated double bonds,which are just in the radiant energy region of ultraviolet light, and the higher the degree of unsaturation, the higher the degree of conjugation, and the greater the stabilizing effect on free radicals.There are often impurities,such as peroxides or ketones that easily generate free radicals in the paint layer, so it is very easy for photoaging to occur in the coating.39,43The photoaging mechanism of organic polymers is generally due to the generation of free radicals, which trigger a series of reactions.The general reaction mechanism is62,80

Chain initiation:

where hv represents high energy illumination which induce polymer degradation.

Chain growth:

Chain branch:

Small molecules, such as ketones, alcohols, and acids, are formed during the photo-oxidation process in the entire coating.These small molecules combine with water molecules and are evaporated or washed away.15,52,53As the coating continues to thin and shrink, it leads to embrittlement and cracking.Because the pigment volume concentration on the coating surface will continue to increase with a decrease in the coating thickness, the surface of the coating is prone to embrittlement, and the deep layer of the coating has relatively high elasticity, which is prone to deep cracking of the coating.62,80,81

Based on the analysis results obtained for the microscopic morphology, molecular structure, and impedance change of a polyurethane coating during the UV aging process, Cai et al.38,40,41,47proposed a possible aging mechanism for the coating under UV conditions, as shown in Fig.4.38,40,41,47After 1800 h of UV aging, a large number of wrinkles and bumps appear on the surface of the coating.The coating depth of 20 μm affected by ultraviolet aging was mainly caused by the carboxylation of the polymer caused by ultraviolet light.Oxygen molecules reach the surface of the coating, causing the coating to undergo photoaging reaction, resulting in a large number of bumps and wrinkles on the surface of the coating.This may be due to the fact that the water molecules and oxygen penetrate the interior of the coating,the number of C-N bonds in the molecular functional groups decrease, and the C═O bonds increase.As the longitudinal depth of the coating increases, this change is weakened, which is mainly due to the rupture of the C-N and C-O bonds in the isocyanate groups.C-N bond cleavage will generate amino radicals and alkyl radicals, and release CO2, among which alkyl radicals will form amino groups.In the process of breaking the C-O bond, carbamoyl and alkoxy radicals are generated,and the carbamoyl radicals are decomposed into amino groups and CO2; the alkoxy radicals are oxidized to form aldehydes.The soluble or volatile fragments generated via photooxidation will continue to flow to the outside of the coating,causing the internal stress in the coating to be released and the relative osmotic pressure to rise, thereby causing the coating surface to produce bumps and wrinkles.Finally, the electrolyte is in contact with the substrate after the coating is damaged, causing anodic corrosion of the aluminum alloy.

Liu et al.53,82studied the aging mechanism of epoxy protective coatings under UV irradiation and the results are shown in Fig.5.53,82Similar to Ref.40, Liu et al.53,82found that the aging process of epoxy coatings was related to the generation of a large number of carbonyl groups.However,the difference was that a post-curing effect was found in the aging process.In the middle and early stages of UVA irradiation, the epoxy peaks gradually disappear, indicating that there is a postcuring process in the epoxy coating.Parameter S is a linear parameter that describes the doppler energy broadening.The larger the parameter S, the greater the number of defects in the coating.40During the UV aging process, the parameter S of the coating continuously decreases.When the irradiation time was short, the dense structure generated via post-curing and the appearance of polar groups, such as carbonyl and amide groups, will cause the parameter S to decrease rapidly.As the post-curing process makes the coating structure denser,after a short time of irradiation, the water-blocking performance of the coating is improved when compared to the non-irradiated state.As the duration of UV irradiation increases, microscopic defects are generated on the surface of the coating,resulting in a gradual decrease in the water blocking performance of the coating.

For organic aviation coatings exposed to the atmosphere for a long time,photoaging is always accompanied by varying degrees of loss of gloss, discoloration, chalking, etc.63,83–85Therefore, the main function of the top-coat is to prevent ultraviolet radiation and media corrosion.2The primer also has a certain protective effect,but its main function is reflected in the good adhesion to the substrate.2,41For the internal structure of aviation structure, the top-coat is generally not coated and only the primer is coated, but it will also be exposed to a certain degree of ultraviolet radiation.There are few mentions of the anti-aging research of the primer in China and abroad.

2.2.4.Pigment Volume Concentration (PVC)

Feliu et al.86,87and Shreepathi et al.88used electrochemical impedance spectroscopy to study coatings with different pigment loadings calculated by weight or the PVC,to better evaluate the effect of the pigment loading on the cathodic protection process.During the initial stage of sample immersion, epoxy coatings with higher zinc pigment loadings have higher capacitance, while coatings with lower zinc pigment loadings act more like insulating coatings with lower capacitance.Nevertheless, Kalendova′ et al.89, Wang and Bierwagen90believed that epoxy primers should not exceed the Critical Pigment Volume Concentration (CPVC) to prevent corrosion products from interfering with the conductive pathways.In addition,the corrosion of the pigment will change the pH of the coating,which will change the adhesion of the coating and further reduce the conductive path.In addition,Abreu et al.91found that the high molecular weight polymer organic coating itself has conductivity to a certain extent.In the abovementioned research, it is generally believed that when the CPVC is about 60 %–70 %, it can play a role in enhancing the cathodic protection.The general expression of the CPVC for an organic aviation coating is shown as92

Fig.4 Micromorphology and aging mechanism of polyurethane coatings under UV irradiation.38,40,41,47

Fig.5 Infrared spectra, parameters S, law of electrochemical parameters change, and aging models of epoxy coatings after UV aging.53,82

2.2.5.Corrosion of substrate under coating

Fig.6 Effect of PVC/CPVC ratio on performance of organic anticorrosive coatings.92,94

After the failure and aging of the surface of organic aviation coatings, the anodic oxide film on the surface of the substrate will degrade due to changes in pH,resulting in serious filiform corrosion of the substrate.95This phenomenon often occurs around the fasteners of the skin of aviation structures or edges of the skin.15When the corrosive solution penetrates the base metal,partial cathode and anode regions are formed on the base due to electrochemical action.Due to oxygen depolarization in the cathode area, the solution in the partial cathode area becomes slightly alkaline, causing damage and blistering of the coating.In the local anode zone,the reaction product Mn+-reacts with oxygen,water,and OH–to generate corrosion,such as M(OH)nand MxOy?nH2O,due to the anodic dissolution reaction of the metal; the volume of these corrosion products is much larger than the volume of the dissolved corrosion products,so the coating swells into‘‘bubbles”.When the‘‘bubbles”burst,the so-called‘‘corrosion”defects are formed.96

A‘‘V-shaped”peeling zone is formed in the region where the coating has the first aging characteristic and the‘‘V-shaped”head extends in the form of filaments from the lower part of the coating to the surroundings.As the corrosion reaction progresses,it increases.The length of the filiform corrosion does not develop in the depth direction.The corrosion wire is composed of a head and a body.The head has growth activity,the front end is arc-shaped,and the back is connected with the body in a ‘‘V-shape”.The corrosion reaction takes place on the head,the body is the corrosion product,and corrosion marks are left after the corrosion reaction.As the active head extends forward, the ‘‘V-shape”moves forward, turning the head into a body,and forming a filament via continuous corrosion;the width of the corroded wire remains unchanged.The mechanism of filiform corrosion is shown in Fig.7.97In general,the coating on the periphery of the fastener head will be broken and the blisters on the coating will often become the center of the initiation of filiform corrosion.After the electrochemical corrosion of the metal under the coating,the product of the cathodic reaction will affect the bonding between the coating and the metal substrate,resulting in the separation of the coating from the substrate.15Eq.(10) is the aluminum dissolution as acidic anode in the head of the filament.The cathodic reaction in the natural corrosion state is mainly the oxygen reduction reaction,which is shown in Eq.(11).The cathodic reaction in the cathodic protection state is mainly the hydrogen evolution reaction,which is shown in Eq.(12).Both reactions will cause the pH of the cathode area to increase.

