Mechanistic study of ammonium-induced corrosion of AZ31 magnesium alloy in sulfate solution

doi:10.1016/j.jmst.2020.02.074

Abstract

Abstract:

The influence of NH₄⁺ ions on the corrosion behavior of AZ31 magnesium alloy was investigated by immersion test, hydrogen evolution, electrochemical methods and morphology observation. The results demonstrate the acceleration effect of NH₄⁺ on corrosion of AZ31 magnesium alloy due to the disruption of protective MgO film in NH₄⁺-containing solution. The loose and cracked corrosion products of AZ31 magnesium alloy in NH₄⁺-containing solutions are mainly composed of (Mg_0.833Al_0.167)(OH)₂(CO₃)_0.083·0.75H₂O and Mg₅(CO₃)₄(OH)₂·5H₂O. When the NH₄⁺ concentration is lower than 0.01 M, knife-cut like corrosion occurs in some active area of the surface due to the partial dissolution of MgO layer. As the NH₄⁺ concentration is increased to 0.1 M, the MgO layer is completely disrupted, resulting in the occurrence of uniform corrosion. Both cathodic and anodic reactions are accelerated by NH₄⁺ ions, while the effect of original pH values on the reaction kinetics can be neglected in NH₄⁺-containing solutions.

Key words: Magnesium alloy, Ammonium, Corrosion kinetics, Knife-cut like corrosion

Hui Pan, Liwei Wang, Yi Lin, Feng Ge, Kang Zhao, Xin Wang, Zhongyu Cui. Mechanistic study of ammonium-induced corrosion of AZ31 magnesium alloy in sulfate solution[J]. J. Mater. Sci. Technol., 2020, 54: 1-13.

Figures/Tables 18

Table 1 Chemical composition (wt.%) of AZ31 magnesium alloy used in the present work.

Alloy	Al	Si	Zn	Mn	Fe	Cu	Ni	Mg
AZ31	3.19	0.025	0.81	0.30	0.006	0.002	0.001	Bal.

Fig. 1. Weight loss (a) and average corrosion rates (b) of AZ31 magnesium alloys in vary concentrations of NH4+.

Fig. 2. Hydrogen evolution (a) and corrosion rates (b) of AZ31 magnesium alloys in the solutions with different concentrations of NH4+.

Fig. 3. SEM morphologies of the surface corrosion products on AZ31 magnesium alloy after immersion in the solutions containing 0.001 M (a1)-(a3), 0.01 M (b1)-(b3), and 0.1 M (c1)-(c3) NH4+ for 6 h (a1)-(c1), 24 h (a2)-(c2), and 120 h (a3)-(c3). (The delineated points are detected by EDS.).

Fig. 4. FTIR (a) and XRD (b) spectra of the corrosion products formed on AZ31 magnesium alloy surface after immersion in different solutions for 120 h.

Fig. 5. Cross-sectional morphology of corrosion product and the substrate beneath it of AZ31 magnesium alloys after 120 h immersion in solutions with 0.001 M (a), 0.01 M (b), (d), and 0.1 M NH4+ (c).

Fig. 6. Surface morphologies of AZ31 magnesium alloy without corrosion products after immersion in the solutions containing 0.001 M (a1)-(a3), 0.01 M (b1)-(b3), and 0.1 M (c1)-(c3) NH4+ for 6 h (a1)-(c1), 24 h (a2)-(c2), and 120 h (a3)-(c3).

Fig. 7. Changes of OCPs (a) and pH values (b) of AZ31 magnesium alloy in NH4+ solutions and the corresponding E-pH diagram (c), as well as the overlaid E-pH diagram of the N-H2O and Mg-H2O system (d).

Fig. 8. Polarization curves of AZ31 magnesium alloy in solutions with different concentrations of NH4+: (a) before IR compensation, (b) after IR compensation.

Table 2 Corrosion parameters of AZ31 magnesium alloys in different (NH4)2SO4 solution derived from the IR-compensated polarization curves.

NH₄⁺ concentration	E_corr (V_SCE)	b_c (V dec^-1)	i_corr (mA cm²)
0.001 M	-1.524	-0.618	0.029
0.01 M	-1.559	-0.636	0.157
0.1 M	-1.722	-0.972	1.954

Fig. 9. Nyquist diagrams and fitting lines of AZ31 magnesium alloys after immersion for different time in the solutions containing 0.001 M (a), 0.01 M (b), and 0.1 M NH4+ (c).

