Threshold Chloride Concentration for Passivity Breakdown of Mg-Zn-Gd-Nd-Zr Alloy (EV31A) in Basic Solution

doi:10.1007/s40195-017-0548-9

Abstract

Abstract:

Mg-Zn-Gd-Nd-Zr alloy (EV31A) is a heat-treatable magnesium (Mg) cast alloy that can be used up to 200 °C for automobile and aerospace applications. This alloy has excellent mechanical properties (ultimate tensile strength: 280 MPa at room temperature, and ~230 MPa at 200 °C) and improved corrosion resistance. Electrochemical corrosion studies were conducted on this alloy under different heat treatment conditions in 0.1 M NaOH solution with the addition of 0-1000 ppm of chloride. The alloy showed excellent passivity in the 0.1 M NaOH solution. The passive potential range typically extended to more than 1.2 V_Ag/AgCl. The transpassive potential was observed to be dependent on heat treatment condition of the alloy. More than 80 ppm of chloride was required to induce passivity breakdown in any heat treatment condition. Peak aging at 200 °C for 16 h imparted better resistance for localized corrosion than other heat-treated conditions. The alloy in the as-received condition showed the highest passivation kinetics due to its smaller grain size that possibly increased the diffusion of reactive elements to form protective oxide. The passive film of the EV31A alloy showed n-type semiconductivity with a charge carrier density of ~2 × 10²¹ cm^-3 with no chloride addition. The charge carrier density increased with chloride addition in the electrolyte which could be correlated with the susceptibility to localized corrosion.

Key words: Magnesium alloy, Corrosion, Passivation, Chloride

Jakraphan Ninlachart, Krishnan S. Raja. Threshold Chloride Concentration for Passivity Breakdown of Mg-Zn-Gd-Nd-Zr Alloy (EV31A) in Basic Solution[J]. Acta Metallurgica Sinica (English Letters), 2017, 30(4): 352-366.

Figures/Tables 14

Fig. 1 Optical microstructures of EV31A in different heat treatment conditions: a as-received, b solution heat-treated, c peak-aged (200 °C, 16 h), d overaged (200 °C, 100 h)

Fig. 2 a XRD patterns of EV31A in different heat treatment conditions, b Vickers hardness of EV31A in different heat treatment conditions. AR as-received, ST solution heat-treated, 200/16: peak-aged (PA) at 200 °C for 16 h, 200/100 overaged (OA) at 200 °C for 100 h

Table 1 Cyclic polarization results of EV31A specimens in different heat-treated conditions in 0.1 M NaOH

Sample	OCP	E_Transpassive	E_{Cross over}	Passive current density
Sample	(V vs Ag/AgCl)	(V vs Ag/AgCl)	(V vs Ag/AgCl)	(μA/cm²)
AR	-1.03	1.67	1.07	7.00
ST	-1.50	1.73	1.52	2.67
Peak aged	-1.20	1.52	1.31	3.90
Overaged	-1.21	1.58	1.47	4.11

Table 2 Cyclic polarization results of EV31A specimens in different heat-treated conditions in 80 ppm Cl- solution

Sample	OCP	E_Transpassive	E_{Cross over}	Passive current density
Sample	(V vs Ag/AgCl)	(V vs Ag/AgCl)	(V vs Ag/AgCl)	(μA/cm²)
AR	-1.66	1.64	1.27	2.50
ST	-1.57	1.61	1.37	1.66
Peak aged	-1.55	1.58	1.29	1.21
Overaged	-1.57	1.62	1.38	2.20

Fig. 3 Cyclic polarization results of EV31A specimens in different heat-treated conditions in 0.1 M NaOH with different chloride additions: a 0 ppm Cl-,b 80 ppm Cl-, c 100 ppm Cl-, d 200 ppm Cl-, e 500 ppm Cl-, f 1000 ppm Cl-

Fig. 4 Cathodic polarization plots of EV31A alloy in different heat-treated conditions in 0.1 M NaOH solution with different chloride (NaCl) additions: a no chloride addition, b 80 ppm chloride, c 100 ppm chloride, d 200 ppm chloride

Table 3 Summary of the cathodic polarization results of EV31A alloy in different heat-treated conditions in 0.1 M NaOH with addition of different chloride concentrations

Sample	Cl^- concentration	Tafel slope (V/dec)	Exchange current density for hydrogen evolution, i₀ (A/cm²)	Over potential for 1 mA/cm² (V)
AR	No Cl^-	-0.14	1.3 × 10^-11	-1.11
	80 ppm	-0.21	4.0 × 10^-9	-1.11
	100 ppm	-0.18	1.8 × 10^-9	-1.07
	200 ppm	-0.18	9.0 × 10^-10	-1.08
ST	No Cl^-	-0.16	2.9 × 10^-10	-1.03
	80 ppm	-0.18	2.4 × 10^-9	-1.03
	100 ppm	-0.14	1.3 × 10^-10	-1.04
	200 ppm	-0.18	7.0 × 10^-10	-1.03
Peak aged	No Cl^-	-0.19	4.0 × 10^-9	-1.25
	80 ppm	-0.18	3.8 × 10^-10	-1.05
	100 ppm	-0.16	1.5 × 10^-10	-1.07
	200 ppm	-0.13	1.2 × 10^-11	-1.05
Overaged	No Cl^-	-0.17	5.5 × 10^-10	-1.04
	80 ppm	-0.18	7.7 × 10^-10	-1.07
	100 ppm	-0.14	1.0 × 10^-10	-1.04
	200 ppm	-0.14	9.5 × 10^-12	-1.05