3.Experimental methods for organic aviation coatings

3.1.Corrosion and aging experimental methods

3.1.1.Atmospheric exposure experiments

At present,experimental research on organic aviation coatings is mainly divided into two types: (A) Atmospheric exposure experiments and (B) Artificial corrosion and aging experiments.Due to the use of organic aviation coatings, they are often subject to external corrosion and aging environments,especially in high temperature, high humidity, high salt, and strong ultraviolet light environments.Moreover, aviation structures, especially military aviation structures, are parked on the ground most of the time,so it is very necessary to carry out atmospheric exposure tests on organic aviation coatings.The results of atmospheric exposure experiments are true and credible, but the test period is longer, the results are usually calculated in the units of years, and the test takes a lot of time, manpower, and material costs.98The main purpose of carrying out air exposure tests is to obtain the aging and failure data of organic aviation coatings in order to provide reliable data to evaluate the service safety of aviation structures.98

Fig.7 Corrosion mechanism of metal undercoating.97

Since the beginning of the 20th century, many countries around the world have established many atmospheric environment experimental stations and have carried out exposure tests of various types of materials and structures, which has accumulated a large amount of material corrosion and aging data.The American Society for Testing Materials has carried out atmospheric exposure tests, marking the beginning of largescale atmospheric exposure test research worldwide.98The natural environment test station network of the Atlas weather test service group is the largest and most powerful in the world.According to the statistics published in 2009, the group had a total of 23 test sites worldwide.99Among them, the Phoenix and Miami atmospheric exposure test stations have advanced outdoor accelerated test equipment and a large number of advanced artificial simulation accelerated test equipment, and various environmental tests involving as many as 850 standards.99The Kure beach ocean and atmosphere test site and Wrightsville beach seawater test site under the LaQue corrosion technology research center are world-renowned test sites that can perform comprehensive analysis and evaluation of test samples, such as failure, corrosion, and aging.99There are about 40 atmospheric exposure test stations in the UK,the largest of which is the Carrington exposure site.99The Choshi exposure test site of Japan Natural Exposure Testing Center (JWTC) ranks the second in the world.99Multiple exposure test stations under the Institute of Physical Chemistry of the Russian Academy of Sciences have been significant in terms of national defense, national economic construction,and product export.99

During the ‘‘Sixth Five-Year Plan”period, China began to establish atmospheric environment test stations,100and during the ‘‘Eighth Five-Year Plan”period, it carried out exposure tests on various coated aviation structures at 7 atmospheric environment test stations over 10 years.101,102With the support of the Chinese government,Li Xiaogang’s research team103,104created a corrosion data-sharing platform including 30 atmospheric environmental test stations, which covered standard materials in typical environments (air, soil, and water) in different regions of China.Atmospheric exposure test results at various test stations in China found that the loss of gloss of coatings in Beijing area was lighter than that of other environments, the gloss loss rate was almost zero, and the gloss loss rate of acrylic primer systems alone was 20 %.96Among several environments, the conditions of the Wanning coast have the most severe damage to aviation coatings.In the first 6 months, the gloss of most systems remained basically unchanged, but after half a year, the gloss performance began to decline.After one year of testing, the degree of influence of several environments, such as Jiangjin, Wuhan, and Guangzhou, on the loss of gloss of the coating was between that of Beijing and Wanning,but after three years of testing,the gloss of several coating systems was significantly reduced;the loss of light rate reached as high as more than 80 %.96Deng et al.105,106carried out one-year outdoor exposure tests in Wuhan and Lasa, and found that the bonding strength of acrylic polyurethane coatings changes exponentially with exposure time.In Lasa, photodegradation and aging are the main factors, whereas in Wuhan area it is aging via photodegradation and water degradation.Zhang et al.107found that in a tropical (marine atmosphere) monsoon climate environment, corrosive media (water, chloride ions, etc.) easily penetrates the coating to reach the substrate,and the substrate at the coating interface is prone to metal corrosion.Electrochemical Impedance Spectrum (EIS) of the coatings exposed in Wanning showed that the protective performance of the layer was significantly lower than that in Lasa and Mohe,and the sample basically loses its protective effect after 5 years of exposure, while the protective effect in Lasa and Mohe still exhibits a good protective effect.Fig.8104,107shows the scenes of several atmospheric exposure test stations in China and the results of the atmospheric exposure tests on polyurethane topcoat samples in different regions.

3.1.2.Artificial corrosion and aging tests

Artificial corrosion aging tests generally have two purposes:(A) Test the corrosion aging characteristics of materials in a single environment or a compound damage environment;(B) Simulate atmospheric exposure tests through accelerated aging tests in the laboratory to obtain a more efficient test method.The shortcomings of atmospheric exposure tests have led to the emergence of many accelerated aging test methods.40The requirement of the accelerated aging experiment is not to change the aging mechanism of the organic coating in the natural environment, but it is inevitable to introduce unnatural environmental aging factors in the actual accelerated aging experiment, which will obviously affect the validity of the accelerated aging experiment results.The accelerated corrosion and aging tests in the laboratory are easy to implement,which saves time and effort, but there will be a certain deviation between the test results and the actual atmospheric exposure test results, and the reliability of the test results completely depends on the scientific rationality of the accelerated corrosion method used.

Due to the long duration of the natural aging exposure test,many countries have developed outdoor accelerated aging test methods to meet the requirements of the natural environment of outdoor exposure and shorten the test cycle.The test method was used to strengthen one of the various factors that can affect aging to achieve the purpose of accelerating the aging process.Atlas has developed Equatorial Mount with Mirrors for Acceleration (EMMA) with mirrors and Equatorial Mount with Mirrors for Acceleration with Water Spray Cycles and Mirrors (EMMAAQUA) can automatically track the sun.108A highly polished mirror was used to reflect sunlight to the accelerated sample, which is equivalent to 8 times the solar radiation intensity (calculated according to the total amount of ultraviolet radiation), which reduces the required exposure time from several years to 12–18 months.109

Fig.8 Atmospheric exposure experimental station site and microscopic appearance of polyurethane top-coat samples after 5 years of atmospheric exposure in different areas.104,107

The outdoor accelerated aging test method still has the disadvantage of a long test period, so artificial accelerated aging experiments that simulate the outdoor natural environment(ultraviolet light, temperature, humidity, sulfur oxide, etc.)have been developed, for example, experimental methods of simulating a single aging factor (such as ultraviolet light,humidity, temperature, etc.) and multiple factors (such as ultraviolet and water, humidity, and SO2).

(1) Simulated light

Exposing the tested coating to a light source with a known spectral power distribution and corresponding humidity and temperature under certain conditions causes the coating to age in a period of time.Commonly used light sources are carbon arc lamps, ultraviolet lamps, xenon lamps, and highpressure mercury lamps.Among the light sources used, xenon lamps are considered to be the light source that best matches sunlight.108Many scholars use the Q-Panel Laboratory Ultraviolet (QUV) testing aging box to study the effects of ultraviolet light, water, and temperature on the aging of organic coatings,but there are few studies on the effects of single ultraviolet light on the aging of organic coatings.Signor et al.110used a 1000 W xenon lamp as a source of ultraviolet light to perform different degrees of aging tests on vinyl ester resin samples at room temperature and 30 %RH–50 %RH,and subsequently tested the chemical, physical, and mechanical effects of ultraviolet light on the resin to analyze the impact of the destruction process.Fig.9(a)110shows the xenon lamp aging test device,macroscopic morphology and load–displacement curve after aging.

(2) Simulated sulfur oxide pollution

O¨ zcan et al.76carried out an artificial accelerated aging test in a stable SO2atmosphere on polyurethane-coated galvanized steel.Fig.9(b)76shows the test device and the Nyquist curves obtained under different humidity during the test.Considering the influence of an SO2environment under different humidity at room temperature((25±2)°C)on the aging of the organic coatings, the air is adjusted to obtain a constant SO2concentration during the entire exposure process.To obtain a threeelectrode electrochemical cell, a polyvinyl chloride cylinder was glued to the substrate and filled with a 0.1 mol/L SO2solution,which was obtained by dissolving SO2gas in water.Hendricks and Balik111proposed the functional relationship between the amount of SO2adsorbed in the alkyd resin and time.The solubility and diffusion coefficient (D) of SO2in the coating were extracted and it was found that the dissolution of SO2in each sample follows Henry’s law; the diffusion was exponentially related to the concentration of SO2based on Fick’s law.The data obtained can be used to calculate the permeation level of SO2in the coating sample in the atmosphere.

Davis et al.77studied the effect of SO2combined with a high humidity environment on alkyd resins.SO2will give rise to spots and discoloration in the sample,and condensation on the surface of the sample will accelerate the reduction of the bonding force between the primer and steel substrate during the drying process.When the condensation evaporates,brown spots with higher sulfur content appear in areas where H2SO3or H2SO4was more concentrated.Samples that were not exposed to a high-humidity environment also have areas with higher sulfur concentrations, but they do not change color.A single high-humidity environment will gradually reduce the bonding force at the primer-metal interface and trigger the coating to peel, but SO2will increase the peeling speed.

Fig.9 Artificial corrosion aging test method.76,110

(3) Simulated temperature or relative humidity

When the temperature environment of the organic coating decreases from the glass transition temperature (Tg), the coating is in a non-equilibrium state due to the fluidity of the polymer segment,and the volume value is also at a non-equilibrium value.106This will lead to the degradation of the coating properties (mechanical, thermal, and insulating properties).Perera112found that, compared with the gradual increase in temperature and relative humidity, when the temperature and relative humidity gradually decrease from the highest point, the aging of the organic coating is more significant.The reason for this phenomenon is that the hot and humid environment affects the coating in terms of the enthalpy relaxation value and thermal stress value of the layer.

