Effect of metallurgical factors on the pitting corrosion behavior of super austenitic stainless steel weld in an acidic
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Seung Gab Hong POSCO Steel Solution Marketing Division, Geidong-Dong, Pohang 790-704, Republic of Korea
Min-suk Ohb) Automotive Components & Materials R&D Group, Korea Institute of Industrial Technology, Gwanju 500-480, Republic of Korea (Received 3 January 2017; accepted 6 February 2017)
This study examined the effects of a variety of metallurgical factors on the electrochemical corrosion behavior of superaustenitic stainless steel welds. First, the effects of the sigma (r)-phase on the corrosion behavior were studied by means of a three-dimensional-atom probe. Cr and Mo depletion areas formed around the r-phases which are precipitated in the interdendritic area were clearly observed. Second, the effects of oxide inclusion on the pitting corrosion of the steel welds were analyzed. The utilization of high resolution transmission electron microscope clearly demonstrated that the thickness and Cr content of the passive film formed on the steel surface decreased significantly with decreasing distance to the oxide inclusion, resulting in a deterioration of the corrosion resistance. Third, the effects of alloying elements, Cu and Al, were evaluated using an electrochemical polarization technique. This confirmed that Cu has a detrimental effect on the resistance to localized corrosion of the steel. The addition of Al up to 0.25 wt% had no significant effects on corrosion resistance in a chloride environment despite the presence of an Al-based oxide layer (Al2O3) on the outermost surface.
I. INTRODUCTION
Superaustenitic stainless steel exhibits superior resistance to localized and stress corrosion as well as outstanding mechanical properties.1,2 In particular, its superior corrosion resistance to localized corrosion has been attributed to the high concentration of Cr, Mo, and N. As its pitting resistance equivalent number (PREN 5 % Cr 1 3.3% Mo 1 30% N) is approximately 50,1 it has been regarded a good alternative to Ti alloys. Nevertheless, its superior corrosion resistance can be aggravated when the steel is welded by autogenous gas tungsten arc welding (GTAW).1 Generally, the welding of stainless steels essentially leads to compositional heterogeneity,3–5 resulting in the precipitation of various intermetallic phases.6–8 Consequently, a steel weld can be more susceptible to localized corrosion. The first aim of this study was to clarify the corrosion initiation site in the weld, employing the electrochemical polarization technique and three-dimensional-atom probe (3DAP) analysis. Contributing Editor: Jürgen Eckert Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2017.65
The high corrosion resistance of austenitic stainless steel has been attributed to the formation of a passive film acting as a protective barrier against a corrosive environment, including Cl ions on the steel surface.9,10 Such a passive film, however, is often susceptible to pitting or crevice corrosion by the local breakdown of passivity at some chemical or physical heterogenei
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