Inf l uence of chloride ion concentration on immersion corrosion behaviour of plasma sprayed alumina coatings on AZ31B magnesium alloy

*

Department of Manufacturing Engineering,Annamalai University,Annamalainagar,Chidambaram 608 002,Tamil Nadu,India

Inf l uence of chloride ion concentration on immersion corrosion behaviour of plasma sprayed alumina coatings on AZ31B magnesium alloy

D.Thirumalaikumarasamy*,K.Shanmugam1,V.Balasubramanian2

Department of Manufacturing Engineering,Annamalai University,Annamalainagar,Chidambaram 608 002,Tamil Nadu,India

Corrosion attack of aluminium and magnesium based alloys is a major issue worldwide.The corrosion degradation of an uncoated and atmospheric plasma sprayed alumina(APS)coatings on AZ31B magnesium alloy was investigated using immersion corrosion test in NaCl solutions of different chloride ion concentrations viz.,0.01 M,0.2 M,0.6 M and 1 M.The corroded surface was characterized by an optical microscope and X-ray diffraction.The results showed that the corrosion deterioration of uncoated and coated samples were signif i cantly inf l uenced by chloride ion concentration.The uncoated magnesium and alumina coatings were found to offer a superior corrosion resistance in lower chloride ion concentration NaCl solutions(0.01 M and 0.2 M NaCl).On the other hand the coatings and Mg alloy substrate were found to be highly susceptible to localized damage,and could not provide an effective corrosion protection in solutions containing higher chloride concentrations(0.6 M and 1 M).It was found that the corrosion resistance of the ceramic coatings and base metal gets deteriorated with the increase in the chloride concentrations.

Atmospheric plasma spraying;Magnesium alloy;Chloride ion concentration;Corrosion;NaCl

1.Introduction

Growing concern for reducing greenhouse gas emissions and lowering fuel consumption have been major driving forces to develop lightweight materials for automotive and aerospace applications[1,2].Magnesium(Mg)is the lightest structural metal currently available in the world and therefore it remains a promising material for such applications.Mg and its alloys have high specif i c strength,high damping capacities,good castability and machinability[3].Besides,Mg alloys are considered to be promising materials in the f i eld of electronic industries,owing to their other unique advantages such as good electrical conductivity(good electromagnetic shielding characteristics),high thermal conductivity and good recycling potential compared with engineering plastics.However,the widespread application of Mg and its alloys has been fairly limited compared to other lightweight metals(e.g.,Al,Ti). However,a critical limitation for the extensive usage of magnesium alloys is their high susceptibility to corrosion, especially in aggressive environments,which is primarily attributed to the high chemical activity of magnesium and the unstable passive f i lm on the surface of these alloys[4].Many researchers have addressed the inf l uence of various corrosive environments on the corrosion behaviour of pure magnesium and/or magnesium alloys for the understanding of environmental factors controlling corrosion[5].

Abbreviations

APS atmospheric plasma spraying process

Cchloride ion concentration,mol

Ttime,h

CR corrosion rate,mm/year

Surface coating technology is one of the most effective methods to protect the Mg alloys against corrosion.Different coating processes are described in the literature for protection of Mg alloys,such as electro/electroless plating[6,7],anodizing [8,9],chemical conversion coatings[10,11],gas-phase deposition[12],laser surface alloying/cladding[13]and organic coatings[14,15].ThesemethodswerereviewedindetailbyGrayand Luan[16].Amongthem,atmosphericplasmaspraying(APS)has beenmostcommerciallyusedonMgandMgalloys.BytheAPS process a relatively thick,dense and hard oxide coating can be produced on the surface of magnesium alloys to improve their corrosion resistance remarkably[17].

Dhanapal et al.[18]explored the friction stirs welded AZ61A magnesium alloy welds corroded more seriously with the increase in Cl-concentrations.More the Cl-promoted the corrosion along with the rise in corrosion rate.Merino et al. [19]have investigated the inf l uence of chloride ion concentration and temperature on the corrosion of Mg-Al alloys in salt fog.According to salt fog tests,they concluded that corrosion attack of Mg,AZ31,AZ80 and AZ91D materials under the salt fog test increased with increasing temperature and Cl-concentration.The corrosion behaviour of an AZ91 alloy in dilute chloride solutions was studied recently in which a corrosion map as in term of the electrode potential and Clwas obtained using electrochemical measurement.It was found that there is corrosion and passivation zones in diluted NaCl solutions and open circuit potential were located in the passivation zone when the Cl-is less than 0.2 M and the corrosion zone as the Cl-is higher than 0.2 M[20].GUO Hui-Xia et al.[21]studied the corrosion behaviour of micro-arc oxidation coating on AZ91D magnesium alloy in NaCl solutions with different concentrations.The results of their investigation showed that the MAO coating on AZ91D magnesium alloy had a better corrosion protection in dilute NaCl solution than in higher concentration NaCl solution.The inf l uence of chloride concentration on the corrosion behaviour of MAO coated AM50 has been studied[22].Yanhong Gu et al.[23]reported that the magnitude of the corrosion potential increased with increasing chloride ion concentration, suggesting the MAO coated AZ31 alloys are more reactive in higher chloride ion concentrated solutions.It is well known that chloride ion is one of the most important factors of the corrosion of magnesium alloys in many desirable applications.

