Effect of K2HPO4 Concentration on the Formation, Structure, Composition and Protectiveness of Conversion Coating Deposited on AZ31 Magnesium Alloy

doi:10.1007/s40195-025-01815-3

Abstract

Abstract:

In deaerated 0.05 M, 0.1 M, 0.2 M and 0.5 M K₂HPO₄ solutions with pH 9.5, AZ31 magnesium (Mg) alloy was subjected to potentiostatic polarization at − 0.8 V_SCE to deposit a phosphate conversion coating. The morphology, structure, chemical composition, and protective performance of the conversion coating during the formation process were characterized. The results showed that amorphous Mg(OH)₂ and MgHPO₄ and crystallized KMgPO₄·6H₂O (struvite-K) were successively deposited on the surface of AZ31 Mg alloy in the four solutions. MgHPO₄ was converted from Mg(OH)₂, while struvite-K was transformed from MgHPO₄. The distribution of Mg(OH)₂, MgHPO₄ and struvite-K along the thickness of the coating differs with K₂HPO₄ concentration. As the concentration of K₂HPO₄ increased, the coating time was gradually shortened, and the coating was gradually thinned. Meanwhile, the ratio of the nucleation rate to the growth rate of struvite-K crystal nuclei increased, resulting in a decrease in the size of struvite-K crystal. When the concentration of K₂HPO₄ increased from 0.05 to 0.1 M, the content of struvite-K increased, and the protective performance of the coating was enhanced. However, as the concentration of K₂HPO₄ continued to increase to 0.5 M, the content of struvite-K and the protective performance of the coating decreased.

Key words: Magnesium alloy, Potentiostatic technique, Phosphate coating

Liping Wu, Junhua Dong, Xianfei Zheng, Zhongying Wang, Xiaoying Sun, Xiuling Shang, Wei Ke, Changgang Wang. Effect of K₂HPO₄ Concentration on the Formation, Structure, Composition and Protectiveness of Conversion Coating Deposited on AZ31 Magnesium Alloy[J]. Acta Metallurgica Sinica (English Letters), 2025, 38(3): 481-496.

Figures/Tables 13

Fig. 1 Current decay of AZ31 Mg alloy after polarization at − 0.8 VSCE in pH 9.5 of deaerated solutions with different concentrations of K2HPO4 [18]

Fig. 2 Surface morphologies of AZ31 Mg alloy after polarization at − 0.8 VSCE in pH 9.5 of deaerated a1-a4 0.05 M, b1-b4 0.1 M [18], c1-c4 0.2 M and d1-d4 0.5 M K2HPO4 solutions for a1 151 s, a2 453 s, a3 1335 s, a4 50 ks, b1 100 s, b2 200 s, b3 590 s, b4 24 ks, c1 86 s, c2 205 s, c3 670 s, c4 5 ks, d1 5 s, d2 40 s, d3 318 s, d4 3 ks

Table 1 Elemental content of different positions on the corresponding polarized surfaces

C_{K2HPO4 (M)}	Time (s)	Position	Mg (at.%)	O (at.%)	P (at.%)	K (at.%)
0.05	151		90.95	9.05	×	×
	453	A	82.40	16.57	1.03	×
	453	B	17.64	61.48	12.56	8.32
	1335	C	85.38	14.11	0.51	×
	1335	D	11.70	67.00	12.80	8.50
	50 k		6.27	70.57	13.54	9.62
0.1	100		91.32	8.68	×	×
	200	E	85.79	13.34	0.86	×
	200	F	48.69	38.64	8.18	4.50
	590	G	87.29	12.12	0.59	×
	590	H	38.54	45.86	9.81	5.80
	24 k		15.58	64.07	12.31	8.04
0.2			91.46	8.54	×	×
	86	I	83.30	15.27	1.43	×
	86	J	36.92	52.58	6.91	3.59
	205	K	85.37	13.62	1.01	×
	205	L	31.52	56.35	7.82	4.31
	5 k		25.59	57.21	10.92	6.28
0.5	5	Q	86.08	13.45	1.17	×
	5	R	37.06	52.60	7.18	3.14
	40	M	87.20	12.32	0.48	×
	40	N	8.39	58.30	9.20	4.11
	318	O	88.58	11.22	0.20	×
	318	P	8.07	75.52	10.91	5.50
	3 k		3.18	76.25	12.07	8.50

Fig. 3 Cross-section morphologies of AZ31 Mg alloy after respective polarization at − 0.8 VSCE in deaerated a 0.05 M, b 0.1 M [18], c 0.2 M, d 0.5 M K2HPO4 solutions with pH 9.5 for 50 ks, 24 ks, 5 ks, 3 ks

