Review on the phosphate-based conversion coatings of magnesium and its alloys

Abstract

Magnesium (Mg) and its alloys are lightweight as well as biocompatible and possess a high strength-to-weight ratio, making them suitable for many industries, including aerospace, automobile, and medical. The major challenge is their high susceptibility to corrosion, thereby limiting their usability. The considerably lower reduction potential of Mg compared to other metals makes it vulnerable to galvanic coupling. The oxide layer on Mg offers little corrosion resistance because of its high porosity, inhomogeneity, and fragility. Chemical conversion coatings (CCs) belong to a distinct class because of underlying chemical reactions, which are fundamentally different from other types of coating. Typically, a CC acts as an intermediate sandwich layer between the base metal and an aesthetic paint. Although chromate CCs offer superior performance compared to phosphate CCs, yet still they release carcinogenic hexavalent chromium ions (Cr⁶⁺); therefore, their use is prohibited in most European nations under the Registration, Evaluation, Authorization and Restriction of Chemicals legislation framework. Phosphate-based CCs are a cost-effective and environment-friendly alternative. Accordingly, this review primarily focuses on different types of phosphate-based CCs, such as zinc, calcium, Mg, vanadium, manganese, and permanganate. It discusses their mechanisms, current status, pre-treatment practices, and the influence of various parameters—such as pH, temperature, immersion time, and bath composition—on the coating performance. Some challenges associated with phosphate CCs and future research directions are also elaborated.

Keywords

phosphate conversion coatings / magnesium alloys / corrosion / zinc phosphate conversion coatings / calcium phosphate conversion coatings / magnesium phosphate conversion coatings

References

[1] X.B. Chen, N. Birbilis, and T.B. Abbott, Review of corrosion-resistant conversion coatings for magnesium and its alloys, Corrosion, 67(2011), No. 3, art. No. 035005

[2] Song G. Recent progress in corrosion and protection of magnesium alloys. Adv. Eng. Mater., 2005, 7(7): 563.

[3] Gray JE, Luan B. Protective coatings on magnesium and its alloys—A critical review. J. Alloys Compd., 2002, 336(1–2): 88.

[4] D.D. Zhang, F. Peng, and X.Y. Liu, Protection of magnesium alloys: From physical barrier coating to smart self-healing coating, J. Alloys Compd., 853(2021), art. No. 157010.

[5] O. Gharbi, S. Thomas, C. Smith, and N. Birbilis, Chromate replacement: What does the future hold? npj Mater. Degrad., 2(2018), art. No. 12.

[6] Cho KW, Rao VS, Kwon HS. Microstructure and electrochemical characterization of trivalent chromium based conversion coating on zinc. Electrochim. Acta, 2007, 52(13): 4449.

[7] European Chemicals Agency (ECHA), Understanding REACH, 2007 [2021-12-31]. https://echa.europa.eu/regulations/reach/understanding-reach

[8] Chen WK, Bai CY, Liu CM, Lin CS, Ger MD. The effect of chromic sulfate concentration and immersion time on the structures and anticorrosive performance of the Cr(III) conversion coatings on aluminum alloys. Appl. Surf. Sci., 2010, 256(16): 4924.

[9] S. Jana, M. Olszta, D. Edwards, M. Engelhard, A. Samanta, H.T. Ding, P. Murkute, O.B. Isgor, and A. Rohatgi, Microstructural basis for improved corrosion resistance of laser surface processed AZ31 Mg alloy, Corros. Sci., 191(2021), art. No. 109707.

[10] M. Doerre, L. Hibbitts, G. Patrick, and N.K. Akafuah, Advances in automotive conversion coatings during pretreatment of the body structure: A review, Coatings, 8(2018), No. 11, art. No. 405

[11] Pommiers S, Frayret J, Castetbon A, Potin-Gautier M. Alternative conversion coatings to chromate for the protection of magnesium alloys. Corros. Sci., 2014, 84, 135.

[12] Yong ZY, Zhu J, Qiu C, Liu YL. Molybdate/phosphate composite conversion coating on magnesium alloy surface for corrosion protection. Appl. Surf. Sci., 2008, 255(5): 1672.