From the literature survey[18-23],it was understood that most of the published works have focused on the effect of Cllevel on the corrosion performance of uncoated and MAO coated magnesium alloys in NaCl solutions.However,up to now,there is not much published information on the corrosion performance of thermal sprayed coatings on magnesium alloys with different chloride ion concentrations.Hence the present investigation was carried out to study the inf l uence of chloride ion concentration on the corrosion behaviour of uncoated and plasma sprayed alumina coatings on AZ31B magnesium alloy in differentconcentrationsfor8hrwere assessed and discussed.

2.Experimental details

The chemical composition of the AZ31B alloy,substrate material,was found by the optical emission spectroscopy method used in this investigation are as follows(in wt.%):Al 3.0,Zn 0.1,Mn 0.2 and Mg balance.The cut sectional surface of AZ31B magnesium alloy rod(16 mm in diameter and 15 mm in thickness)was grit blasted using cabinet type grit blasting machine prior to plasma spraying.Grit blasting was carried out using corundum grits of size of 500+320 μm and subsequently cleaned using acetone in an ultrasonic bath and dried.The optimized plasma spraying parameters,presented in Table 1,were used to deposit the coatings.In this investigation,alumina powders with size range from-45+20 μm have been deposited on grit blasted magnesium alloy substrates.The plasma spray deposition of the alumina powders were carried out using a semi-automatic 40 kW IGBT-based Plasmatron(Make:Ion Arc Technologies;India.Model: APSS-II).Coating thickness for all the deposits were maintained at 200 ± 15 μm.

The uncoated substrate and coated samples were immersed in 1000 ml NaCl solutions with mass ion concentrations of 0.01 M,0.2 M,0.6 M and 1 M for 8 h.For each experimental condition two coated specimens were prepared and tested.Fig.1 presents the test set up and specimen during the immersion corrosion test.The specimens were ground with 500#,800#,1200#,1500#grit SiC paper washed with distilled water and dried by warm f l owing air.The corrosion rates of the uncoated and as coated specimens were estimated through the weight loss measurement. The original weight(WO)of the specimen were recorded and then immersed in the solution of 3.5%NaCl solution for 8 h. Finally,the corrosion products were removed by immersing the specimens for one minute in the solution prepared by using 50 g chromium trioxide(CrO3),2.5 g silver nitrate (AgNO3)and 5 g barium nitrate(Ba(NO3)2)for 250 ml distilled water.The f i nal weight(wt)of the specimen was measured and the net weight loss was calculated using the following equation[24]:

Table 1Optimized plasma spray parameters used to coat alumina.

Fig.1.Test set up and specimen during the immersion corrosion test.

whereW=weight loss in mg,A=surface area of the specimen in cm2,D=density of the uncoated and coated specimen,T=corrosion time in h.

The main phases in the alumina coating were detected using X-ray diffraction(XRD)experiment,in which the angle of the incident beam was f i xed at 2°against the sample surface.The XRD prof i les were recorded using Cu Kα radiation at 40 kVand 20 mA.A SEM(JSM 6400,JEOL,Tokyo,Japan) was used to examine the surface and the cross section morphologies of the coatings.The changes of surface micrographs were observed by an optical microscope(MEIJI,Japan; Model:ML7100).

3.Results and discussion

3.1.Phase and microstructure

The SEM image of the feedstock taken at 100x magnif ication with an image resolution of 1024 × 768 pixels shows fused and then crushed,which gives its characteristic angular shape as shown in Fig.2.The SEM images shown in Fig.3 revealed the surface and cross sectional morphologies of the as deposited coating.From these f i gures it is found that the coating has low porosity.The micro pores and the micro cracks(Fig.3a)are observed in the coating.Good adhesion between the coating and the substrate is seen without any visible boundary from the cross-sectional morphology as shown in Fig.3b.The XRD spectrum of the as sprayed coating is shown in Fig.4 reveals the coating was mainly constituted of both α-Al2O3and β-Al2O3.