Fig. 4 Elemental cross-section EPMA images of AZ31 Mg alloy after respective polarization at − 0.8 VSCE in a 0.05 M, b 0.1 M [18], c 0.2 M, d 0.5 M K2HPO4 solutions with pH 9.5 for 50 ks, 24 ks, 5 ks, 3 ks

Fig. 5 Variation of a P, b K, c Mg, d O content along the thickness of the coatings deposited on AZ31 Mg alloy after respective polarization at − 0.8 VSCE in deaerated 0.05 M, 0.1 M, 0.2 M and 0.5 M K2HPO4 solution with pH 9.5 for 50 ks, 24 ks, 5 ks, 3 ks

Fig. 6 XPS spectraof the coatings deposited on AZ31 Mg alloy after respective polarization at − 0.8 VSCE in deaerated 0.05 M, 0.1 M [18], 0.2 M and 0.5 M K2HPO4 solutions with pH 9.5 for 50 ks, 24 ks, 5 ks, 3 ks

Fig. 7 XRD patterns of AZ31 Mg alloy after respective polarization at − 0.8 VSCE in deaerated 0.05 M, 0.1 M [18], 0.2 M and 0.5 M K2HPO4 solutions with pH 9.5 for 50 ks, 24 ks, 5 ks, 3 ks

Fig. 8 a Struvite-K, b MgHPO4, c Mg(OH)2 content across the coating deposited on AZ31 Mg alloy after respective polarization at − 0.8 VSCE in deaerated 0.05 M, 0.1 M, 0.2 M and 0.5 M K2HPO4 solutions with pH 9.5 for 50 ks, 24 ks, 5 ks, 3 ks

Fig. 9 a and b Bode plots measured in 0.1 M NaCl solution after respective polarization of AZ31 Mg alloy at − 0.8 VSCE in deaerated 0.05 M, 0.1 M, 0.2 M and 0.5 M K2HPO4 solutions with pH 9.5 for 50 ks, 24 ks, 5 ks and 3 ks; c equivalent circuit

Table 2 Fitting parameters of the EIS spectra in Fig. 9

C_{K2HPO4 (M)}	Y_s (mS·sⁿ·cm⁻²)	n_s	R_s (Ω·cm²)	Y_H (mS·sⁿ·cm⁻²)	n_H	R_H (Ω·cm²)	Y_f (mS·sⁿ·cm⁻²)	n_f	R_f (Ω·cm²)	Y_d (mS·sⁿ·cm⁻²)	n_d	R_d (Ω·cm²)	R_L (Ω·cm²)	L (H)
0.05	5.1 × 10⁻⁸	0.9	45.0	2.5 × 10⁻⁵	0.9	3806.1	1.2 × 10⁻⁵	0.9	97.1	8.3 × 10⁻⁴	0.9	1590.2	127,000.2	43,900.2
0.1	0.9 × 10⁻⁸	0.8	54.3	2.1 × 10⁻⁵	0.7	15,710.6	1.6 × 10⁻⁵	0.4	10,160.2	1.0 × 10⁻⁶	0.9	3208.4	127,500.3	106,200.3
0.2	9.8 × 10⁻⁸	0.8	56.1	3.2 × 10⁻⁵	0.8	4859.3	2.7 × 10⁻⁵	0.7	1841.0	3.1 × 10⁻⁴	0.6	2878.8	4151.0	60,290.1
0.5	5.7 × 10⁻⁸	1	18.2	3.7 × 10⁻⁵	0.9	3971.8	2.6 × 10⁻⁵	0.2	246.9	3.4 × 10⁻⁴	0.5	2217.9	4076.2	90,720.1

Fig. 10 H2 volume collected in 0.1 M NaCl solution as a function of immersion time after respective polarization of AZ31 Mg alloy at − 0.8 VSCE in deaerated 0.05 M, 0.1 M, 0.2 M and 0.5 M K2HPO4 solutions with pH 9.5 for 50 ks, 24 ks, 5 ks and 3 ks

Table 3 Standard Gibbs free energy of the compounds and ions

Compounds and ions	Gibbs free Energy (kJ・mol⁻¹)
HPO₄²⁻[35]	−1089.26
OH⁻[35]	−157.244
MgHPO₄ [36]	−1566.87
Mg(OH)₂ [37]	−834
K⁺[38]	−283.27
H⁺[35]	0
H₂O [35]	−237.129
Struvite-K [37, 39]	−3263

