Influence of pH on the Formation, Composition and Protectiveness of Fluoride Conversion Film Deposited on AZ31 Magnesium Alloy in KF Solution

doi:10.1007/s40195-023-01571-2

Abstract

Abstract:

In deaerated 0.1 M KF solution at pH 5, pH 7.5 and pH 9, fluoride films were deposited on AZ31 magnesium (Mg) alloy at − 1.4 V vs saturated calomel electrode (SCE). The morphology, phase composition, elemental and compositional depth distribution of different fluoride films and their influences on the corrosion resistance of AZ31 Mg alloy were explored in detail. The results indicated that the films deposited at weak acidic and nearly neutral pH consisted of two layers with amorphous Mg(OH)₂ and MgF₂ as the inner layer, and Mg(OH)₂, MgF₂ and KMgF₃ crystals the outer layer. The film deposited at alkaline pH was a monolayer structure consisting of amorphous Mg(OH)₂ as the major constitute and MgF₂ as the minor constitute. The protectiveness of fluoride film was intimately related to its composition and structure. The content and crystal regularity of KMgF₃ were the two main factors influencing the protectiveness of fluoride film. The fluoride film deposited at pH 7.5 offered the best protection to AZ31 Mg alloy.

Key words: Magnesium alloy, Potentiostatic deposition, Fluoride film, KF solution

Liping Wu, Chen Liu, Jie Wei, Junhua Dong, Lin Zhao, Chao Li, Wei Ke, Yiqing Chen, Changgang Wang. Influence of pH on the Formation, Composition and Protectiveness of Fluoride Conversion Film Deposited on AZ31 Magnesium Alloy in KF Solution[J]. Acta Metallurgica Sinica (English Letters), 2023, 36(9): 1397-1408.

Figures/Tables 12

Fig. 1 a Current decay of AZ31 Mg alloy polarized at − 1.4 V vs SCE in deaerated 0.1 M KF solution with various pH values, b equivalent circuit of electric double layer capacitor. Reproduced with permission from Ref [25]

Fig. 2 Surface morphologies of AZ31 Mg alloy polarized at − 1.4 V vs SCE in deaerated 0.1 M KF solution with various pH values for different time: a, a1, a2, a3 pH 5: 304 s, 1264 s, 2830 s, 16 ks; b, b1, b2, b3 pH 7.5: 1200 s, 1800s, 2700 s, 16 ks; c, c1 pH 9: 530 s, 21.2 ks

Table 1 Elemental compositions of the polarized surfaces presented in Fig. 2

	Time	Mg (at.%)	O (at.%)	F (at.%)	K (at.%)
pH 5	304 s	94.6	5.4
	1264 s	89	6.6	4.4
	2830 s	89.1	7.1	3.8
	16 ks	80.1	6.8	6.8	6.3
pH 7.5	1200 s	93.2	6.8
	1800s	88	9.2	2.8
	2700 s	87.1	9.7	3.2
	16 ks	79.6	7.2	5.6	7.6
pH 9	530 s	93.9	6.1
pH 9	21.2 ks	75.4	16.1	8.5

Fig. 3 High resolution XPS spectra of AZ31 Mg alloy polarized at − 1.4 V vs SCE in deaerated 0.1 M KF solutions with various pH values for different time: a pH 5, 16 ks; b pH 7.5, 16 ks [25]; c pH 9, 21.2 ks. Reproduced with permission from Ref [25]

Fig. 4 XPS elemental depth profiles of AZ31 Mg alloy polarized at − 1.4 V vs SCE in deaerated 0.1 M KF solutions with various pH values for different time: a pH 5, 16 ks; b pH 7.5, 16 ks [25]; c pH 9, 21.2 ks. Reproduced with permission from Ref [25]

Fig. 5 a Mg(OH)2, b MgF2, c KMgF3, d MgF2 + KMgF3 depth profiles of AZ31 Mg alloy polarized at − 1.4 V vs SCE in deaerated 0.1 M KF solutions with various pH values for different time: pH 5, 16 ks; pH 7.5, 16 ks; pH 9, 21.2 ks. Reproduced with permission from Ref [25]

Fig. 6 XRD patterns of AZ31 Mg alloy polarized at − 1.4 V vs SCE in deaerated 0.1 M KF solution with various pH values for different time: pH 5, 16 ks; pH 7.5, 16 ks; pH 9, 21.2 ks. Reproduced with permission from Ref [25]

Fig. 7 Cross-section morphologies of AZ31 Mg alloy polarized at − 1.4 V vs SCE in deaerated 0.1 M KF solutions with various pH values for different time: a pH 5, 16 ks; b pH 7.5, 16 ks [25]; c pH 9, 21.2 ks. Reproduced with permission from Ref [25]

Fig. 8 a, b Bode plots in 0.1 M NaCl solution of AZ31 polarized at − 1.4 V vs SCE in deaerated 0.1 M KF solution with pH 5, 7.5 and 9 for 16 ks, 16 ks and 21.2 ks, respectively; c the equivalent circuit of the FCF coated AZ31 Mg alloy-NaCl solution interface

Table 2 Fitting parameters of the EIS data as shown in Fig. 8

	pH 5	pH 7.5	pH 9
Y_m (mS sⁿ cm⁻²)	1.308 × 10^-5	3.914 × 10^-9	6.918 × 10^-9
n_m	0.9989	0.6023	1
R_m (Ω cm²)	34.5	35.852	37.73
Y_H (mS sⁿ cm⁻²)	7.745 × 10^-9	3.61 × 10^-9	8.822 × 10^-6
n_H	0.9856	0.9454	0.9982
R_H (Ω cm²)	39.44	54.32	19.55
Y_d (mS sⁿ cm⁻²)	8.9 × 10^-6	7.569 × 10^-6	1.965 × 10^-5
n_d	0.8257	0.8056	0.8712
R_d (Ω m²)	539.2	679.3	12.16
Y_f (mS sⁿ cm⁻²)	9.534 × 10^-9	3.63 × 10^-10	8.066 × 10^-9
n_f	1	0.9123	0.9671
R_f (Ω cm²)	11.63	1403	291.3
R_L (Ω cm²)	1116	1088	467.1
L (H)	893.2	1002	426.5

Fig. 9 Hydrogen volume in 0.1 M NaCl of AZ31 Mg alloy polarized at − 1.4 V vs SCE in deaerated 0.1 M KF solutions with various pH values for different time: pH 5, 16 ks; pH 7.5, 16 ks; pH 9, 21.2 ks

Table 3 Standard Gibbs free energy of the compounds and ions mentioned in reaction (11)

Compounds and ions	Gibbs free energy (kJ mol⁻¹)
OH⁻	− 157.244
MgF₂	− 1071
F⁻	− 262.34
Mg(OH)₂	− 834

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