The high humidity environment of the aviation structure in service will cause a thin liquid film to form on the surface of the organic coating,which is mixed with the corrosive ions present in the atmosphere.When the droplets penetrate the interface between the coating and substrate, the substrate will corrode,so that the coating gradually peels off.106In addition,the surface of the coating immersed in the liquid film will undergo hydrolysis,causing molecular chain scission.106Dehri and Erbil113investigated the influence of different RH on the aging of zinc-coated low-carbon steel coated with defective polyester coatings and used the electrical simulation and semi-elliptic model developed by Erbil to fit the Nyquist curves.The results showed that the coating resistance decreases with an increase in the humidity and the coating capacitance will exhibit an opposite trend.

3.1.3.Research on equivalence

To achieve the purpose of the artificial corrosion aging test to quickly predict the actual service life of organic aviation coatings, many scholars have engaged in work seeking the equivalent relationship or correlation coefficient between artificial corrosion aging and atmospheric exposure.Currently, the most commonly used method is the time correlation coefficient, i.e., the ratio of the time experienced by the two tests when one or several properties of the coating reach a predetermined change during the process of atmospheric exposure and artificial corrosion and aging.This is the acceleration multiple or acceleration factor of the artificial corrosion aging test,which indicates how many hours of artificial aging is equivalent to how many months or years of atmospheric exposure.1At present, the generally selected properties are loss of gloss, chromaticity change, contact angle, etc., measuring the required time and change trend when the performance degradation values are equal, and drawing the relationship curve between performance change and time, as shown in Fig.10.The correlation coefficient can be obtained by comparing the tangent of the angle between the two.Since the curve is often non-linear, it is necessary to request the average value of the correlation coefficient at each stage as the final correlation coefficient, as shown in

Fig.10 Performance change vs time curves.

where Knis the total equivalence coefficient; knis the equivalence coefficient; αn1is the angle between the artificial aging curve at a given moment; αn2 is the angle between the atmospheric exposure curve at a given moment.

However,it should be noted that due to the variability and instability of atmospheric conditions, artificial corrosion and aging can only simulate a few main conditions and there are large errors in the results.Moreover,the method of using only one performance index to establish the equivalent relationship is not highly reliable, so the equivalent relationship calculated using this method cannot be used rashly and can only be used as a reference.Deflorian et al.114pointed out that due to the different meteorological conditions experienced in different years,there is no way to fully characterize all of the influencing factors of atmospheric exposure, but several typical environmental variables can be used to define the atmospheric exposure environment, such as the total energy of ultraviolet light, total time for the RH above the threshold, and environmental temperature higher than Tgof the polymer, to verify the correlation between artificial corrosion and aging (ultraviolet light,thermal cycling,salt spray,etc.).Santos et al.115proposed that the results of an accelerated aging test can only be used to characterize the performance of atmospheric exposure and cannot be used as proof to predict the life of the coating.Although current research on the correlation coefficient between air exposure and artificial corrosion and aging of organic aviation coatings has several bottlenecks,there are still a large number of researchers seeking breakthroughs.For future research, the following aspects should be considered:

(1) The coating samples used for atmospheric exposure and artificial corrosion aging tests should be the same type,prepared using the same conditions, and be in the same initial state before the test is carried out.

(2) The instruments and equipment used for the testing related performance should be the same and the testing methods should be the same to reduce human error.

(3) The number of coating samples should follow the minimum number of test pieces required in statistics and the test method used for characterizing the performance should be repeatable and reproducible.

(4) The test results and correlation coefficients obtained according to statistical analysis methods are only applicable to specific types of coatings.The aging mechanism and degradation rate of coatings of different types and formulations are different, so they cannot be extended to other coatings at will.

3.2.Performance characterization method

3.2.1.Macroscopic morphology characterization method

The significant macroscopic characteristics of the corrosion and aging of organic aviation coatings are loss of gloss, chromaticity change, chalking, thinning, etc.Therefore, the loss of gloss,color difference,contact angle,and thickness changes in the coating are generally characterized to macroscopically characterize the change law of corrosion and aging.116

(1) Glossiness

The gloss change of the coating before and after aging can be measured using a gloss meter and the evaluation of the loss of gloss level refers to the ISO 2813 standard.117The gloss under geometric conditions of 20°,60°,and 85°can be selected according to the characteristics of the coating.The 60°geometric condition is suitable for all coatings,the 20°geometric condition can give better resolution in the case of high-gloss coatings,and the 85°geometric condition can give better resolution in the case of low-gloss coatings.117The formula for calculating the loss of light is given as

where GLis the loss of gloss;G0is the gloss of the unaged coating; G is the gloss of the coating after aging.

(2) Chromaticity

It can be seen from Eq.(18)that the sum of squares of lightness difference, chroma difference, and hue difference is equal to the square of the total color difference.

Some scholars have studied the process of coating aging by measuring the Yellowness Index (YI) of the coating.Hu et al.119found that acrylic polyurethane coatings will turn yellow in both ultraviolet and xenon lamp environments because the coatings absorb certain wavelengths of ultraviolet light during photooxidation, forming a highly colored chromophore, such as quinone and stilbene quinone.

(3) Contact angle

The change in the wettability of the organic coating surface can be observed by measuring the contact angle.120–122The types of contact angles are generally divided into Water Contact Angle(WCA),Diiodomethane Contact Angle(DCA),and Ethanol Contact Angle (ECA).The surface free energy (γ),polar component (γp), and dispersion component (γd) of the coating can also be calculated to make a preliminary judgment on the degree of coating aging, as

where the subscripts S and L represent the solid and liquid,respectively; θ is the contact angle; WaSLis the work of adhesion.Zhang et al.56measured the contact angle and surface free energy of epoxy coatings exposed to the atmosphere for 7,12,and 20 years,and found that the aging of the epoxy coating produces hydrophilic groups, which enhance the surface wettability.It was also found that the dispersion component increases with the exposure time, indicating that bond rupture occurred in the epoxy coating during the aging and degradation process, which promotes the cross-linking reaction and increases the surface density.

3.2.2.Microscopic morphology characterization method

(1) Laser Scanning Confocal Microscopy (LSCM)

LSCM also plays an important role in materials science.In addition to its obvious use in high-resolution reflected and transmitted light optical microscopy, it also has many advantages in materials research.The LSCM can be used for pseudo-infinite depth of field imaging, topographic imaging,photoluminescence imaging, and Raman spectroscopy imaging.123LSCM uses a collimated laser as a‘‘point”illumination source, in which the point is focused on the coating and the reflected light, transmitted light, or fluorescence is focused through a pinhole to provide a ‘‘point”detector.The pinhole defines the second cross focus and serves as the wide-angle filter for the detector.If the scattered light does not pass through the pinhole,it is not focused.Once the light passes through the pinhole, it is usually divided into different wavelengths using dichroic mirrors and filters, and then detected using a Photomultiplier Tube (PMT).The large-area imaging of the sample is achieved using a mirror controlled by a piezoelectric actuator to rasterize the beam on the sample.Fig.11123shows the principle of the optical path of a laser confocal microscope.Wang et al.124used LSCM to study the early aging surface characteristics of organic coatings, found defects and voids caused by photooxidation during the early stage of epoxy coatings, and compared the initial aging resistance characteristics of different coatings.

Fig.11 Optical path principle of LSCM.123

(2) Atomic Force Microscopy (AFM)

One end of the cantilever of the AFM probe is fixed and the other end is a tip that is in slight contact with the coating surface.The cantilever is very sensitive to signal feedback of weak loads.41When the probe is scanned, a tiny repulsive load is generated between the tip of the needle tip and the atoms on the coating surface.The system will control this load to be constant and the probe will drive the needle tip to undulate in a direction perpendicular to the coating surface.Then, the position change of the probe during the scanning process is fed back using the optical detection method or the tunnel current detection method and the microscopic morphology of the coating surface obtained.In addition,AFM-Infrared Spectroscopy(IR) analysis uses top-down illumination to test the feedback of the coating using fast pulses (10 ns duration, 1 kHz repetition rate) from a tunable infrared source (optical parametric oscillator).The absorption of infrared radiation will cause a sudden thermal expansion of the coating surface,which is then detected by the deflection of the AFM probe in contact with the surface.The recorded AFM-IR signal is either the maximum peak-to-peak deflection observed during the cantilever ringing period or the induced oscillation amplitude after fast Fourier transform,as shown in Fig.12.62Morsch et al.62used the peak force mode to characterize the morphological changes of the coating with water adsorption and ion penetration,and then carried out AFM-IR analysis to determine the location of the aging coating that absorbs water.Moreover, the research team also determined the uneven curing of epoxy resin on the nanoscale using AFM-IR and proved for the first time that chemical heterogeneity is the basis of the characteristic nodular structure in epoxy resin.125

3.2.3.Micro-area electrochemical analysis method

Fig.12 AFM-IR test using top-down illumination.62

EIS has been widely used to characterize the corrosion protection performance of coatings.However,characterization of the coating properties measured using traditional EIS only provides average information for large-area coatings.Therefore,the local electrochemical processes at the micro-defects, such as pinholes in the coating, are ‘‘a(chǎn)veraged”.126Therefore, EIS can be used to characterize the general state of the coating,but cannot be used to characterize local changes.The use of some new micro-area electrochemical techniques that perform local measurements can help understand the anti-corrosion performance of a defective coating in more details.