References 39

[1]	G.Q. Wang, H.H. Wan, Z. Rao, G.F. Li, H.F. Liu, J. Alloy. Compd. 1005, 176026 (2024)
[2]	J. Singh, A.W. Hashmi, S. Ahmad, Y.B. Tian, Inorg. Chem. Commun. 169, 113111 (2024)
[3]	L.P. Wu, J.H. Dong, W. Ke, Electrochim. Acta 105, 554 (2013)
[4]	J. Yuan, X.F. Cui, B. Dai, X.L. Fu, Y.F. Cui, J.H. Peng, J. Alloy. Compd. 1010, 177113 (2025)
[5]	S. Kannan, K. Madhu, M.A. Alotaibi, Surf. Coat. Technol. 493, 131260 (2024)
[6]	Y.Q. Li, R.Q. Hou, P.L. Jiang, K. Li, J. Wang, D. Mei, L.H. Wu, S.J. Zhu, R. Willumeit-Romere, S.K. Guan, Corros. Sci. 239, 112387 (2024)
[7]	D.F. Chen, D. Mei, L. Chen, C. Wang, J. Bai, F. Xue, C.L. Chu, L.G. Wang, S.J. Zhu, S.K. Guan, Appl. Surf. Sci. 672, 160790 (2024)
[8]	L.T. Wang, R.T. Xu, L.J. Meng, Q.Y. Zhang, Z. Qian, J. Chen, C.J. Pan, Biomater. Adv. 163, 213960 (2024)
[9]	X.P. Li, E.L. Lin, K.X. Wang, R.G. Ke, S.Z. Kure-Chu, X.F. Xiao, Ceram. Int. 50, 36838 (2024)
[10]	W.Y. Li, Y.H. Wang, C.J. Che, X.Y. Fu, Y. Liu, D.Z. Xue, S. Zhang, R. Niu, H. Zhang, Y. Cao, S.Y. Song, L.R. Cheng, H.J. Zhang, Bioact. Mater. 40, 474 (2024)
[11]	A.S. Gnedenkov, S.L. Sinebryukhov, A.D. Nomerovskii, V.S. Marchenko, AYu. Ustinov, S.V. Gnedenkov, J. Magnes. Alloy. 12, 2909 (2024)
[12]	M.Y. Ma, D.B. Pokharel, J.H. Dong, L.P. Wu, R.Y. Zhao, Y. Zhu, J.R. Hou, J.Y. Xie, S.H. Sui, C.G. Wang, W. Ke, J. Alloy. Compd. 848, 156506 (2020)
[13]	S. Graeser, W. Postl, H.P. Bojar, P. Berlepsch, T. Armbruster, T. Raber, K. Ettinger, F. Walter, Eur. J. Mineral. 20, 629 (2008)
[14]	A.S. Wagh, S.Y. Sayenko, V.A. Shkuropatenko, R.V. Tarasov, M.P. Dykiy, Y.O. Svitlychniy, V.D. Virych, E.A. Ulybkina, J. Hazard. Mater. 302, 241 (2016)
[15]	F. Tamimi, D.L. Nihouannen, D.C. Bassett, S. Ibasco, U. Gbureck, J. Knowles, A. Wright, A. Flynn, S.V. Komarova, J.E. Barralet, Acta Biomater. 7, 2678 (2011) DOI PMID
[16]	S. Ibasco, F. Tamimi, R. Meszaros, D. Le Nihouannen, S. Vengallatore, E. Harvey, J.E. Barralet, Acta Biomater. 5, 2338 (2009)
[17]	T. Ishizaki, R. Kudo, T. Omi, K. Teshima, T. Sonoda, I. Shigematsu, M. Sakamoto, Electrochim. Acta 62, 19 (2012)
