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Acta Metall Sin  2019, Vol. 55 Issue (7): 840-848    DOI: 10.11900/0412.1961.2018.00558
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Effect of Mo Element and Heat Treatment on Corrosion Resistance of Ni2CrFeMox High-Entropy Alloyin NaCl Solution
Lin WEI1,Zhijun WANG1(),Qingfeng WU1,Xuliang SHANG2,Junjie LI1,Jincheng WANG1
1. State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China
2. Citic Daika Co. , Ltd. , Qinhuangdao 066011, China
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Abstract  

As a new alloy design concept, the high-entropy alloy (HEA) and the formation of simple solid solution introduce excellent properties such as high hardness, high strength and corrosion resistance. Investigations have shown that the single solid solution CrCoFeNi alloy possesses good corrosion resistance. The addition of Mo is beneficial to the corrosion resistance of the HEAs for potential industrial applications in 3.5%NaCl (mass fraction) simulating seawater type environments. The major effect of Mo is to promote the pitting potential of the alloy and inhibit the dissolution of the passivation film by forming and retaining molybdenum oxyhydroxide or molybdates (MoO42-). Considering that the cost of pure Co is higher, Ni and Co elements have similar atomic size and valence electron concentration, and the corrosion resistance of pure Ni is higher than that of pure Co, Ni2CrFeMox HEA was designed by replacing Co element with Ni element in CoCrFeNiMox HEA. As the Mo content increases in the Ni2CrFeMox HEAs, the interdendrite is a Cr and Mo rich σ phase, and the dendrite is a Cr and Mo depleted fcc phase. The potential difference between interdendrites and dendrites leads to galvanic corrosion, which accelerates the localized corrosion of alloys. Here, a solution heat treatment process is selected to reduce the precipitation phase and improve the corrosion resistance of the alloy. The effects of Mo element and heat treatment on the corrosion resistance of Ni2CrFeMox HEA in 3.5%NaCl solution were tested. The results show that the corrosion resistance of as-cast Ni2CrFeMox HEA is obviously higher than that of 316L stainless steel. The Ni2CrFeMo0.2 alloy has the best corrosion resistance because of its minimum dimensional passive current density and corrosion current density. However, the addition of excessive Mo leads to the precipitation of σ phase and galvanic corrosion, which reduces the corrosion resistance of the alloy. After solution treatment, the uniformity of alloy structure and element distribution weakens galvanic corrosion, and the corrosion resistance is obviously improved.

Key words:  Mo element      high entropy alloy      solution treatment      corrosion resistance     
Received:  21 December 2018     
ZTFLH:  TG132  
Fund: National Natural Science Foundation of China(Nos.51471133);National Natural Science Foundation of China(51771149)
Corresponding Authors:  Zhijun WANG     E-mail:  zhjwang@nwpu.edu.cn

Cite this article: 

Lin WEI,Zhijun WANG,Qingfeng WU,Xuliang SHANG,Junjie LI,Jincheng WANG. Effect of Mo Element and Heat Treatment on Corrosion Resistance of Ni2CrFeMox High-Entropy Alloyin NaCl Solution. Acta Metall Sin, 2019, 55(7): 840-848.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2018.00558     OR     https://www.ams.org.cn/EN/Y2019/V55/I7/840

Fig.1  Cyclic polarization curves of the as-cast Ni2Cr-FeMox alloys and 316L stainless steel in 3.5%NaCl solution (Eprot—protection potential, Eb—breakdown potential)
MaterialEcorr / mVicorr / (μA·cm-2)ipass / (μA·cm-2)Eb / mVEprot / mV
Ni2CrFeMo0.1-1352.13018.350986380
Ni2CrFeMo0.2-1790.8965.404920792
Ni2CrFeMo0.3-1031.95915.118896780
Ni2CrFeMo0.4-1211.71113.492954774
Ni2CrFeMo0.5-1252.01414.130966750
NiCoCrFeMo0.2[12]-1310.07216.000941747
316L-762.46322.135439-
Table 1  The electrochemical parameters of the as-cast Ni2CrFeMox alloys and 316L stainless steel in 3.5%NaCl solution
Fig.2  SEM images of the as-cast Ni2CrFeMox alloys with x=0.1 (a), x=0.2 (b), x=0.3 (c), x=0.4 (d), x=0.5 (e) and 316L stainless steel (f) polarized in 3.5%NaCl solution
Fig.3  Cyclic polarization curves of the heat treated Ni2CrFeMox alloys in 3.5%NaCl solution
MaterialEcorr / mVicorr / (μA·cm-2)ipass / (μA·cm-2)Eb / mVEprot / mVErp / mV
Ni2CrFeMo0.1-1470.8017.302915802401
Ni2CrFeMo0.2-1830.3261.612952785497
Ni2CrFeMo0.3-2121.7458.548945780628
Ni2CrFeMo0.4-2361.8079.152890763382
Ni2CrFeMo0.5-2221.3238.810870744367
Table 2  The electrochemical parameters of the heat treated Ni2CrFeMox alloys in 3.5%NaCl solution
Fig.4  SEM images of the heat treated Ni2CrFeMox alloys with x=0.1 (a), x=0.2 (b), x=0.3 (c), x=0.4 (d), x=0.5 (e) polarized in 3.5%NaCl solution
Fig.5  EDS maps of as-cast (a) and heat treated (b) Ni2FeCrMo0.2 alloy (unpolarized)
Fig.6  Nyquist plots (a) and Bode plots (b) of as-cast Ni2FeCrMox alloys in 3.5%NaCl solution
Fig.7  Nyquist plots (a) and Bode plots (b) of as-cast and heat-treated Ni2FeCrMo0.2 alloys in 3.5%NaCl solution
Fig.8  XPS spectra of as-cast (a1~a4) and heat-treated (b1~b4) Ni2CrFeMo0.2 alloy in 3.5%NaCl solution after constant potential+450 mV (vs SHE) polarization for 4 h
ConditionCr(+3)/(0)Fe(+3)/(0)Mo(+6)/(+4)/(0)Ni(+2)/(0)
As-cast14.465.4661.29∶15.54∶23.170.46
Heat-treated17.991.5267.5∶15.18∶17.321.01
Table 3  Atomic ratio between different oxidation states of each element in as-cast and heat-treated Ni2CrFeMo0.2 alloy after XPS analysis
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