(1) Local Electrochemical Impedance Spectroscopy (LEIS)

Similar to the traditional AC impedance method,LEIS also applies a perturbation voltage to the working electrode via a potentiostat to generate an alternating current between the working electrode and remote counter electrode, and then measures the AC using a probe composed of two platinum microelectrodes.The potential difference is used to derive the local AC density to reflect the local impedance changes.112,113In general, the planar distribution image of the local AC or local AC potential can be obtained using LEIS measurements,and the impedance, admittance and other related parameters of the local area can be calculated to obtain a threedimensional microscopic image of the area under the coating.Huang et al.127used a self-made potentiostat and four-channel Frequency Response Analyzer (FRA) to construct a LEIS measurement device,which can simultaneously measure global and local impedance changes,as shown in Fig.13.127The dualprobe electrode is driven by a 3-axis positioning system,which breaks through the limitation that traditional LEIS equipment cannot measure the global impedance.

(2) Scanning Kelvin Probe (SKP) method

Traditional reference electrode technology needs to establish a conductive pathway between the working and reference electrodes.The SKP method can overcome this difficulty.128The SKP uses a vibrating condenser to measure the work function.In some cases, the work function is determined by the electrode potential,so the SKP can measure the local electrode or corrosion potential.129In principle, the SKP consists of a metal reference electrode, which is separated from the sample by a dielectric and connected to the sample via a metal wire.130The biggest difference between the SKP and traditional electrochemical equipment is that it can measure the potential distribution at defects without touching the coating.

(3) Scanning Electrochemical Microscopy (SECM)

SECM can display the reactivity distribution of the coating surface so that the degradation rate at a certain point on the coating surface can be calculated.131In addition to the traditional three-electrode system, SECM uses Ultramicroelectrodes (UME) with a radius of 5 μm as probes.The probe is driven to the surface of the sample to accurately determine the gap distance between the probe and sample.When the UME current (iT) reaches a specified value, the tip automatically stops.The normalized current (IT, the measured tip current divided by the measured tip current in the bulk solution)and the normalized distance (L, the distance between the tip and the sample divided by the radius of the UME) are drawn as the Probe Proximity Curve(PAC).Fig.14131is a schematic diagram of the SECM measurement system, where CE means Counter Electrode.The Working Electrode (WE) with two defects is sandwiched between two pieces of polytetrafluoroethylene to complete the electrochemical cell.Xia et al.131observed a single defect of an organic coating using SECM and found that the surface reactivity of the defect area was significantly higher than that of the complete area, and the PAC modeling results showed that due to the formation of corrosion products on the substrate,the rate constant at the coating defect first increased and then decreased.Jensen et al.132obtained an SECM probe scan curve in redox competition and feedback mode to provide information on the corrosion process of organic coating substrate defects, but found that when the probe was too close to the substrate, the accumulation of corrosion products would produce interference.

Fig.13 LEIS measuring device and partial enlarged view of probe.127

Fig.14 Schematic diagram of local Electrochemical Noise(EN)measurements using SECM.131

(4) Electrochemical Noise Method (ENM)

Electrochemical noise can be used to monitor the corrosion of organic coatings without external potential or under constant potential or current conditions; it is a non-invasive and non-destructive non-interference technology.And ENM has simple circuit settings and various data analysis methods,which have great potential applications in laboratory or field conditions.131The basic principle of ENM is to measure both the potential and current fluctuations caused by the selfgeneration chemical reaction and then conduct mathematical analysis on the potential and current data.133There is also research to propose a ‘‘single-cell”configuration to eliminate the problems caused by the asymmetry of the two electrodes,which has good consistency with EIS, as shown in Fig.15.134The simplest quantitative method used in the data analysis was to calculate the ratio of the standard deviation of the noise signal, i.e., the noise resistance Rn, as

Fig.15 ENM illustration of ‘‘single-cell”configuration.134

where σ(v) and σ(i) are the standard deviations of potential and current fluctuations, respectively.It should be noted that Rnper unit area needs to be normalized when calculating the absolute corrosion rate.135At present, the Rnvalue has been proven to change with the degradation of organic coatings,but whether ENM can be as repeatable as DC resistance measurements or EIS remains to be further studied.

3.2.4.Spectral characterization methods,such as FTIR,Raman,and XPS

(1) Fourier Transform Infrared (FTIR) spectroscopy

The main components of organic aviation coatings have characteristic absorption peaks in the infrared region.136,137When the coating is in contact with the IR radiation absorbing material (Attenuated Total Reflectance (ATR) crystal) with a high refractive index,part of the radiation beam will penetrate the coating and the other part will be reflected.The attenuation of the coating can be measured using a spectrophotometer to obtain the characteristic absorption spectrum of the coating.138This method can qualitatively and quantitatively analyze the changes in the coating polymer, thereby assessing its aging characteristics and using them to analyze its aging mechanism.However,it should be noted that if the contact between the coating material and the ATR crystal is not good, it will significantly reduce the signal-to-noise ratio, thereby reducing the quality of the spectrum obtained.139Moreover, when studying colored coatings, the results may be disturbed due to the high absorption coefficient of the pigment particles.

(2) Raman spectroscopy

Raman spectroscopy also analyzes the chemical composition of the coating by detecting the molecular vibration or rotational energy levels.84,85,140Unlike infrared spectroscopy,Raman spectroscopy is based on inelastic scattering measurements due to molecular vibration,and the frequency of scattering is different from the incident frequency.141If the molecule vibrates symmetrically and thus changes the polarizability,this vibration is suitable for measurement by Raman spectroscopy.If the molecule undergoes an anti-symmetric vibration and thus changes the dipole moment, this vibration is suitable for measurement by infrared spectroscopy.142When analyzing thin coatings, it is necessary to analyze the spatial Raman information and measure the Raman scattered light at different positions of the coating using Confocal Raman Microscopy (CRM).142As shown in Fig.16,40Cai et al.40studied the aging process of polyurethane coatings along the depth direction under UVA irradiation using CRM.

Fig.16 CRM depth spectrum of an fluorinated polyurethane coating.40

(3) X-ray Photoelectron Spectroscopy (XPS)

X-ray photoelectron spectroscopy irradiates the surface of the coating by an X-ray beam to excite the electrons in the coating atoms into photoelectrons.During this process, the energy of the photoelectrons is calculated and the element composition, chemical state, and chemical bonds of the coating can be analyzed.143Since the photoelectron beam is less destructive to the sample, this method is very suitable for analyzing polymeric materials, such as organic coatings.144,145

3.2.5.Differential Scanning Calorimetry (DSC) and thermo gravimetric analysis methods

DSC can obtain Tg, crystallization rate, and other parameters by measuring the relationship between the power difference before and after coating aging and temperature.Relevant studies have shown that Tgof an epoxy coating is closely related to the degree of crosslinking: the higher the degree of crosslinking,the higher the Tg.By increasing the degree of crosslinking,a denser coating structure can be obtained, and thereby better waterproof permeability will be obtained.146

Thermo Gravimetric Analysis (TGA) can obtain thermal stability and thermal decomposition information on a coating by measuring the relationship between the quality of the coating and temperature,thereby obtaining the weight loss change of the coating before and after aging.147Magnetic Thermo Gravimetric Analysis (MTGA) is a new experimental technique that allows the activation energy of different processes involved in the degradation process to be obtained from a single experiment, so there is no need to assume any type of kinetic model in advance.148The conventional TGA experiment is carried out at a heating rate of 1–30 K/min to obtain the kinetic parameters of MTGA.

4.Research on calendar life and life prediction method of organic aviation coatings

4.1.Coating aging assessment and failure judgment

The failure modes of organic aviation coatings are generally loss of gloss, chromaticity change,chalking,peeling, cracking,etc.149But often,before the above-mentioned features are visually observed,the coating has lost its ability to protect the substrate.150Therefore,it is necessary to evaluate the aging degree of organic aviation coatings and develop a set of authoritative standards to determine the failure of coatings, to predict the remaining life of the coating, and to better guide maintenance work in this field.21,151This has always been a problem.152,153

The EIS is the most widely used non-destructive testing method for the aging assessment and failure judgment of organic aviation coatings.The more commonly used methods for evaluation and judgment using the EIS are the equivalent circuit fitting method, characteristic frequency method, and low-frequency impedance method.The equivalent circuit fitting method is the most widely used, but it has the following two drawbacks: (A) It requires a basic understanding of the failure mechanism of the coating/matrix system and it is impossible to set a reasonable effective equivalent circuit model for the coating without knowing its aging process;(B) There is a certain error in the process of extracting the coating capacitance and coating impedance using fitting the model.Therefore, the characteristic frequency method and low-frequency impedance method are fast and effective methods for evaluating the aging of coatings.