[13] Saji VS. Review of rare-earth-based conversion coatings for magnesium and its alloys. J. Mater. Res. Technol., 2019, 8(5): 5012.

[14] Shadanbaz S, Dias GJ. Calcium phosphate coatings on magnesium alloys for biomedical applications: A review. Acta Biomater., 2012, 8(1): 20.

[15] ASTM International. Standard Specification for Magnesium-Alloy Die Castings, 2018, West Conshohocken, PA, ASTM International ASTM Standard B94-18

[16] S.A. Salman, R. Ichino, and M. Okido, A comparative electrochemical study of AZ31 and AZ91 magnesium alloy, Int. J. Corros., 2010(2010), art. No. 412129.

[17] Yang HY, Guo XW, Chen XB, Birbilis N. A homogenisation pre-treatment for adherent and corrosion-resistant Ni electroplated coatings on Mg-alloy AZ91D. Corros. Sci., 2014, 79, 41.

[18] Chen XB, Yang HY, Abbott TB, Easton MA, Birbilis N. Corrosion protection of magnesium and its alloys by metal phosphate conversion coatings. Surf. Eng., 2014, 30(12): 871.

[19] L. Pezzato, D. Vranescu, M. Sinico, C. Gennari, A.G. Settimi, P. Pranovi, K. Brunelli, and M. Dabalà, Tribocorrosion properties of PEO coatings produced on AZ91 magnesium alloy with silicate- or phosphate-based electrolytes, Coatings, 8(2018), No. 6, art. No. 202.

[20] Liu ZM, Gao W. Electroless nickel plating on AZ91 Mg alloy substrate. Surf. Coat. Technol., 2006, 200(16–17): 5087.

[21] Brunelli K, Dabalà M, Calliari I, Magrini M. Effect of HCl pre-treatment on corrosion resistance of cerium-based conversion coatings on magnesium and magnesium alloys. Corros. Sci., 2005, 47(4): 989.

[22] Tomlinson WJ, Mayor JP. Formation, microstructure, surface roughness, and porosity of electroless nickel coatings. Surf. Eng., 1988, 4(3): 235.

[23] Zhang CY, Liu B, Yu BX, Lu XP, Wei Y, Zhang T, Mol JMC, Wang FH. Influence of surface pretreatment on phosphate conversion coating on AZ91 Mg alloy. Surf. Coat. Technol., 2019, 359, 414.

[24] T. Li, Z.J. Leng, S.F. Wang, X.T. Wang, R. Ghomashchi, Y.S. Yang, and J.X. Zhou, Comparison of the effects of pre-activators on morphology and corrosion resistance of phosphate conversion coating on magnesium alloy, J. Magnes. Alloys, (2021). DOI: https://doi.org/10.1016/j.jma.2021.03.012

[25] Yang HY, Chen XB, Guo XW, Wu GH, Ding WJ, Birbilis N. Coating pretreatment for Mg alloy AZ91D. Appl. Surf. Sci., 2012, 258(14): 5472.

[26] Zhang YK, You J, Lu JZ, Cui CY, Jiang YF, Ren XD. Effects of laser shock processing on stress corrosion cracking susceptibility of AZ31B magnesium alloy. Surf. Coat. Technol., 2010, 204(24): 3947.

[27] Lu JZ, Luo KY, Zhang YK, Cui CY, Sun GF, Zhou JZ, Zhang L, You J, Chen KM, Zhong JW. Grain refinement of LY2 aluminum alloy induced by ultra-high plastic strain during multiple laser shock processing impacts. Acta Mater., 2010, 58(11): 3984.

[28] Zhang XC, Zhong F, Li XP, Liu B, Zhang CY, Buhe B, Zhang T, Meng GZ, Wang FH. The effect of hot extrusion on the microstructure and anti-corrosion performance of LDHs conversion coating on AZ91D magnesium alloy. J. Alloys Compd., 2019, 788, 756.

[29] Hu GQ, Guan K, Lu LB, Zhang JR, Lu N, Guan YC. Engineered functional surfaces by laser microprocessing for biomedical applications. Engineering, 2018, 4(6): 822.