3.2.Effect of chloride ion concentration on corrosion rate

The inf l uence of chloride ion concentration on corrosion rates of the base metal and alumina coatings are illustrated in Fig.5.It is seen that the coatings exhibited a rise in corrosion rate with the increase in Cl-concentration.In this way,the change of Cl-concentration affected the corrosion rate much more in higher concentration solutions than that in lower concentration solutions.When more Cl-in NaCl solution promoted the corrosion,the corrosive intermediate(Cl-)would be rapidly transferred through the outer layer and reached the specimen surface.Hence,the corrosion rate was increased[25].

Fig.2.SEM image of alumina powder.

Fig.3.SEM images of the alumina coating produced on AZ31 Mg alloy:(a)surface morphology and(b)cross-section morphology.

Fig.6 represents the macroscopic appearance of the corroded surface after 8 h of testing in different corrosive electrolytes and Fig.7 shows the scanning electron micrographs of corroded area corresponding to the specimens/ regions labelled in Fig.6.From Fig.7a,it can be seen that at lower chloride ion concentration,less corrosion pits were formed on the surface of the AZ31B magnesium alloy.If the chloride ion concentration was increased,some obvious pits appeared on the surface of the specimen as represented in Fig.7g.The highest corrosion rate is observed at the chloride ion concentration of 1 M as could be inferred from Fig.5.It shows that the corrosion rate is increased with the increase in the chloride ion concentration.This is because the corrosion becomes severe owing to the penetration of the hydroxide f i lm by the Cl-ion,and hence the formation of the metal hydroxyl chloride complex,which is governed by the following reaction,given in Eq.(1).

Fig.4.XRD pattern of the as sprayed coating.

This hydroxyl complex would break through the protective layer which causes the Cl-ion to penetrate into the layer, causing cracks in the outer layer,which symbolizes the enhancement of corrosion and its rate.Furthermore,with the decrease of the Cl-ions the activity of the corrosion is depressed and the OH-ions dominate over the Cl-ions by forming an insoluble hydroxide layer,composed of oxides and hydroxides.It is also observed that the rising rate of corrosion was reduced with the increase in the chloride ion concentration[26].Yamasaki et al.proposed that during pit formation, the chloride ion tends to be concentrated inside the pit,causing an anodic dissolution of magnesium,not the surface of the substrate.Thus,it is clear that the,rising rate of corrosion was reduced with the increase in the chloride ion concentration [27].Song et al.also have pointed out that,the rising rate of the corrosion was reduced with the increase of the chloride ion concentration,leading to the conclusion that the β-phase was stable in the NaCl solution,and it is more inert to corrosion; the β-phase was itself,however,an effective cathode[28].

As shown in the SEM micrograph Fig.7b,it is also observed that at lower chloride ion concentrations,coating has no pronounced deterioration in this condition.At this stage,because the pores and defects were not interconnecting and chloride ion concentration in 0.01 M NaCl solution was low,the corrosive electrolyte permeated slowly into the coating through these intrinsic defects.In lower chloride ion concentration solutions (0.01 M NaCl),the corrosive electrolytes are too mild to break down the coatings.The corrosion deterioration of coated specimens was dictated by the degradation of coatingsespecially in inner regions of the coating.There was also no macroscopic damage on the alumina coated surface after 8 h of immersion testing in 0.01 M NaCl solution(Fig.6).Therefore, due to the denser and more compact inner layer in the alumina coating was superior and the corrosion deterioration was slower in mild corrosive electrolytes(Fig.7b).

Fig.5.Effect of chloride ion concentration on corrosion rate.

In more concentrated NaCl solutions,the permeation of higher concentration of chloride ions into the coating/substrate interface induced the quick break down of alumina coatings. The localized damage was evident in 1 M NaCl solution as seen in Figs.6 and 7h.The level of corrosion damage increased with the increase of chloride ion concentration of NaCl solution.At the concentration not more than 0.01 M,the coating was only deteriorated lightly on the edge of the samples(Figs.6 and 7b).In the case of the higher chloride ion concentration,however,the corrosion damage was evident in the macroscopic morphology in Fig.6,in which localized corrosion damage was observed on the corroded surface,as represented in Fig.7h.At the concentration more than 6 M,a large amount of chloride ions penetrate the coating and contact with the substrate,resulting in heavy corrosion reaction and a larger level of corrosion damage(Fig.7h).This suggests that the alumina coated AZ31 alloys corroded much more heavily when chloride ion concentration is higher than 6 M.This is due to more corrosive ions in 6 M and 1 M NaCl solutions have been in contact with the Mg substrate through pores and defects in the coatings,resulting in more conversion of Mg into Mg(OH)2[29].The deposit of Mg(OH)2may propagate and further form a passive layer when the ions completely contact with Mg alloy substrate.The passive layer will inhibit the diffusion of NaCl solution and to some extent protect Mg alloy from degrading quickly[30].Based on this investigation,it is concluded that the alumina coatings cannot provide a long term protection to the magnesium alloy substrate in neutral environments containing high chloride concentrations.