Fig.17 Time correlation of φmin, fmin, and fb.154

Mansfeld154conducted an electrochemical analysis on a coating after a 5-week exposure test and found the minimum value of the phase angle (φmin), its corresponding frequency(fmin),and breaking point frequency(fb)(when the phase angle drops to 45°) change with an increase in the coating damage.The values of fminand fbwill be larger and larger, and the value of φminwill be smaller and smaller, as shown in Fig.17.154This is a quick way to evaluate the anti-corrosion performance of coatings.Haruyama and Sudo155also found in a later study that the frequency at which the phase angle was attenuated to 45° can be used as fb, which can reflect the distance between the coating defect and the nearest edge of the local corrosion peeling site,to evaluate the corrosion resistance of the layer.Fig.18155shows the Bode and Nyquist simulation diagrams of the local corrosion in samples of different sizes.

Feng and Frankel156used the characteristic frequency method to evaluate paint systems and found that when the power failure frequency was more than 102Hz, visual corrosion would occur.At the same time,the limitations of the characteristic frequency method were discovered, i.e., it is only applicable to the initial stage of coating aging or very small defect areas, and the accuracy is low when evaluating the long-term aging stage of the coating or a large range of defect areas.

Many studies have taken low-frequency impedance modulus data as an indicator of the corrosion resistance of coatings.According to experience, the corrosion resistance of the coating can be assessed as failure, fair, and good.157The lowfrequency impedance modulus of the coating system is rated as good when it is 109–1010cm-2, fair when it is 106–109cm-2, and invalid when it is less than 106cm-2.157

Fig.18 Bode and Nyquist simulation diagrams of local corrosion in samples of different sizes.155

Bierwagen et al.157combined EIS with thermal cycling tests to provide a faster and quantitative evaluation method for the coating corrosion resistance.The natural aging process of military organic aviation coatings was also simulated using ultraviolet-condensation and salt spray-drying cycle tests.Fig.19158shows the change curves obtained for the lowfrequency impedance of several conventional organic aviation coatings with the exposure time.The author used the characteristic frequency electrochemical impedance data to establish the exponential failure model:158

where |Z|(t)is the impedance at a given moment; |Z|mis the impedance limit value of the pure metal; |Z|0is the coating impedance value at time t = 0; q is a time-dimensional constant, which can be regarded as the characteristic decay time of the coating, and is also the reciprocal of the slope of the exponential fitting curve.Assuming that the decay time to a specific failure impedance value needs to be calculated,Eq.(24) is used in terms of the coating failure time tfailas a function of the selected failure modulus:

where |Z|failis the impedance at failure.

Fig.19 Plot of |Z| = 0.012 Hz vs exposure time for several organic aviation coatings.158

The coating failure model (BG model) established by Bierwagen has been widely used to determine the aging degree of organic aviation coatings, whether the coating fails, and predict its remaining life.7,95,157,159–161However, the prediction results obtained using this model are often conservative and may not be conducive to determining the remaining life of the coating.

Sekine162summarized three methods for evaluating the aging degree of coatings using electrochemical measurements,as shown in Fig.20.162Breakpoint time method is to evaluate the aging of the coating with the time point(tb)when the coating resistance Rfsuddenly drops.Substrate impedance time method is to use the time (ts) when Rfdrops to be equal to the substrate impedance.Constant Rfmethod is to use the time when Rfdrops to a certain value for evaluation.Both tband tscan be regarded as the apparent lifetime of the coating.In addition, by comparing tb1(obtained from the curve of the maximum phase angle corresponding to fθmaxversus time,where fθmaxis the maximum phase angle frequency) and tb2,it was found that the two have a good linear relationship, so in addition to Rf, fθmaxcan also be used to evaluate the coating’s degradation performance.

Mahdavian and Attar163extracted the high-frequency phase angle (10 kHz) of epoxy coatings during the aging process using EIS and found that its change has a good agreement with the coating resistance.Because the method of extracting the high-frequency phase angle is relatively simple, the error of this method is smaller than that of extracting the coating resistance and coating capacitance using the fitted equivalent circuit model.Zuo et al.164found that the phase angles at intermediate and high frequencies can show good agreement with the changes in the coating resistance.When the phase angle of 15 kHz rapidly drops to below 70°,the protective performance of the coating system is basically lost.

In addition to the EIS, there have been many studies performed to evaluate the deterioration of coatings using other macro-characteristics.Dr.Takashi Yamamoto of Japan is the inventor of the corrosion tester.After consulting more than 26000 articles on coatings worldwide,he divided the coating life into two stages and proposed a coating life prediction formula under natural conditions and electrolysis.He believed that the rust area rate of a coated steel plate(or the area rate of open holes in the coating) reached its end of life when it reached 5 %, which has been verified using a large number of experiments.165The results of this evaluation model are dangerous because corrosive media may have penetrated into the openings of the coating leading to corrosion of the substrate.Zhou et al.166reduced nine individual indicators including loss of gloss, chromaticity change value, degree of chalking, number of cracks, and size of blistering, determined the weight of each evaluation indicator based on rough set theory,and finally established the military comprehensive evaluation model for corrosion damage of organic aviation coatings.The idea of this model is very novel and can fully meet the requirements of coating aging evaluation.However, a large amount of data needs to be measured and many of the indicators are evaluated using visual methods, so the contingency caused by manual errors is relatively large.

Fig.20 Three methods used to analyze corrosion resistance of coatings through change curve of coating impedance with time.162

In summary, the apparent characteristics of the coating’s gloss loss and pitting area rate can be used to evaluate the aging degree of a coating, but this method does not evaluate the aging degree of the coating mechanically.The breakpoint frequency, maximum/minimum phase angle and its corresponding frequency,and low-frequency impedance of the coating at different aging cycles can also be used to evaluate the degree of aging of a coating, and the degradation level of the coating can be evaluated from the mechanism.There is no uniform failure criterion to effectively and accurately determine whether the coating has expired.

4.2.Calendar life prediction of coating

The service environment of organic aviation coatings is very complicated and their corrosion resistance is affected by many factors, such as temperature, humidity, salt ion deposition,ultraviolet light radiation, sand and dust erosion, porosity,resistivity, and so on.167–170It is currently not possible to quantitatively describe how these factors affect the calendar life of these coatings.At present, there are two ideas for the calendar life prediction of organic aviation coatings:(A) Ignore the failure mechanism and influencing factors of the coating, and use pure mathematical theory to establish parameter models of input variables and output variables,such as greyscale system theory method and artificial neural network method and(B)Analyze the influence law of each factor to establish a mathematical model based on the failure mechanism of the coating.The drawback of the Approach B is that it often only conducts research based on a single factor variable.When creating the model, the aging process of the coating is too idealized and singular,and there is no in-depth study on the coupling effect of each influencing factor,weight distribution, etc., so it is often not widely used.At present, there is not much literature on the calendar life prediction of organic aviation coatings, which also shows that the life prediction of coatings has always been a difficult problem in this field.At present,common methods for predicting the life of organic coatings include diffusion theory,grey system theory,and artificial neural networks.

4.2.1.Diffusion theory

Cmaitland and Mayne165first proposed the polarization resistance control theory of organic coatings based on the Fick diffusion model.It was first proved that the coating has good water and oxygen permeability, so the corrosion current can pass through the coating at the local anode and cathode.If the resistance value of the coating is increased, the corrosion current will be reduced.Electrochemical analysis has been carried out for a coated steel plate and the formula for the calendar life of the organic coating was deduced as

where CL is the calendar life of the organic coating; l is the thickness of the coating; DCis the diffusion coefficient of the corrosive medium in the coating; φ is the coefficient; psis the bonding force between the coating and the substrate; σnis the interface pressure between the substrate and coating.The difficulty of this method is how to accurately measure the diffusion coefficient of corrosive media in the coating.Since the composition and concentration of corrosive media will change with the corrosion process,the complexity and difficulty of the measurement process will increase.

4.2.2.Grey system theory

The principle of the grey system theory method is to process several uncertain and irregular factors into a regular model,in order to achieve the purpose of prediction.This method is the same as the diffusion theory method.It is necessary to measure the basic parameters of the coating system in advance,such as the low-frequency impedance.The GM(1,1)model is a typical representative of grey system theory.This is a singlesequence first-order grey differential equation.When modeling, the original data must be equally spaced.The general differential equation of the grey dynamic GM (1,1) model is14

Grey system theory has been widely used in the field of corrosion.For example, Li et al.171,172used the grey correlation method to study the influence of the structure of amine corrosion inhibitors on the corrosion rate of carbon steel in acidic solutions.Meng et al.173established a characterization model of Epoxy Varnish (EV)/steel and Epoxy Glass Flake (EGF)coating/steel wet adhesion based on the grey GM (1,1) model,as shown in Fig.21.173

where W is the critical value of wet adhesion.

