Corrosion Study of Base Material and Welds of a Ni-Cr-Mo-W Alloy

doi:10.1007/s40195-017-0559-6

Abstract

Abstract:

Alloys containing chromium (Cr) and molybdenum (Mo), as the major alloying elements, are widely used in various industries where the material experiences corrosive environments. Chromium (Cr), when added in an optimum amount, forms a Cr₂O₃ passive film which protects the underlying metal in aggressive solutions. Molybdenum (Mo) forms its oxides in the low pH solutions, thus, enhances the uniform corrosion resistance of an alloy in reducing acids and assists in inhibition to localized corrosion. Minor alloying elements, like tungsten (W) and copper (Cu), also improve the overall corrosion resistance of an alloy in specific solutions. In the present study, corrosion resistance behavior of commercial iron-based alloys (316L SS, 254 SMO and 20Cb3) and nickel-based alloys (Monel 400, Alloy 625 and C-276) was studied in the acidic solutions. While the corrosion behavior of wrought alloys has been widely studied, there is little to no information on the corrosion performance of their welds, typically being the weak regions for corrosion initiation and propagation. Therefore, an attempt was undertaken to investigate the uniform and localized corrosion performance of base metal, simulated heat-affected zone and all-weld-metal samples of a Ni-Cr-Mo-W alloy, C-276. The study was conducted in aggressive acidic solutions. Various corrosion and surface analytical techniques were utilized to analyze the results.

Key words: Iron-chromium-molybdenum, Nickel-chromium-molybdenum-tungsten, All-weld-metal, Heat-affected-zone, Acids Acidified ferric chloride, Potentiodynamic, Scanning electron microscopy (SEM)

Ajit Mishra. Corrosion Study of Base Material and Welds of a Ni-Cr-Mo-W Alloy[J]. Acta Metallurgica Sinica (English Letters), 2017, 30(4): 326-332.

Figures/Tables 12

Table 1 Chemical composition of the corrosion-resistant alloys (wt% of major alloying elements)

Alloys (UNS number)	Cr	Mo	W	Cu	Ni	Fe
316L SS (S31603)	17	2.5	-	-	12	Bal.
254 SMO (S31254)	20	6.1	-	0.7	18	Bal.
20Cb3 (N08020)	20	2.5		3.5	35	Bal.
Monel 400 (N04400)	-	-	-	31	Bal.	1.5
Alloy 625 (N06625)	21	9	-	-	Bal.	4.0
C-276 (N10276)	16	16	4	-	Bal.	5.0

Fig. 1 Corrosion rate of Fe- and Ni-based alloys in reagent-grade HCl solutions at an ambient temperature

Fig. 2 Corrosion rate of Fe- and Ni-based alloys in reagent-grade 5% HCl solution at different temperatures

Table 2 Corrosion rate (in mm/y) of corrosion-resistant alloys in reagent-grade HCl

Solution	Temperature °F (°C)	316L SS	254 SMO	20Cb3	Monel 400	Alloy 625	C-276
5% HCl	75 (23)	0.50	0.24	0.26	0.12	0.001	0.002
	100 (38)	2.07	0.26	0.44	0.29	0.003	0.003
	125 (52)	11.66	2.01	1.23	0.49	0.03	0.02
	150 (66)	23.20	9.17	2.14	0.75	2.05	0.31
10% HCl		1.75	0.43	0.30	0.18	0.15	0.05
15% HCl	75 (23)	4.14	0.43	0.26	0.30	0.19	0.07
20% HCl		6.48	0.27	0.25	0.61	0.08	0.04

Fig. 3 Corrosion rate of Fe- and Ni-based alloys in reagent-grade H2SO4 solutions at 66 °C (150 °F)

Fig. 4 Microstructure of mill-annealed C-276 alloy

Fig. 5 Microstructure of simulated HAZ C-276 alloy

Fig. 6 Microstructure of AWM C-276 alloy

Fig. 7 Corrosion rate of BM, HAZ and AWM C-276 alloy in HCl solutions at 38 and 66 °C

Table 3 Corrosion rate and critical pitting temperature (CPT) of C-276 alloy using ASTM G-48 technique

Alloy	Temp. (°C)	BM (mm/y)	HAZ (mm/y)	AWM (mm/y)
C-276	100	0.10	0.12	0.33
C-276	110	0.13	0.12	0.38 (pitted)
Critical pitting temperature (CPT)		>120 °C	>120 °C	110 °C

Fig. 8 Potentiodynamic polarization graph of base metal (BM) and all-weld-metal (AWM) specimens of C-276 alloy in 20% HCl at room temperature (RT) and 38 °C

Fig. 9 SEM image of the simulated HAZ microstructure of C-276 alloy. The composition of grain and secondary phases, obtained using EDX, is shown in the image

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