Effect of Mo Addition on Corrosion Behavior of High-Entropy Alloys CoCrFeNiMox in Aqueous Environments

doi:10.1007/s40195-018-0812-7

Abstract

Abstract:

The corrosion behavior of CoCrFeNiMo_x alloys was investigated in aqueous environments, NaCl and H₂SO₄ solutions, respectively, to simulate typical neutral and acidic conditions. The cyclic polarization curves in NaCl and the potentiodynamic curves in H₂SO₄ clearly reveal the beneficial effects of Mo and the detrimental effect of σ-phase on the corrosion resistance. The X-ray photoelectron spectroscopy results of CoCrFeNiMo_x alloys in H₂SO₄ solution indicate that Cr and Mo predominate the corroded surface of the alloys, where Mo primarily exists in the form of MoO₃.

Key words: High-entropy alloys, Corrosion, σ-phase;, Molybdenum

Xu-Liang Shang, Zhi-Jun Wang, Qing-Feng Wu, Jin-Cheng Wang, Jun-Jie Li, Jia-Kang Yu. Effect of Mo Addition on Corrosion Behavior of High-Entropy Alloys CoCrFeNiMo_x in Aqueous Environments[J]. Acta Metallurgica Sinica (English Letters), 2019, 32(1): 41-51.

Figures/Tables 13

Fig. 1 Cyclic polarization curves of CoCrFeNiMox alloys in 3.5 wt% NaCl solution: ax?=?0.1; bx?=?0.2; cx?=?0.3; dx?=?0.4; ex?=?0.5

Table 1 Electrochemical parameters of CoCrFeNiMox alloys in 3.5 wt% NaCl solution

Designation	E_corr (mV vs. RE)	i_corr (μA/cm²)	E_pp (mV vs. RE)	i_pass (μA/cm²)	E_b (mV vs. RE)	E_prot (mV vs. RE)	E_rp (mV vs. RE)	ΔE (mV)
Mo_0.1	-?263	0.381	-?134	38	949	-	-?182	1083
Mo_0.2	-?131	0.072	-?28	16	941	747	184	969
Mo_0.3	-?257	0.766	-?181	50	955	11	-?178	1136
Mo_0.4	-?261	0.082	-?261	16	948	695	237	1209
Mo_0.5	-?261	0.738	-?100	24	965	370	63	1065

Fig. 2 SEM images of CoCrFeNiMox alloys in 3.5 wt% NaCl solution: ax?=?0.1; bx?=?0.2; cx?=?0.3; dx?=?0.4; ex?=?0.5; b’-e’ high magnification of Fig. 2b-e, respectively

Fig. 3 LSCM images of CoCrFeNiMox alloys after cyclic polarization in 3.5 wt% NaCl solution: a CoCrFeNiMo0.1, corrosion morphology; b CoCrFeNiMo0.1, topographic map; c CoCrFeNiMo0.3, corrosion morphology; d CoCrFeNiMo0.3, topographic map

Fig. 4 EDS maps of CoCrFeNiMo0.5 after cyclic polarization in 3.5 wt% NaCl for different elements

Fig. 5 a Bode, b Nyquist plots of CoCrFeNiMox alloys in the 3.5 wt% NaCl solution after different amounts of Mo addition; c, d corresponding equivalent electrical circuits (Z: impedance; Zre: real part of impedance; and Zim: imaginary part of impedance)

Fig. 6 aPotentiodynamic polarization curves of CoCrFeNiMox alloys in 0.5 mol/L H2SO4 solution, b magnification of curves around corrosion potential Ecorr

Table 2 Electrochemical parameters of CoCrFeNiMox alloys in 0.5 mol/L H2SO4 solution

Designation	E_corr (mV vs. RE)	i_corr (μA/cm²)	E_pp (mV vs. RE)	i_pass (μA/cm²)	E_b (mV vs. RE)	ΔE (mV)
Mo_0.1	-?688	9.666	-?638	30	469	1107
Mo_0.2	-?682	2.926	-?661	9	460	1121
Mo_0.3	-?694	8.626	-?656	30	453	1109
Mo_0.4	-?663	0.712	-?576	6	462	1038
Mo_0.5	-?632	1.526	-?536	12	450	986

Fig. 7 SEM images of CoCrFeNiMox alloys in 0.5 mol/L H2SO4 solution: ax?=?0.1; bx?=?0.2; cx?=?0.3; dx?=?0.4; ex?=?0.5; (c’-e’) high magnification of Fig. 7c-e, respectively

Fig. 8 LSCM images of CoCrFeNiMox alloys after potentiodynamic polarization in 0.5 mol/L H2SO4 solution: a CoCrFeNiMo0.1, corrosion morphology; b CoCrFeNiMo0.1, topographic map; c CoCrFeNiMo0.5, corrosion morphology; d CoCrFeNiMo0.5, topographic map

Table 3 Cationic fractions analyzed by XPS for CoCrFeNiMo0.4 potentiostatically polarized for 1 h in 0.5 mol/L H2SO4 solution

Potential (mV vs. RE)	Co	Cr	Fe	Ni	Mo
-?550	8.52	44.89	8.14	6.42	32.03
-?100	8.02	35.82	11.73	6.79	37.65
350	6.18	52.24	8.44	5.27	27.87

Fig. 9 Analytical results of XPS for CoCrFeNiMo0.4 potentiostatically polarized for 1 h in 0.5 mol/L H2SO4

Table 4 Summary of ratios for main elements with different oxidation states detected by XPS

Voltage (mV vs. RE)	Co²⁺/Co⁰	Cr³⁺/Cr⁰	Fe³⁺/Fe⁰	Ni²⁺/Ni⁰	Mo⁶⁺: Mo⁴⁺: Mo⁰
-?550	0.34	5.38	2.94	0	47.61: 16.59: 35.80
-?100	0.37	6.90	1.60	0.10	48.93: 17.11: 33.96
350	1.33	14.46	5.46	0.46	61.29: 15.54: 23.17

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