[18]	L.P. Wu, L. Zhao, J.H. Dong, W. Ke, N. Chen, Electrochim. Acta 145, 71 (2014)
[19]	R.J. Kirkpatrick, Am. Mineral. 60, 798 (1975)
[20]	L.P. Wu, L. Zhao, J.H. Dong, W. Ke, X.F. Li, Res. Rev. J. Mater. Sci. 3, 1 (2015)
[21]	T.S.N. Sankara Narayanan, L.L.S. Park, M.H. Lee, J. Mater. Chem. B 2, 3365 (2014) DOI PMID
[22]	J. Farzidayeri, R. Taylor, V. Bedekar, Appl. Energy 351(12180), 8 (2023)
[23]	S.H. Mohamed, M. Raaif, Surf. Coat. Technol. 205, 525 (2010)
[24]	F. Ahimou, C.J. Boonaert, Y. Adriaensen, P. Jacques, P. Thonart, M. Paquot, P.G. Rouxhet, J. Colloid Interf. Sci. 309, 49 (2007)
[25]	J. Christophe, P. Boonaert, P.G. Rouxhet, Appl. Environ. Microb. 66, 2548 (2000) DOI PMID
[26]	S. Feliu, A. Pardo, M.C. Merino, A.E. Coy, F. Viejo, R. Arrabal, Appl. Surf. Sci. 255, 4102 (2009)
[27]	B. Lothenbach, B.W. Xu, F. Winnefeld, Appl. Geochem. 111, 104450 (2019)
[28]	R.L. Frost, S.J. Palmer, R.E. Pogson, J. Therm. Anal. Calori. 107, 1143 (2011)
[29]	L.P. Wu, C. Liu, J. Wei, J.H. Dong, L. Zhao, C. Li, W. Ke, Y.Q. Chen, C.G. Wang, Acta Metall Sin. -Engl. Lett. 36, 1397 (2023)
[30]	L.P. Wu, Z.D. Yang, G.W. Qin, J. Alloy. Compd. 694, 1133 (2017)
[31]	S. Fintová, J. Drábiková, F. Pastorek, J. Tkacz, I. Kuběna, L. Trško, B. Hadzima, J. Minda, P. Doležal, J. Wasserbauer, P. Ptáček, Surf. Coat. Technol. 357, 638 (2019)
[32]	J.P. Zhu, C.W. Fan, C.H. Ning, W. Wang, Ceram. Int. 49, 37604 (2023)
[33]	W. Luo, K. Qi, Y.B. Qiu, X.P. Guo, Prog. Org. Coat. 178, 107507 (2023)
[34]	A. Jangde, S. Kumar, C. Blawert, Corros. Sci. 240, 112386 (2024)
[35]	L. Montastruc, C. Azzaro-Pantel, L. Pibouleau, S. Domenech, Chem. Eng. Process. Process Intensif. 43, 1289 (2004)
[36]	E. Méndez, Biochem. Mol. Biol. Educ. 48, 247 (2020)
[37]	A. La Iglesia, Estud. Geol. 65, 109 (2009)
[38]	H.E. Barner, R.V. Scheuerm, Handbook of Thermochemical Data for Compounds and Aqueous Species (Wiley, Hoboken, 1979)
[39]	M.I. Lelet, M.L. Yakunkova, D.A. Mikhailov, J.N. Lelet, J. Chem. Eng. Data 66, 2723 (2021)