Fig.21 Changes in logarithm of adhesion-immersion time of coating/steel system under normal pressure (0.1 MPa) and hydrostatic pressure (3.5 MPa).173

The prediction accuracy of the GM(1,1) model is higher than that of traditional statistical model.However, the limitation of this method is that it can only predict when the number of samples was small and the prediction time was short.When the number of samples is large and the prediction period is long, the accuracy will decrease.96

4.2.3.Artificial neural networks

The Artificial Neural Network (ANN) method deals with the connection between objective things in the real world in the same way as the biological nervous system.174ANN uses electronic components or computer software to simulate the structure and function of human brain nerve cells.It is a spatially irregular topological structure formed using a large number of simple neurons (processing units) connected to each other.14,175,176Its advantages are non-linearity, distributed storage, fault tolerance, learning functions, massively parallel processing, and powerful computing capabilities.It is a very powerful and efficient information processing system.14,177,178The ANN model does not include the pre-conceived influence relationship, but attempts to identify and quantify the hidden relationship between the dependent and the independent variables.The main advantage is that the prediction results are completely based on the data instead of a pre-set influence relationship and the network can infer the effect through learning and training without specifying the mathematical model for inference.179,180

ANN is more and more widely used in the field of corrosion and protection.Mansfeld181used neural network technology to evaluate the corrosion resistance of organic coatings for long-distance marine facilities and evaluated the corrosion resistance of organic coatings from the experimental impedance spectra.The Backpropagation (BP) model is the most used algorithm in the field of corrosion protection and its principle is the BP learning theory of the forward multilayer neural network.177The commonly used BP network model is composed of an input layer, hidden layer, and output layer, and its basic structure is shown in Fig.22.179In theory,any continuous function in a closed interval can be approximated using a hidden layer BP network.

Tian et al.182used the pressure value, immersion time, and impedance modulus as input variables and established a BP Artificial Neural Network (BP-ANN) to predict the calendar life of organic coatings.In the three-layer BP-ANN model,assuming that the input element of the input layer is pj, then the hidden element and output element of the hidden layer and output layer can be expressed by

Fig.22 Schematic diagram of BP-ANN model structure and each neuron in network.179

where a1iand a2kare the output elements of the hidden and output layers, respectively; f1and f2are the transfer functions of the hidden and output layers,respectively;w1ijis the weighting factor connecting the input layer to the hidden layer (the probability of data transmission along the path); w2kiis the weighting factor that connects the hidden layer to the output layer; b1iand b2kare the bias values that connect the input layer to the hidden layer and the hidden layer to the output layer, respectively; r is the number of hidden layer iterations;s1is the number of output layer iterations.The goal of the training process is to model the mapping between input and output, not the noise in the data.In the training process, the weight factors and bias values of the BP-ANN model were adjusted to minimize the error between the model output and the target based on a training data set containing a set of input vectors and corresponding output values.The performance function or error function should be minimized during training.A typical performance function takes182

where Ntiopis the number of training input and output pairs;tkis the actual output (target).

Shi et al.179developed an ANN model based on variables such as temperature, pH, Electrochemical Corrosion Potential (ECP), stress intensity factor, and yield strength to predict the main water profitable corrosion crack growth rate of Alloy 600.In addition, the contribution of various independent variables to the determination of the crack growth rate was determined based on sensitivity analysis using the‘‘fuzzy curve”and finally, the temperature was found to be the main influencing variable that determines the Crack Growth Rate (CGR).Lin and Cunningham183adopted the fuzzy curve method using fuzzy logic to establish the relationship between the input and output variables.Assuming that there are Ntiopinput variables X = (x1,x2,...,xN) in the system, one output variable Y = [yi] and the number of training modes (or training data points) is P.Gaussian function was used to generate the fuzzy curve:183

where φikis the membership function used to form the fuzzy curve for each input candidate; xikand xiare the kth pattern;b is the model parameter;I is the number of data bars;Ciis the value of fuzzy curve space; ykis the output of the kth neuron.

In the fuzzy curve graph in the xi-Ci(xi)space,the input significance is related to the range of the fuzzy curve on the C axis.A larger range means a larger span and greater salience.On the contrary,a small range means a smaller span and lower significance.

Prudhvi Sai et al.184developed a feedforward BP network with Levenberg–Marquardt algorithm to predict the corrosion potential and corrosion rate of the AA5083 friction stir processed samples and obtained the prediction of the ANN model through the error histogram and fitting effect map.The results are in good agreement with the experimental results.Kenny et al.185established an artificial neural network corrosion model for low-carbon steel,copper,and aluminum in an equatorial climate based on linear and sigmoidal functions, and predicted the corrosion rate based on environmental coefficients.Cai et al.186constructed a 5-8-1 neural network model to predict the atmospheric corrosion behavior of steel and zinc.The input factors used were temperature, relative humidity,exposure time, SO2concentration, and Cl–concentration,and the output result is the corrosion depth.It turns out that the neural network prediction method can take into account the change of more than 70 % of atmospheric corrosion.At the same time, Cai et al.186pointed out that due to ignoring other important influencing factors and the inherent dispersion of the data, the forecast has a certain deviation.

Although the ANN provides a convenient and accurate prediction method, it is a pure empirical model and is often criticized because it cannot explain the relationship between input and output in terms of the mechanism.Therefore, the ANN is also called the‘‘black box”model.179At present,some scholars combine ANNs with grey scale system theory and use the characteristics of the former’s non-linear mapping and the ability of data parallel processing to correct output errors and improve the GM(1,1)model,thereby improving the prediction accuracy.187

The characteristics of simplification and idealization are reflected in the current research on the failure judgment of coatings and the prediction of calendar life, and it is difficult to comprehensively consider the coupling effect of various parameters and weight distribution.How to determine the failure time of the coating and predict the calendar life while considering the failure mechanism and comprehensively weighing various characteristic parameters is the key breakthrough direction of future research.

5.Modification methods of organic aviation coating

The fillers of traditional organic aviation anticorrosive coatings are mostly based on chromate, phosphate, etc., which not only are toxic to the human body, but also pollute the environment.49,88,188–190At present, a large amount of literature has introduced research on the modification of organic aviation coatings,including organic fillers and inorganic fillers,conductive and non-conductive fillers, etc.(such as polyaniline, phytic acid, carbon black, carbon nanotubes, aluminum pigments,mica iron oxide,zinc nanoparticles,titanium dioxide nanoparticles, nano clays, etc.).Sections 5.1–5.3 will focus on several advanced and widely researched fillers in organic aviation coatings.These fillers can improve the corrosion resistance of the coating to a certain extent,which give rise to fewer negative effects on the ecological environment.

5.1.Modification using nano-cerium oxide

CeO2has strong activity, is hard to dissolve in water and acid, and has good chemical and thermal stability.As shown in Fig.2,8,22,24,25,31,38–43CeO2has a face-centered cubic structure.At present, the more commonly used CeO2preparation methods are sol–gel, microemulsion, chemical precipitation, and hydrothermal methods.191Nano-CeO2-modified organic coatings can inhibit the corrosion of the substrate,mainly due to the following reasons: (A) When nano-CeO2is in contact with the metal substrate, it can form a passivation film on the active dissolution site of the substrate,thereby hindering ion transfer and reduce the rate of electrochemical corrosion;38,192(B) The Ce element in nano-CeO2has two forms, Ce3+and Ce4+, which can be transformed into each other, which gives CeO2strong redox properties;(C) The OH–and cerium particles produced in the cathode area will generate more stable precipitates, thereby reducing the corrosion of the substrate.193

Montemor and Ferreira194found that Ce-activated nanoparticles inhibit the corrosion activity at the defects using Scanning Vibrating Electrode Technology(SVET)and showed that adding CeO2nanoparticles was more effective than adding SiO2nanoparticles,as shown in Fig.23,194where scan size is 2 mm×2 mm.Because CeO2nanoparticles are stable over a wide range of pH, they possess good corrosion inhibition properties on the coating surface.In addition, they can complex with other substances, which contributes to the stability of the passivation film.

Kumar et al.195prepared a series of polypyrrole nanocomposite coatings with different CeO2contents on an AA2024 aluminum alloy substrate and found that the formation of oxides between Al and Ce greatly enhances the protective performance of the coating:

The protective mechanism of the nanocomposite coating on the aluminum alloy substrate is shown in Fig.24.195The research conclusions show that nano-CeO2modified polypyrrole nanocomposite coatings have great potential in the field of aviation aluminum alloy protective coatings.195Ashraf and Shibli196also confirmed that the synergistic effect of Al and CeO2can greatly inhibit the corrosion rate.Hinton et al.197,198found that cerium cations form a cerium oxide conversion complex on the surface of AA7075 high-strength aluminum alloy; the main component is Al-Ce-O and it also contains Ce(IV) species (CeO2and Ce(OH)4).The mixture with Ce(III) (Ce(OH)3) always maintains a stable corrosion resistance in the pH range of 4.5–8.0.More interestingly, Hill et al.199found that the free Ce site on the inhibitor complex can also coordinate with atoms on the Al surface to form an Al/Ce bimetallic complex to form a protective insoluble film.

