Effect of valence state of chromium in molten chloride salt on corrosivity of nickel-based alloy

doi:10.11949/0438-1157.20220156

Abstract

Abstract:

The corrosivity of molten chloride salt is an important factor limiting its application. Its corrosion to chromium-rich metal materials mainly results in the preferential loss of chromium from metals to molten salt. The effect of chromium valence on the subsequent corrosion of metal in molten salt is the key to understand the continuous corrosion of metal. In this paper, the effects of Cr⁰, Cr²⁺ and Cr³⁺ on the corrosion of a chromium-poor Hastelloy B-2 (HB-2) and two chromium-rich Hastelloy C-276 (HC-276), Hastelloy X (HX) nickel-based alloys are studied by immersion corrosion test. By analyzing the results of mass change, X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), the corrosion difference of molten salts containing different valence chromium on chromium-rich and chromium-poor metals are discussed. The experimental results show that Cr⁰ and Cr²⁺ can significantly reduce the corrosion mass loss rate and inhibit their corrosion by consuming active oxides such as H₂O and O₂. Cr³⁺ can inhibit the corrosion of chromium-poor alloy HB-2, but can promote the corrosion of chromium-rich alloy HC-276 and HX. The results of SEM and XRD show that Cr³⁺ not only enhance the preferential loss of chromium, but also enhance the loss of Fe. The standard Gibbs free energy variation $? r G m ?$ of oxidation of CrCl₃ oxidized Ni, Cr, Fe are calculated respectively. The results show that the oxidation of Cr and Fe by CrCl₃ are -142.9 kJ/mol and -87.4 kJ/mol, indicating that the reaction is carried out thoroughly. The oxidation of Ni and Mo by CrCl₃ is limited. Therefore, CrCl₃ in molten salt promotes the corrosion of chromium-rich alloy by oxidizing Cr and Fe in the alloy, while Cr⁰ and CrCl₂ inhibit corrosion by consuming oxidizing species in molten salt.

Key words: molten chloride salt, chromium, corrosion, nickel base alloy

摘要：

氯化物熔盐的腐蚀性是制约其应用的重要因素，对富铬金属材料的腐蚀会导致金属中铬元素优先流失到熔盐中。探讨进入熔盐中不同价态的铬对后续腐蚀的影响，是了解熔盐长期运行中金属持续受腐蚀的关键。通过浸没腐蚀实验研究在三元NaCl-MgCl₂-CaCl₂熔盐中引入Cr⁰、Cr²⁺与Cr³⁺后，对一种贫铬Hastelloy B-2(HB-2)和两种富铬Hastelloy C-276(HC-276)、Hastelloy X(HX)镍基合金腐蚀性的影响。通过比较腐蚀前后质量变化、X射线衍射(XRD)、扫描电子显微镜(SEM)以及能谱分析(EDS)的结果，探讨含不同价态铬的熔盐对贫铬与富铬金属的腐蚀性差异。实验结果表明，Cr⁰和Cr²⁺能消耗熔盐中H₂O、O₂等氧化性物种，从而有效抑制腐蚀；Cr³⁺会抑制贫铬HB-2的腐蚀，但能促进富铬HC-276和HX的腐蚀；SEM和XRD分析结果表明，Cr³⁺在增强富铬金属铬优先流失的同时也会增强铁流失。热力学理论计算结果表明，CrCl₃氧化Cr、Fe的反应进行得很彻底；而CrCl₃氧化Ni、Mo的反应进行程度有限。因此，熔盐中含CrCl₃会氧化合金中Cr和Fe从而促进富铬合金腐蚀，而含Cr⁰和CrCl₂能降低熔盐中的氧化性物质含量而抑制腐蚀。

关键词: 氯化物熔盐, 铬, 腐蚀, 镍基合金

CLC Number:

TQ 124.42

Xiaolan WEI, Wenjie QI, Jing DING, Jianfeng LU, Weilong WANG, Shule LIU. Effect of valence state of chromium in molten chloride salt on corrosivity of nickel-based alloy[J]. CIESC Journal, 2022, 73(7): 3182-3192.