Effect of K₂HPO₄ Concentration on the Formation, Structure, Composition and Protectiveness of Conversion Coating Deposited on AZ31 Magnesium Alloy

RichHTML

PDF (PC)

Knowledge

Abstract

Cite this article

share this article

Figures/Tables 13

References 39

Related Articles 15

Recommended Articles

Metrics

Comments

[1]	Wei-Peng Chen, Jia-Qi Pei, Hua Hou, Yu-Hong Zhao. Phase-field simulation of α-Mg dendrite growth in magnesium alloys: A review [J]. Metals Advances, 2026, 40(2): 48-61.
[2]	Shudong Yang, Xiaoqian Guo, Chao Ma, Lu Shen, Lingyu Zhao, Wei Zhu. Anisotropic Mechanical Behavior in an Extruded AZ31 Magnesium Alloy: Experimental and Crystal Plasticity Modeling [J]. Acta Metallurgica Sinica (English Letters), 2025, 38(9): 1527-1544.
[3]	Dongchao Li, Fen Zhang, Lanyue Cui, Yueling Guo, Rongchang Zeng. Accelerated Corrosion Rate of Wire Arc Additive Manufacturing of AZ91D Magnesium Alloy: The Formation of Nano-scaled AlMn Phase [J]. Acta Metallurgica Sinica (English Letters), 2025, 38(7): 1069-1082.
[4]	Yuxuan Li, Xi Zhao, Shuchang Li, Yihan Gao, Rui Guo. Precipitation Behavior and Strengthening and Toughening Mechanisms of Pre-fabricated Strong Basal Texture AZ80 + 0.4%Ce Alloy Under Room-Temperature Pre-deformation Coupled with Dual-Stage Aging Conditions [J]. Acta Metallurgica Sinica (English Letters), 2025, 38(7): 1127-1144.
[5]	Wei Qiu, Shuang-Long Li, Zhao-Yuan Lu, Sen-Mao Zhang, Jian Chen, Wei Chen, Lang Gan, Wei Li, Yan-Jie Ren, Jun Luo, Mao-Hai Yao, Wen Xie. Effects of CeO₂ Content on the Microstructure and Mechanical Properties of ZK60 Mg Alloy [J]. Acta Metallurgica Sinica (English Letters), 2025, 38(2): 287-298.
[6]	Hong Ju, Cheng Wang, Wei-Jiang Guo, Zhao-Yuan Meng, Peng Chen, Hui-Yuan Wang. Solute Segregation and Grain Boundary Cohesion of Magnesium Binary Alloys: A First-Principles Study [J]. Acta Metallurgica Sinica (English Letters), 2025, 38(12): 2179-2196.
[7]	Li-Lan Gao, Jiang Ma, Yan-Song Tan, Xiao-Hao Sun, Qi-Jun Gao, De-Bao Liu, Chun-Qiu Zhang. Effect of Free-End Torsion on the Corrosion and Mechanical Properties for Mg-3Zn-0.2Ca Alloy [J]. Acta Metallurgica Sinica (English Letters), 2025, 38(1): 59-70.
[8]	Zhenfei Jiang, Bo Hu, Zixin Li, Fanjin Yao, Jiaxuan Han, Dejiang Li, Xiaoqin Zeng, Wenjiang Ding. A Review of Magnesium Alloys as Structure-Function Integrated Materials [J]. Acta Metallurgica Sinica (English Letters), 2024, 37(8): 1301-1338.
[9]	Ze-Song Wei, Zi-You Ding, Lei Cai, Shao-Xia Ma, Dong-Qing Zhao, Lan-Yue Cui, Cheng-Bao Liu, Yuan-Sheng Yang, Yuan-Ding Huang, Rong-Chang Zeng. Exfoliation Corrosion of As-Extruded Mg-1Li-1Ca: the Influence of the Superficial Layer [J]. Acta Metallurgica Sinica (English Letters), 2024, 37(8): 1339-1353.
[10]	Gang Zeng, Hong Liu, Jing-Peng Xiong, Jian-Long Li, Yong Liu. Enhanced Grain Refining Effect of Mg-Zr Master Alloy on Magnesium Alloys via a Synergistic Strategy Involving Heterogeneous Nucleation and Solute-Driven Growth Restriction [J]. Acta Metallurgica Sinica (English Letters), 2024, 37(8): 1354-1366.
[11]	Bishan Cheng, Depeng Li, Baikang Xing, Ruiqing Hou, Pingli Jiang, Shijie Zhu, Shaokang Guan. Effect of Ca Micro-Alloying on the Microstructure and Anti-Corrosion Property of Mg0.5Zn0.2Ge Alloy [J]. Acta Metallurgica Sinica (English Letters), 2024, 37(7): 1147-1160.
[12]	Xiaoxue Wang, Jingjing Guo, Zihao Zeng, Peng Zhou, Rongqiao Wang, Xiuchun Liu, Kai Gao, Jingli Sun, Yong Yuan, Fuhui Wang. A Semi-Mechanistic Model for Predicting the Service Life of Composite Coatings on VW63Z Magnesium Alloy [J]. Acta Metallurgica Sinica (English Letters), 2024, 37(7): 1161-1176.
[13]	Qian Wang, Peng Yu, Haoran Lin, Chongzhi Guo, Xiaoqiang Hu. Joined AZ31B Magnesium Alloys with Ag Interlayer by Ultrasonic-Induced Transient Liquid Phase Bonding in Air [J]. Acta Metallurgica Sinica (English Letters), 2024, 37(7): 1177-1185.
[14]	Qian-Long Ren, Shuai Yuan, Shi-Yu Luan, Jin-Hui Wang, Xiao-Wei Li, Xiao-Yu Liu. High-Temperature Stability of Mg-1Al-12Y Alloy Containing LPSO Phase and Mechanism of Its Portevin-Le Chatelier (PLC) Effect [J]. Acta Metallurgica Sinica (English Letters), 2024, 37(6): 982-998.
[15]	Zhaochen Yu, Kaixuan Feng, Shuyun Deng, Yang Chen, Hong Yan, Honggun Song, Chao Luo, Zhi Hu. Quasi-in-situ Observation and SKPFM Studies on Phosphate Protective Film and Surface Micro-Galvanic Corrosion in Biological Mg-3Zn-xNd Alloys [J]. Acta Metallurgica Sinica (English Letters), 2024, 37(4): 648-664.