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

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

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