Fig.23 SVET obtained in a scratched filled film upon immersion in 0.005 mol/L NaCl.194

Fig.24 Schematic diagram of corrosion protection mechanism of a PPy/CeO2nanocomposite coating on AA2024 alloy in 0.6 mol/L NaCl solution.195

Fedel et al.193speculated two possible reasons why CeO2improves the corrosion resistance of epoxy-polyester polymers.One was to consider the combination of soluble cerium oxide and hydroxide ions (OH–) produced by the cathode on the metal, and the cathode site precipitate.The second is that the ground cerium oxide particles act as a complexing agent to react with the zinc ions corresponding to the anode site,resulting in the formation of stable corrosion products.Fedel et al.200further studied the above speculation and found that nano-CeO2particles reduce the electrochemical performance of the substrate surface and form a passivation film, which has a strong anti-corrosion effect.In addition, the corrosion resistance stability of the sample can be maintained for the longest time when the particle content was 3wt%.

Zhang42summarized several aspects of the impact of nano-CeO2particles on the anti-corrosion performance of composite epoxy composite coatings.First, CeO2nano-particles are dispersed and filled in the gap, making the diffusion pathway for the corrosive medium more tortuous.In addition, the oxidation–reduction reaction of Ce4+/Ce3+is very active and the effect of inhibiting corrosion is significant.The specific reaction is shown in Fig.25.42

5.2.Modification using carbon nanotubes

Using Chemical Vapor Deposition(CVD),Single-Walled Carbon Nanotubes (SWCNTs), Double-Walled Carbon Nanotubes (DWCNTs), and Multi-Walled Carbon Nanotubes(MWCNTs) can be obtained by decomposing carbon sources in the presence of metal-supported catalysts at high temperature.201,202The aspect ratio and specific surface area of carbon nanotubes are large and they also possess good electrical conductivity, thermal stability, and mechanical properties.203–205In particular, a small number of carbon nanotubes dispersed in the organic coating can effectively improve the corrosion resistance,thermodynamic properties,and mechanical properties including the stiffness,fracture toughness,and elastic modulus206–208In general, the anti-corrosion principles of carbon nanotube modified organic coatings are as follows:(A)Carbon nanotubes can be filled into the micropores on the surface of and inside the coating to make the coating denser and reduce microcracking in the coating;38,209(B) Carbon nanotubes can enhance the adhesion strength between the organic coating and the substrate.209–212

Fig.25 Anti-corrosion mechanism of nano-CeO2composite epoxy coatings.42

At the same time,it should be noted that carbon nanotubes will agglomerate via van der Waals forces to form ‘‘tube bundles”due to its high aspect ratio and weak interface interaction with the polymer matrix.213This will cause the mechanical properties and corrosion resistance of the coating to decrease.Therefore, research on the modification of carbon nanotubes must first solve the problem of whether they can be uniformly dispersed.For example, epoxy resins often used in organic aviation coatings which belong to mediumpolarity organic solvents and dispersants such as Tris,N-Methylpyrrolidone (NMP), carbon nanotube alcohol dispersant(TNADIS),etc.are needed.A large number of studies have shown that physical adsorption or chemical bonding can improve the interface interactions formed between the nanoparticles and polymer to promote the dispersion of carbon nanotubes in organic coatings.49

The modification principle of chemical bonding is to allow the nano-ions to directly react with the molecular chains of the polymer so that the organic functional groups are bonded to the end or middle part of the carbon nanotubes.The introduction of oxygen-containing functional groups(such as hydroxyl,carboxyl, carbonyl, amine, acryloyl chloride, ester, silane groups, etc.) on the surface of carbon nanotubes via strong acid oxidation can enhance the interface interactions formed between the nanoparticles and resin matrix to achieve a uniform dispersion.214,215Ezzeddine et al.216have used organosilanes as a silane coupling agent to react with hydroxyl functional groups generated via the oxidation or reduction on the surface of carbon nanotubes to increase the adhesion strength of carbon nanotubes to the matrix.Choosing appropriate organic molecules for modification can achieve the most favorable covalent functionalization, thereby producing good adhesion to the substrate.

The use of inorganic particles to modify or coat carbon nanotubes is also beneficial to the dispersion of CNTs and can combine the performance of coated particles with Carbon Nanotubes(CNTs).Pham et al.217found that the use of Fe3O4coated carbon nanotubes can show good dispersion in epoxy coatings, which were compared to pure epoxy resin, epoxy resin/carbon nanotubes, and epoxy resin/Fe3O4.The Fe3O4/CNTs combination can significantly enhance the corrosion resistance of epoxy coatings.Similarly, the bonding strength was also increased by about 71 % when compared with pure epoxy resin, and for the epoxy resin/carbon nanotube combination, it was increased by about 22 %.Jitianu et al.218also found that the sol–gel method can be used to add inorganic particles to improve the dispersibility of carbon nanotubes.

The arguments for the modification method and anticorrosion mechanism of the above carbon nanotube modification are as follows:Cai et al.38found in their research that carbon nanotubes can be used as a crosslinking agent to inhibit the propagation of microcracks and improve the adhesion between the coating and substrate.Moreover, carbon nanotubes coated with polydopamine and cerium oxide can distort the permeation pathways of water, oxygen, and aggressive ions, thereby reducing the permeability.Khun et al.209found that as the content of MWCNTs increases, the adhesion strength of the epoxy composites gradually increases, and the solid lubrication and rolling effect of the MWCNTs can reduce the friction coefficient of the epoxy coating and improve the wear resistance.Using EIS, it was found that an increase in the MWCNTs content increases the pore resistance of the coating by reducing the pore density and finally leads to an increase in the total resistance of the epoxy coating.Jeon et al.210found that the surface properties of epoxy coatings with multi-walled carbon nanotubes change from hydrophilic to hydrophobic, and the diffusion and absorption of water were significantly lower than that of epoxy coatings prepared without MWCNTs, which proved that the MWCNTs have a delay effect on water transmission.Wei et al.219found that the addition of MWCNTs increases the thermal stability of polyurethane coatings and the decomposition temperature increased by 21 K,indicating that MWCNTs have a retarding effect on the movement of the polyurethane molecular chain.Moreover, when compared with the bare substrate, the corrosion potential of the nanocomposite coating has a significant positive offset,which is close to 1 V;the corrosion current also drops from 286 nA to 13.5 nA and the corrosion rate drops from 1.885 mm/year to 0089 mm/year.Zachariah and Liu220used a Radical Transfer and Addition Reaction (RTAR) to realize the electrochemical activation process of chemical bonding between the Polybenzoxazine (PBz) chains and the MWCNT bundles.This process requires the use of the traditional thermal initiator, Benzoyl Peroxide (BPO), to carry out the thermal process of free radical polymerization, as shown in Figs.26(a)220and (b).220As shown in the polarization curve of Fig.26(c),220the PBz-MWCNTs coating reduces the corrosion rate from 20 μm/year to 0.179 μm/year, mainly because the degree of crosslinking increases from 90.28 % to 99.91 %.

Meng and Soucek221carried out electrochemical experiments on a series of composite coatings prepared using several nano-modification technologies and found that the nanocomposite coatings of fullerenes and carbon nanotubes exhibit high conductivity and corrosion resistance.The effect of water absorption is very obvious.Meng and Soucek221proposed a possible film formation mechanism for the nanocomposite coatings, as shown in Fig.27.221It was found that due to the good hydrophobicity of carbon fibers, the barrier effect of the coating was significant and when the filler content was 1wt%, its water absorption and corrosion resistance were the strongest.Qiu et al.222added carbon nanotube-polyaniline brush-like nanostructures to epoxy acrylic resin,which showed higher electrochemical activity in acidic, neutral, and alkaline environments, and provided good passivation protection for the substrate.

Fig.26 Synthesis of PBz and PBz functionalized multi-walled carbon nanotubes via a thermally activated RTAR and polarization curves obtained for cross-linked PBz and its nano-hybrid coated on a stainless steel sheet.220

Fig.27 Protective mechanism of carbon/modified epoxy hybrid coatings on metal substrates.221

5.3.Modification using graphene

Since the discovery of graphene materials, research on its structure and properties has developed rapidly.Graphene is also regarded to be a revolutionary material for the future.213,223–226Graphene is a hexagonal two-dimensional carbon nanomaterial composed of carbon atoms and sp2hybrid orbitals.It is currently the thinnest nanomaterial that can be prepared.It has the characteristics of good electrical conductivity, large specific surface area, high mechanical strength, good chemical stability, and super hydrophobicity.30,227For a long time, graphene modification technology has also been a research hotspot in organic coating modification.228,229The unique structure and impermeability of graphene nanosheets make them be one of the nanofillers most suitable for enhancing the barrier and permeability of organic aviation coatings.In addition,an increase in the graphene content can significantly enhance the conductivity of the organic coating and enhance the cathodic protection of the filler.230Graphene can also improve the bonding force between the organic coating and substrate, and it has self-healing properties.However, like carbon nanotubes, graphene is very easy to agglomerate due to its higher surface energy and van der Waals forces between layers,and it is not easy to disperse uniformly in the organic coating.In addition,due to its good electrical conductivity, galvanic corrosion is likely to occur in the later stage of coating aging, which promotes the corrosion of defects.Therefore, current research is mostly focused on taking advantage of graphene to avoid agglomeration problems and subsequent corrosion promotion problems.