[30] H.L. Liu, Z.P. Tong, Y. Yang, W.F. Zhou, J.N. Chen, X.Y. Pan, and X.D. Ren, Preparation of phosphate conversion coating on laser surface textured surface to improve corrosion performance of magnesium alloy, J. Alloys Compd., 865(2021), art. No. 158701.

[31] H.L. Liu, Z.P. Tong, W.F. Zhou, Y. Yang, J.F. Jiao, and X.D. Ren, Improving electrochemical corrosion properties of AZ31 magnesium alloy via phosphate conversion with laser shock peening pretreatment, J. Alloys Compd., 846(2020), art. No. 155837.

[32] Hafeez MA, Farooq A, Zang A, Saleem A, Deen KM. Phosphate chemical conversion coatings for magnesium alloys: A review. J. Coat. Technol. Res., 2020, 17(4): 827.

[33] Pragatheeswaran A, Ananthapadmanabhan PV, Chakravarthy Y, Bhandari S, Chaturvedi V, Nagaraj A, Ramachandran K. Plasma spray-deposited lanthanum phosphate coatings for protection against molten uranium corrosion. Surf. Coat. Technol., 2015, 265, 166.

[34] Cheng YL, Wu HL, Chen ZH, Wang HM, Li LL. Phosphating process of AZ31 magnesium alloy and corrosion resistance of coatings. Trans. Nonferrous Met. Soc. China, 2006, 16(5): 1086.

[35] Amini R, Sarabi AA. The corrosion properties of phosphate coating on AZ31 magnesium alloy: The effect of sodium dodecyl sulfate (SDS) as an eco-friendly accelerating agent. Appl. Surf. Sci., 2011, 257(16): 7134.

[36] Niu LY, Lin JX, Li Y, Shi ZM, Xu LC. Improvement of anticorrosion and adhesion to magnesium alloy by phosphate coating formed at room temperature. Trans. Nonferrous Met. Soc. China, 2010, 20(7): 1356.

[37] Li Q, Xu SQ, Hu JY, Zhang SY, Zhong XK, Yang XK. The effects to the structure and electrochemical behavior of zinc phosphate conversion coatings with ethanolamine on magnesium alloy AZ91D. Electrochim. Acta, 2010, 55(3): 887.

[38] Phuong NV, Lee KH, Chang D, Moon S. Effects of Zn²⁺ concentration and pH on the zinc phosphate conversion coatings on AZ31 magnesium alloy. Corros. Sci., 2013, 74, 314.

[39] Shang W, He CB, Wen YQ, Wang YY, Zhang Z. Performance evaluation of triethanolamine as corrosion inhibitor for magnesium alloy in 3.5 wt% NaCl solution. RSC Adv., 2016, 6(115): 113967.

[40] Leeuwenburgh SCG, Heine MC, Wolke JGC, Pratsinis SE, Schoonman J, Jansen JA. Morphology of calcium phosphate coatings for biomedical applications deposited using Electrostatic Spray Deposition. Thin Solid Films, 2006, 503(1–2): 69.

[41] Su YC, Guo YT, Huang ZL, Zhang ZH, Li GY, Lian JS, Ren LQ. Preparation and corrosion behaviors of calcium phosphate conversion coating on magnesium alloy. Surf. Coat. Technol., 2016, 307, 99.

[42] Liu B, Zhang X, Xiao GY, Lu YP. Phosphate chemical conversion coatings on metallic substrates for biomedical application: A review. Mater. Sci. Eng. C, 2015, 47, 97.

[43] Su YC, Su YC, Lu YB, Lian JS, Li GY. Composite microstructure and formation mechanism of calcium phosphate conversion coating on magnesium alloy. J. Electrochem. Soc., 2016, 163(9): G138.

[44] Amaravathy P, Sampath Kumar TS. Novel strontium doped zinc calcium phosphate conversion coating on AZ31 magnesium alloy for biomedical applications. J. Biomimetics Biomater. Biomed. Eng., 2017, 34, 57.

