Effects of Nickel on the Stress Corrosion Susceptibility of High Chromium Ferritic Stainless Steels in a Hot Chloride Solution

Abstract

Effects of nickel (Ni) on the stress corrosion susceptibility of high-chromium (Cr) ferritic stainless steels (SS) in boiling solution of 42% lithium chloride (LiCl) plus thiourea (NH2CSNH2) were investigated using a uniaxial tension-constant load tester. The influence of Ni on film breakdown potential, repassivation rate, creep behavior, and susceptibility to hydrogen (H)-induced failure of the alloys was evaluated. Commercial alloys of 29% Cr-4% molybdenum (Mo) (UNS S44700) and 29% Cr-4% Mo-2% Ni (UNS S44800) ferritic SS were compared. The 29% Cr-4% Mo alloy was immune to stress corrosion cracking (SCC) at open-circuit potential (OCP) but became susceptible when polarized anodically to critical cracking potential (E(cc), -450 mV(sce)) which was 20 mV(sce) more noble to the stabilized corrosion potential (E(corr) -470 mV(sce)). Addition of 2% Ni to the 26% Cr-4% Mo alloy did not affect E(cc), but did induce susceptibility to cracking by increasing E(corr) to a value 80 mV(sce) noble to E(cc). The 2% Ni addition to 29% Cr-4% Mo alloy reduced film breakdown potential (E(b)) and the repassivation rate and significantly increased susceptibility to H embrittlement (HE). These factors contributed to greater susceptibility to SCC of 29% Cr-4% Mo-2% Ni alloy compared to the 29% Cr-4% Mo alloy. Similarities in fracture morphology between samples that failed by SCC in a hot Cl- solution and those that failed under load during H charging in 1 N sulfuric acid (H2SO4) plus sodium arsenite (NaAsO2) solution suggested HE was involved in the propagation mechanism of SCC in the ferritic SS studied.