Fig.6.Macroscopic morphologies of corroded surface after 8 h exposure in NaCl solutions of different chloride ion concentrations.

Fig.7.SEM micrographs of corroded surface after 8 h immersion in NaCl solutions of different chloride ion concentrations.

3.3.Characterization of corroded surfaces

Fig.8 shows the SEM,EDAX and XRD analysis of the immersion corrosion test specimens immersion in NaCl solutions with chloride ion concentrations(a)0.01 M and(b)1 M. The surface of the specimen exposed to lower Cl-concentration appears spongy,and the adherent corrosion product is displayed in Fig.8a.The corrosion behaviour of the AZ31B magnesium alloy is governed by the partially protective surface f i lm.However,with a chloride ion concentration of 0.01 M,the Gibb's free energy to form the metal chloride layer is -591.8 kJ/mol.But,the free energy of the initial protective layer MgO is-596.3 kJ/mol.Hence,at this concentration,it f i nds it hard to break down the protective layer[31].Hence the Cl-concentration of 0.01 M cannot promote the corrosion much.The specimen exposed to higher Cl-concentration of 1 M is shown in Fig.8b.When the chloride ion concentration is 1 M,the Gibb's free energy formed is higher,compared to thefree energy of the protective f i lm.The surface of the specimen shows more cracks over the corrosion products,where the Clpenetrates into the surface.More Cl-in the NaCl solution promotes corrosion.The corrosive intermediate(Cl-)rapidly inf i ltrates through the outer layer to reach the substrate of the, alloy surface.Hence,the corrosion rate increases with the increase in the chloride ion concentration.Fig.8c exhibits the EDAX of the immersion corrosion test specimens with a chloride ion concentration of 0.01 M.It shows that the corrosion products contain Mg and O compounds.It means that the specimen underwent a milder attack.Fig.8d shows certain peaks of Cl-,which indicate the corrosion products having chloride ions.These chloride ions remain in contact with the magnesium throughout the exposure time.Also,the surface of the pit shows more cracks over the corrosion products,where the Cl-penetrate into the surface.

Fig.8.SEM,EDAX and XRD analysis of AZ31B magnesium alloy after immersion in a NaCl solution with different chloride ion concentrations of 0.01 M and 1 M.

Fig.8e presents the XRD analysis of the specimen that underwent the immersion corrosion test in a NaCl solution with a chloride ion concentration of 0.01 M;the characteristic peaks originate from the metallic Mg substrate.More peaks of Mg(OH)2are observed,which suggest that the protective action is enhanced by the decrease in the chloride ion concentration.However,the intensity of the Mg(OH)2peaks is slightly diminished.This means that the resistance towards corrosion is reasonable.Also,the peaks of β-phases are seen along with the Mg(OH)2.This means that the β-phases arealso still active.The β-phases dominate with higher peaks in the specimen,immersed in 1 M NaCl,as can be observed in Fig.8f.This means that the microgalvanic coupling enhanced the corrosion attack,leaving the β-phases undermined.During pitting corrosion,the β-phases are fall out and are undermined more than the general corrosion.These undermined β-phases are found at the substrate of the AZ31B magnesium alloy, during the spraying phase[32,33].

Fig.9.(a)SEM micrograph from surface of alumina coating after 8 h of immersion(a surface pore has been shown by a circle)and(b)high magnif i cation SEM micrograph of the pore.