[45] W. Zai, X.R. Zhang, Y.C. Su, H.C. Man, G.Y. Li, and J.S. Lian, Comparison of corrosion resistance and biocompatibility of magnesium phosphate (MgP), zinc phosphate (ZnP) and calcium phosphate (CaP) conversion coatings on Mg alloy, Surf. Coat. Technol., 397(2020), art. No. 125919.

[46] Y.T. Guo, Y.C. Su, R. Gu, Z.H. Zhang, G.Y. Li, J.S. Lian, and L.Q. Ren, Enhanced corrosion resistance and biocompatibility of biodegradable magnesium alloy modified by calcium phosphate/collagen coating, Surf. Coat. Technol., 401(2020), art. No. 126318.

[47] D. Liu, Y.Y. Li, Y. Zhou, and Y.G. Ding, The preparation, characterization and formation mechanism of a calcium phosphate conversion coating on magnesium alloy AZ91D, Materials (Basel), 11(2018), No. 6, art. No. 908.

[48] Chen XB, Birbilis N, Abbott TB. Effect of [Ca²⁺] and $\left[{{\rm{PO}}_4^{3 -}} \right]$ levels on the formation of calcium phosphate conversion coatings on die-cast magnesium alloy AZ91D. Corros. Sci., 2012, 55, 226.

[49] Zeng RC, Lan ZD, Kong LH, Huang YD, Cui HZ. Characterization of calcium-modified zinc phosphate conversion coatings and their influences on corrosion resistance of AZ31 alloy. Surf. Coat. Technol., 2011, 205(11): 3347.

[50] R.X. Sun, S.K. Yang, and T. Lv, Corrosion behavior of AZ91D magnesium alloy with a calcium—phosphate—vanadium composite conversion coating, Coatings, 9(2019), No. 6, art. No. 379.

[51] Morks MF. Magnesium phosphate treatment for steel. Mater. Lett., 2004, 58(26): 3316.

[52] van Phuong N, Moon S. Comparative corrosion study of zinc phosphate and magnesium phosphate conversion coatings on AZ31 Mg alloy. Mater. Lett., 2014, 122, 341.

[53] Fouladi M, Amadeh A. Comparative study between novel magnesium phosphate and traditional zinc phosphate coatings. Mater. Lett., 2013, 98, 1.

[54] Zai W, Su YC, Man HC, Lian JS, Li GY. Effect of pH value and preparation temperature on the formation of magnesium phosphate conversion coatings on AZ31 magnesium alloy. Appl. Surf. Sci., 2019, 492, 314.

[55] Jayaraj J, Amruth Raj S, Srinivasan A, Ananthakumar S, Pillai UTS, Dhaipule NGK, Mudali UK. Composite magnesium phosphate coatings for improved corrosion resistance of magnesium AZ31 alloy. Corros. Sci., 2016, 113, 104.

[56] van Phuong N, Gupta M, Moon S. Enhanced corrosion performance of magnesium phosphate conversion coating on AZ31 magnesium alloy. Trans. Nonferrous Met. Soc. China, 2017, 27(5): 1087.

[57] Zhou WQ, Shan DY, Han EH, Ke W. Structure and formation mechanism of phosphate conversion coating on die-cast AZ91D magnesium alloy. Corros. Sci., 2008, 50(2): 329.

[58] Zhou WQ, Tang W, Zhao Q, Wu SW, Han EH. Influence of additive on structure and corrosion resistance of manganese phosphate film on AZ91 magnesium alloy. Mater. Sci. Forum, 2011, 686, 176.

[59] Cui XJ, Liu CH, Yang RS, Fu QS, Lin XZ, Gong M. Duplex-layered manganese phosphate conversion coating on AZ31 Mg alloy and its initial formation mechanism. Corros. Sci., 2013, 76, 474.

[60] Cui XJ, Liu CH, Yang RS, Lin XZ, Gong M. Preparation and characterization of phosphate film for magnesium alloy AZ31. Phys. Procedia, 2012, 25, 194.