魏小兰, 戚文杰, 丁静, 陆建峰, 王维龙, 刘书乐. 氯化物熔盐中铬的价态对镍基合金腐蚀性的影响[J]. 化工学报, 2022, 73(7): 3182-3192.

Figures/Tables 13

Table 1 Chemical composition of three nickel base alloys

镍基合金	元素组成/%
镍基合金	Ni	Cr	Mo	Fe	W	Co	Mn	V
HB-2	65.0~70.0	0.4~0.7	26.0~30.0	1.6~2.0	—	≤1.0	≤1.0	—
HC-276	51.0~59.0	14.5~16.5	15.0~17.0	4.0~7.0	3.0~4.5	≤2.5	≤1.0	≤0.35
HX	42.0~52.0	20.5~23.0	8.0~10.0	17.0~20.0	0.2~1.0	0.5~2.5	≤1.0	—

Fig.1 Mass loss of HB-2, HC-276 and HX after corrosion in four molten salts for 100 h

Fig.2 XRD patterns of HB-2, HC-276 and HX before and after corrosion for 100 h in molten salts of BS-Cr0、BS-Cr2+ and BS-Cr3+

Fig.3 Micromorphology of surface of HB-2, HC-276 and HX after corrosion at 600℃ for 100 h in BS (a), BS-Cr0 (b), BS-Cr2+ (c) and BS-Cr3+ (d) molten salts

Table 2 Content of main metal elements before and after corrosion of HB-2， HC-276 and HX analyzed by EDS

熔盐	含量/%(质量)
	HB-2				HC-276				HX
	Cr	Fe	Mo	Ni	Cr	Fe	Mo	Ni	Cr	Fe	Mo	Ni
腐蚀前	0.5	1.8	28.0	69.8	14.0	5.3	13.9	53.4	22.0	18.1	8.5	50.1
加Cr⁰	1.3	1.6	29.3	68.5	8.7	3.3	18.7	57.5	21.6	17.0	8.8	52.1
加CrCl₂	0.8	1.8	26.5	70.1	10.7	4.2	14.0	54.2	17.2	11.9	8.6	62.2
加CrCl₃	0.6	1.8	29.2	68.4	5.4	2.0	13.7	75.1	14.9	6.9	6.6	71.6

Fig.4 Microstructure of cross section of HB-2, HC-276 and HX after corrosion at 600℃ for 100 h in BS (a), BS-Cr0 (b), BS-Cr2+ (c) and BS-Cr3+ (d) molten salts

Fig.5 EDS line scanning results (along yellow line) and cross section element distribution of HC-276 after corrosion in BS molten salt for 100 h

Fig.6 EDS line scanning results (along yellow line) and cross section element distribution of HC-276 after corrosion in BS-Cr0 molten salt for 100 h

Fig.7 EDS line scanning results (along yellow line) and cross section element distribution of HC-276 after corrosion in BS-Cr2+ molten salt for 100 h

Fig.8 EDS line scanning results (along yellow line) and cross section element distribution of HC-276 after corrosion in BS-Cr3+ molten salt for 100 h

Fig.9 EDS line scanning results (along yellow line) and cross section element distribution of HX after corrosion in BS molten salt for 100 h

Fig.10 EDS line scanning results (along yellow line) and cross section element distribution of HX after corrosion in BS-Cr3+ molten salt for 100 h

Table 3 Molar Gibbs free energy of formation of metal chloride at 600℃

金属氯化物	$? f G m ?$ /(kJ/mol)
CrCl₂	-273.7
CrCl₃	-339.0
NiCl₂	-169.8
MoCl₂	-153.3
MoCl₃	-245.1
FeCl₂	-225.8
FeCl₃(g)	-237.7

Table 3 Molar Gibbs free energy of formation of metal chloride at 600℃

金属氯化物	$? f G m ?$ /(kJ/mol)
CrCl₂	-273.7
CrCl₃	-339.0
NiCl₂	-169.8
MoCl₂	-153.3
MoCl₃	-245.1
FeCl₂	-225.8
FeCl₃(g)	-237.7

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