The SEM micrograph from the surface of alumina coating after 8 h immersion is shown in Fig.9a.A surface pore can be observed in this f i gure(as shown by a circle).The high magnif i cation micro-graph of this pore is shown in Fig.9b. The corrosion products are visible inside the pore.The EDX analysis showed that corrosion products contain aluminium and oxygen.It seems that this pore has been plugged by corrosion products formed due to corrosion of substrate.The corroded surfaces of the coated samples were examined using SEM and X-ray diffraction techniques immediately after the immersion test.The occurrence ofuniform corrosion (Fig.10a)can be observed.However,in the as-coated sample, an additional thicker top layer at discrete locations can be noted(Fig.10a)indicative of higher corrosion rate.X-ray diffraction results obtained from the corroded surfaces of the samples are presented in Fig.10b.The main corrosion products formed are bayerite(Al(OH)3)(JCPDS 33-0018)and aluminium oxide(AlO)(JCPDS 10-173)as conf i rmed by EDX.The kinetics of Al(OH)3formation greatly depends on the content of aluminium in the coating and also became dominant at high chloride ion concentration.

Fig.11a and b displays the cross section and EDS analysis of as-sprayed alumina coating on AZ31B magnesium alloy after 8 h of immersion in NaCl solution.The cross section images of as-sprayed coatings revealed signif i cant signs of degradation in the coating/substrate interface Fig.11a evidences the extent of the corrosion process that occurs in the chloride medium,since the as-sprayed alumina coating was detached from the AZ31B substrate after 8 h of immersion. Examination of the coating/substrate interface showed the presence of corrosion products in this area,although only a part of them remained over the substrate or in the coating after the immersion tests.This behaviour is produced because the as-sprayed coating is highly porous,so that,there is a high number of pathways through this coating and the electrolyte rapidly reaches the magnesium alloy surface,giving rise to the substrate corrosion.Afterwards,the corrosion process progresses along the interface area,giving rise to the formation of corrosion products on the metal surface,which will f i nally cause the detachment of the coating.The growth of corrosion products would separate the coating from the substrate and their low mechanical properties would allow its detachment [34].According to EDX analysis(Fig.11b),corrosion products rich in Mg and O were mainly detected in the interfacearea,along with a small amount of Al and of Cl.The main corrosion products responsible for the detachment of the coatings in immersion environment were identif i ed as MgO (JCPDS 77-2179)(Fig.11c).

Fig.10.SEM micrographs(a)and x-ray diffraction analysis(b)of the corroded surface after 8 h exposure of coatings.

Fig.11.(a)Cross section of as-sprayed alumina coating on AZ31B magnesium alloy after immersion in NaCl solution for 8 h(b and c)EDX analysis and XRD pattern of coating-substrate interface.

4.Conclusions

Based on the results obtained in this investigation,the following conclusions can be drawn:

(1)The uncoated and alumina coated samples were found to offer a superior corrosion resistance in lower chloride ion concentration NaCl solutions(0.01 M NaCl).

(2)The corrosion rates of the uncoated magnesium and alumina coatings were increased with increasing chloride ion concentration,suggesting the uncoated and alumina coated AZ31 alloys are more reactive in higher chloride ion concentrated solutions.The level of the corrosion attack is much higher when chloride ion concentration is greater than 0.6 M,which was validated by the surface micrographs and macrographs.

(3)The uncoated and plasma sprayed alumina coatings on AZ31B magnesium alloy were found to be highly susceptible to localized damage,and could not provide an effective corrosion protection in solutions containing higher chloride concentrations.It means that the both the coatings and substrate had a better corrosion protection in NaCl solution than in higher concentration NaCl solution.

Acknowledgements

The authors wish to place their sincere thanks on record to Dr.C.S.Ramachandran,Post Doctoral Fellow,State University of New York,USA for the assistance rendered during deposition ofthe coatings.The authorsalso wish to acknowledge Mr.R.Selvendiran,Technical Assistant,Annamalai University for his help in carrying out this investigation.

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Received 26 June 2014;revised 29 October 2014;accepted 6 November 2014 Available online 4 December 2014

*Corresponding author.Tel.:+91 09894319865(mobile);fax:+91 4144 238080,+91 4144 238275.

E-mail addresses:tkumarasamy412@gmail.com(D.Thirumalaikumarasamy), drshanmugam67@gmail.com (K. Shanmugam), balasubramanian.v.2784@ annamalaiuniversity.ac.in(V.Balasubramanian).

Peer review under responsibility of National Engineering Research Center for Magnesium Alloys of China,Chongqing University.

1Tel.:+91 09443556585.

2Tel.:+91 09443412249(mobile).

http://dx.doi.org/10.1016/j.jma.2014.11.001.

2213-9567/Copyright 2014,National Engineering Research Center for Magnesium Alloys of China,Chongqing University.Production and hosting by Elsevier B.V.All rights reserved.

Copyright 2014,National Engineering Research Center for Magnesium Alloys of China,Chongqing University.Production and hosting by Elsevier B.V.All rights reserved.