[61] Cui XJ, Liu CH, Yang RS, Li MT, Lin XZ, Gong M. Phosphate film free of chromate, fluoride and nitrite on AZ31 magnesium alloy and its corrosion resistance. Trans. Nonferrous Met. Soc. China, 2012, 22(11): 2713.

[62] Li T, Wang SF, Liu HT, Wu JH, Tang SQ, Yang YS, Wang XT, Zhou JX. Improved corrosion resistance of Mg alloy by a green phosphating: Insights into pre-activation, temperature, and growth mechanism. J. Mater. Sci., 2021, 56(1): 828.

[63] Chen XB, Zhou X, Abbott TB, Easton MA, Birbilis N. Double-layered manganese phosphate conversion coating on magnesium alloy AZ91D: Insights into coating formation, growth and corrosion resistance. Surf. Coat. Technol., 2013, 217, 147.

[64] Hamdy AS, Doench I, Möhwald H. Assessment of a one-step intelligent self-healing vanadia protective coatings for magnesium alloys in corrosive media. Electrochim. Acta, 2011, 56(5): 2493.

[65] Chong KZ, Shih TS. Conversion-coating treatment for magnesium alloys by a permanganate—phosphate solution. Mater. Chem. Phys., 2003, 80(1): 191.

[66] Zhao M, Wu SS, Luo JR, Fukuda Y, Nakae H. A chromium-free conversion coating of magnesium alloy by a phosphate-permanganate solution. Surf. Coat. Technol., 2006, 200(18–19): 5407.

[67] Zucchi F, Frignani A, Grassi V, Trabanelli G, Monticelli C. Stannate and permanganate conversion coatings on AZ31 magnesium alloy. Corros. Sci., 2007, 49(12): 4542.

[68] Niu LY, Chang SH, Tong X, Li GY, Shi ZM. Analysis of characteristics of vanadate conversion coating on the surface of magnesium alloy. J. Alloys Compd., 2014, 617, 214.

[69] P. Zhou, B.X. Yu, Y.J. Hou, G.Q. Duan, L.X. Yang, B. Zhang, T. Zhang, and F.H. Wang, Revisiting the cracking of chemical conversion coating on magnesium alloys, Corros. Sci., 178(2021), art. No. 109069.

[70] S.Y. Jian, Y.C. Tzeng, M.D. Ger, K.L. Chang, G.N. Shi, W.H. Huang, C.Y. Chen, and C.C. Wu, The study of corrosion behavior of manganese-based conversion coating on LZ91 magnesium alloy: Effect of addition of pyrophosphate and cerium, Mater. Des., 192(2020), art. No. 108707.

[71] Jayaraj J, Rajesh KR, Amruth Raj S, Srinivasan A, Ananthakumar S, Dhaipule NGK, Kalpathy SK, Pillai UTS, Mudali UK. Investigation on the corrosion behavior of lanthanum phosphate coatings on AZ31 Mg alloy obtained through chemical conversion technique. J. Alloys Compd., 2019, 784, 1162.

[72] Salam HA, Doench I, Moehwald H. The effect of vanadia surface treatment on the corrosion inhibition characteristics of an advanced magnesium elektron 21 alloy in chloride media. Int. J. Electrochem. Sci., 2012, 7(9): 7751

[73] Zeng RC, Sun XX, Song YW, Zhang F, Li SQ, Cui HZ, Han EH. Influence of solution temperature on corrosion resistance of Zn—Ca phosphate conversion coating on biomedical Mg—Li—Ca alloys. Trans. Nonferrous Met. Soc. China, 2013, 23(11): 3293.

[74] Liao SJ, Yu BX, Zhang XL, Lu XP, Zhou P, Zhang CY, Chen XB, Zhang T, Wang FH. New design principles for the bath towards chromate- and crack-free conversion coatings on magnesium alloys. J. Magnes. Alloys, 2021, 9(2): 505.

[75] Esmaily M, Svensson JE, Fajardo S, Birbilis N, Frankel GS, Virtanen S, Arrabal R, Thomas S, Johansson LG. Fundamentals and advances in magnesium alloy corrosion. Prog. Mater. Sci., 2017, 89, 92.