An important prerequisite for graphene to fully give the advantage of enhancing the barrier properties of organic coatings is that graphene can be uniformly dispersed in the organic coating resin matrix.After consulting the literature,it has been found that the three main dispersion methods are:(A)Without surface modification of graphene, the graphene in the resin is physically dispersed using a strong mechanical mixing method;(B)The active agent or polymers are adsorbed onto the surface of graphene via physical adsorption, thereby promoting the dispersion of graphene; (C) In a solvent containing a surface modification group or active agent, the graphene is surface modified using a liquid phase exfoliation method to make it uniformly dispersed.

Li et al.231added graphene oxide to epoxy resin and polyester resin and prepared composite powder coatings utilizing planetary ball milling technology.They found that composite coatings prepared with appropriate amounts of graphene oxide can significantly improve the corrosion resistance of the substrate.Tang et al.232dispersed graphene oxide in water and promoted the extraction process of graphene oxide by adding a phase transfer agent, Triglycidyl-p-Aminophenol(TGPAP), to Bisphenol A Epoxy Resin (DGEBA).This also simplified the high energy consumption or intense stirring process required in the preparation of graphene epoxy coatings,as shown in Fig.28.232Fig.28232shows the phase transfer of Graphene Oxide (GO) from water to DGEBA using TGPAP as the phase transfer agent and the difference between GO before and after manual stirring of the pure DGEBA (EP0)and DGEBA/TGPAP mixtures with 10wt% and 20wt%TGPAP (EP10 and EP20).It is obvious from the water phase that GO was obviously extracted downward to the TGPAP/DGEBA mixture,but GO cannot be transferred to EP0.With an increase in the TGPAP content in DGEBA,the upper water layer is obviously lighter,which indicates that the phase transfer agent has a positive effect on graphene, demonstrating the effectiveness of decentralization.233This graphene dispersion method is the same as strong mechanical stirring, which uses the similar and mutual solubility of the materials,but improves the stability of the dispersion and greatly reduces the time cost.However,this method also has drawbacks,for example,it has high requirements on the process flow,which is not suitable for large-scale production, and will produce aqueous waste liquid that will have a certain impact on the environment.

Fig.28 Schematic diagram of extraction of GO from water to DGEBA using TGPAP as a phase transfer agent.232

In general, functional groups can also be grafted onto active oxidation sites on the surface of graphene oxide to form covalent bonds.This is to use the compatibility of the grafted functional groups and resin polymers to reduce the tendency of graphene to agglomerate.The types of modifying groups include organic small molecules, organic polymers, and inorganic nano-oxides.Commonly used organic small molecules generally include coupling agents, such as silane and titanate.234,235Commonly used organic polymers include polyvinylpyrrolidone, aniline trimer, polyaniline,polyisocyanate, or polyvinyl alcohol.236–240Commonly used inorganic nano-oxides include silica, titania, and alumina.241–243Graphene used in organic coatings has an oxidized state in addition to the reduced state.In general,graphene oxide is more suitable for dispersion by covalent modification due to its abundant surface active oxidation sites and reduced graphene is more suitable for dispersion by physical methods.228

Parhizkar et al.244used 30 aminopropyl triethoxysilane coupling agents to covalently modify Graphene Oxide (fGO)and found that depositing fGO film on the surface of steel significantly improves the bonding strength and corrosion resistance by reducing the cathodic delamination rate of epoxy coating.The high resistance of the fGO coating restricts Cl–and Na+from entering the cathode area.As shown in Fig.29,244the silanol group (—Si—OH—) produced via the hydrolysis of silane is prone to the cross-linking reaction of covalent bonds, forming imino groups on the surface of the substrate, reducing the cathode reaction rate, and greatly improving the cohesion at the interface.

Ding et al.233believed that the main factor for the cathodic protection of zinc-rich coatings is that the zinc filler loses electrons due to oxidation,causing electrons to migrate to the surface of the substrate, and the dissolved oxygen is reduced by the electrons on the surface of the substrate.The electron migration process passes through the metal-graphene barrier and graphene-graphene barrier, as shown in Fig.30.233The process of passing through the metal-graphene barrier must be modeled and calculated using the electric field flux in classical electrodynamics.The results show the difficulty of the electron breaking through the barrier at speed, which can be expressed by245

where η is the corrosion mass ratio;MZnis molar mass of zinc;qeis electronic charge; NAis Avogadro constant; qZnis zinc density; R is the radius of the sphere; f is the function of the total charge; E is the charge of surface of spherical zinc; Ebis dielectric strength.

The process of crossing the graphene-graphene barrier must be described according to the Dirac equation of quantum mechanics.245Based on the relativistic properties of the Dirac linear region in the graphene band structure, i.e., the zero effective mass of electrons and the momentum properties of sub-light speed,Ding et al.245established and calculated a simplified model for the graphene-graphene barrier and electron tunneling barrier.The global probability equation is

where DG(φ)is the global tunneling probabilities; φiais incidence angle; acis model parameter; E is electric field strength;Ueis potential barrier; hsis the shape parameter; vFis volume fraction.

The conclusion is that the presence of graphene improves not only the barrier properties of zinc-rich coatings but also the cathodic protection properties of zinc.

Chaudhry et al.246found that graphene nanosheets can provide effective protection for the physical and chemical mechanisms occurring in the organic protective coating over a short period of time.It has good electrical conductivity, which induces an increase in the porosity and activity of the electrochemical reaction, which promotes corrosion.This research helps to understand the misleading anti-corrosion properties associated with graphene-polymer composite coatings.

6.Conclusions

Organic aviation coatings play an important role in protecting aviation structures and ensuring aviation safety.Due to the unique and harsh service environment, the problems of corrosion, aging, and calendar life are very complicated, and these problems have been the focus of much research.A detailed introduction to current research on corrosion and aging problems with organic aviation coatings from the perspective of the development of organic aviation coatings,corrosion and aging influence factors, test methods, calendar life research, and modification methods are provided.

(1) Organic aviation coatings developed from single coatings,including nitro coatings,alkyd resins,and synthetic resins such as acrylic coatings, epoxy coatings, and polyurethane coatings, have been used to develop to a wide range of products.Fluorine-containing coatings and nano-modified coatings are still at the research stage.Organic aviation coatings are often exposed to harsh environments,such as high salt,high humidity,high temperature, and strong ultraviolet light.The current laws and mechanisms regarding the corrosion and aging of organic aviation coatings in this service environment are discussed.In the application of organic anticorrosive aviation coatings, attention should be paid to a reasonable maintenance plan based on the aging law of the coating,in order to reduce economic costs while ensuring safety.

(2) There are many studies on air exposure and artificial corrosion aging tests of organic aviation coatings, but there is little research on the equivalence and correlation between the two.Due to the variability and instability of atmospheric exposure conditions, it is difficult to establish the equivalent relationship between the two with one performance index.It is also difficult to fully characterize all the influencing factors of atmospheric exposure in an artificial acceleration environment.This paper referred to the previous literature and provided suggestions for future research on the correlation coefficients of the two experiments.

Fig.29 Schematic diagram of chemical bonding between epoxy coating and steel substrate modified using an fGO coating.244

Fig.30 Schematic diagram of two barriers that zinc anode must pass through during conductive process of a zinc-rich coating,simplified spherical wafer model of metal-graphene barrier,and simplified model of graphene energy band,Dirac cone linear region,and graphenegraphene barrier.233

(3) When organic aviation coatings show obvious modes of failure, they have often already lost their function of protecting the substrate.Therefore, new and effective failure criteria are needed to judge the failure of the coating to better guide the maintenance and guarantee work in this field.Current research on the calendar life of coatings often has the disadvantages of simplification and idealization, and it is difficult to comprehensively consider the coupling effects of various parameters and weight distribution.At present, ANNs are the most effective method for predicting coating life, but this method ignores the failure mechanism of the coating.How to determine and predict the calendar life while considering the failure mechanism and comprehensively weighing various characteristic parameters is a key breakthrough direction for future research.

(4) Traditional organic aviation coatings contain chromates and phosphates that are harmful to humans and the environment.The search for alternatives to traditional fillers has become a hot spot for many researchers.Three currently studied nano-modification technologies are selected for discussion.These three fillers can improve the corrosion resistance of organic coatings and the bonding strength with the substrate to a certain extent,but each has some disadvantages: for example, the dispersion problem of carbon nanotubes and graphene in organic coatings,and the problem of local galvanic corrosion at the defects easily caused by graphene.Future research on organic aviation coating modification technology should be directed at solving the above problems on the basis of ensuring the performance of modified fillers.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study was co-supported by the National Science and Technology Major Special Funding, China (No.J2019-I-0016-0015), the National Natural Science Foundation of China (No.52175155), and the Supported Fund for Excellent Doctoral Dissertation of Air Force Engineering University,China (No.KGD082520001).