Fig. 5 I-t plots of EV31A specimens with different heat treatment conditions under potentiostatic control in 0.1 M NaOH solution with different chloride additions: a no chloride, b 80 ppm Cl-c 100 ppm Cl-, d 200 ppm Cl-, e 500 ppm Cl-. The applied potential (typically 0.5 VAg/AgCl) was in the middle of the passivation range of the corresponding heat-treated condition as shown in Fig. 3

Table 4 Passivation kinetic exponent and charge carrier density of EV31A specimens in different heat-treated conditions in different concentrations of chloride

Sample	Cl^- concentration	Passivation kinetic exponent	Charge carrier density, N_D
Sample	Cl^- concentration	α	(1/cm³)
AR	No Cl^-	0.684	2.08 × 10²¹
	80 ppm	0.687	2.46 × 10²¹
	500 ppm	0.55	5.42 × 10²¹
ST	No Cl^-	0.46	1.29 × 10²¹
	80 ppm	0.68	3.54 × 10²¹
	500 ppm	0.57	N/A
Peak aged	No Cl^-	0.48	3.50 × 10²¹
	80 ppm	0.72	2.30 × 10²¹
	500 ppm	0.47	N/A
Overaged	No Cl^-	0.42	1.87 × 10²¹
	80 ppm	0.59	2.50 × 10²¹
	500 ppm	0.55	5.49 × 10²¹

Fig. 6 Optical microstructures of the specimens under potentiostatic condition (1.1 VAg/AgCl) until pit initiation in 500 ppm Cl- + 0.1 M NaOH: a as-received, b solution heat-treated, c peak-aged (200 °C, 16 h), d overaged (200 °C, 100 h)

Fig. 7 SEM images of the solution heat-treated specimen before and after 1 h potentiostatic passivation at 0.5 VAg/AgCl in 0.1 M NaOH solution: a before the test, b after the test, c EDX spectrum of elemental analysis of the circled particle in a, and d EDX spectrum of the circled particle shown in b. Noted that the specimen in Fig. 7a was ground with SiC paper up to 600 grit, and specimen in Fig. 7b was ground up to 1-μm diamond suspension. The images do not show the exact same area

Fig. 8 Nyquist plots of EV31A specimens passivated at mid-potential of their passivation ranges for 1 h in 0.1 M NaOH solution with different chloride additions: a no chloride, b 80 ppm Cl-, c 100 ppm Cl-, d 200 ppm Cl-; e electrical equivalent circuit fitted the data of Nyquist plots with χ2 < 10-4

Table 5 Equivalent circuit values of the specimens in different chloride concentrations

Sample	Cl^- concentration	R_s	Q₁	α	Q₂	β	R₁	R₂
Sample	(ppm)	(Ohm)	(S·s^α)		(S·s^β)		(Ohm)	(Ohm)
AR	0	51.59	9.063 × 10^-6	0.8	2.084 × 10^-4	0.8	5.642 × 10¹¹	4.956 × 10⁴
	80	53.31	8.449 × 10^-6	0.9441	1.616 × 10^-4	0.5918	8.461 × 10¹⁶	4.509 × 10⁴
	500	55.41	9.759 × 10^-6	0.9475	2.536 × 10^-5	0.7867	4.933 × 10⁴	1.011 × 10⁵
ST	0	58.7	5.65 × 10^-6	0.9218	8.943 × 10^-6	0.2355	4.511 × 10⁵	9.939 × 10^-4
	80	50.77	9.583 × 10^-6	0.9475	5.216 × 10^-5	0.8719	8.237 × 10⁴	9.312 × 10⁴
	500	-	-	-	-	-	-	-
Peak aged	0	5.84 × 10^-16	1.393 × 10^-5	2.96 × 10^-10	1.313 × 10^-5	0.9162	2.274 × 10¹²	56.36
	80	63.1	7.567 × 10^-6	0.9459	2.414 × 10^-4	0.7426	6.31 × 10¹³	5.434 × 10⁴
	500	-	-	-	-	-	-	-
Overaged	0	48.64	7.823 × 10^-6	0.9255	8.999 × 10^-7	0.4205	5.732 × 10¹³	3.668 × 10⁴
	80	51.18	8.189 × 10^-6	0.9384	1.664 × 10^-4	0.5575	1.353 × 10¹⁸	4.977 × 10⁴
	500	49.46	1.055 × 10^-5	0.9494	3.477 × 10^-5	0.6005	8.024 × 10⁴	1.009 × 10⁵

Fig. 9 Mott-Schottky plots of overaged (200 °C for 100 h) specimen in solution with different chloride concentrations

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