Fig. 10. Equivalent circuits at different immersion periods (a) and the calculated polarization resistance (b).

Table 3 The fitted electrochemical parameters for AZ31 magnesium in (NH4)2SO4 solution.

NH₄⁺ concentration	Time (h)	R_s (Ω cm²)	CPE_dl 10^-6(Ω ^-1 cm^-2 sⁿ)	n₁	R_ct (Ω cm²)	CPE_dl 10^-6(Ω ^-1 cm^-2 sⁿ)	n₂	R_f (Ω cm²)	R_L1(Ω cm ²)	L1(H cm ²)	R_L2(Ω cm ²)	L2(H cm ²)	R_p(Ω cm²)
0.001 M	0.5	1000	12.37	0.84	1560	3911	1	368.8					1928.8
	3	991.8	12.76	0.87	2377	3044	0.97	869					3246
	6	997	12.8	0.85	3150	2326	1	1077					4227
	12	987.9	11.9	0.85	3219	1670	1	1177					4396
	24	1038	11.6	0.90	4382	4550	0.86	1046					5428
	48	1078	19.6	0.90	3835	3198	0.93	1482					5317
	72	1019	26.9	0.86	3972	4963	1	1184					5156
	96	1011	30.2	0.87	3668	4800	0.88	1272					4940
	120	1009	30.65	0.86	3986	4913	0.91	1326					5312
0.01 M	0.5	223.9	15.98	0.87	292.9	5773	0.96	111.1	454.8	11900			214
	3	220.5	22.5	0.88	333	9399	1	79.5c	890	27720			282
	6	218.6	23.8	0.88	349.8	8718	0.99	88	876	34530			292
	12	221	24.2	0.88	342.5	9315	0.98	88.6	1265	29820			321.5
	24	281.5	24.8	0.88	394.7	9112	1	96	1362	48500			361
	48	222.7	26.29	0.89	482	9363	1	110	1947	83000			454
	72	222.8	29.1	0.89	555.9	11400	1	114	4096	121300			576
	96	222.8	32.3	0.89	586	11610	1	116	1301	21040			456
	120	219	36.67	0.89	599	13740	1	114.8	3649	36220			597
0.1 M	0.5	45	18.9	0.95	14.5	26770	0.81	9	49.9	55.5	8.4	296.5	5.5
	3	43	23.0	0.92	26.6	19460	0.81	20	66.5	115.6	21.6	859.8	12.1
	6	41	24.4	0.91	35	14110	0.93	17.9	89.6	281	34.1	2164	16.9
	12	42.9	24.5	0.92	47	11340	1	21	109	574.6	47.2	5419	22.2
	24	46	24.7	0.92	64.8	10000	1	27	134	1264	68	18200	30.2
	48	48.7	26.6	0.89	95.5	9373	1	34.6	204	3593			79.4
	72	49.7	23.9	0.91	118.5	9314	1	38	294.7	5450			102.4
	96	48	23.2	0.91	145	9303	1	48	399	8546			130.5
	120	50	21.7	0.92	162	9517	1	50	491	10600			148.5

Fig. 11. Comparison of the corrosion rates of AZ31 magnesium alloy during immersion in the NH4+-containing solutions for different time. (b) is the local magnification image marked in (a).

Fig. 12. Schematic diagram of the corrosion process of AZ31 magnesium alloy in NH4+-containing solutions.

Fig. 13. Polarization curves of AZ31 magnesium alloy in different solutions after immersion for 10 min.

Table 4 Corrosion parameters of AZ31 magnesium alloys in different solutions derived from the polarization curves.

Solutions	E_corr (V_SCE)	b_c (V dec^-1)	i_corr (mA cm-2)	P_i (mm a^-1)
(NH₄)₂SO₄ (pH5.96)	-1.559	-0.389	0.149	3.41
Na₂SO₄ (pH6.48)	-1.562	-0.280	0.039	0.895
Na₂SO₄ (pH5.96)	-1.562	-0.290	0.052	1.189

Fig. 14. Normalized impedance diagrams of AZ31 magnesium alloys after immersion in the solutions containing 0.001 M (a), 0.01 M (b), and 0.1 M (c) NH4+ for different time and the comparison of the diagrams in different solutions at 0.5 h (d).

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