Effect of Nickel on Stress Corrosion Cracking of 23% Cr Stainless Steels

Abstract

Effect of nickel content on SCC susceptibility of 23% Cr stainless steels in boiling 42% MgCl₂ solution was examined by SSRT technique. Corrosion behavior was studied by performing electrochemical potential measurement, XPS analysis of surface films and the quantitative analysis of the elements dissolved during the exposure tests. Susceptiblity to TGSCC increased significantly with increasing nickel contents within the single α phase region up to 3% Ni. Such tendency agreed well with that of the formation of passive film. The principal role of nickel on the corrosion process is believed to be the enhancement of the formation of passive film enriched chromium. Nickel was found to have accumulated in the interface between the passive film and the undering alloy. The similar role of nickel was observed in results tested in solution dissolved Ni²⁺ and Cr⁶⁺ ions. Change in the properties of passive film rather than that in mechanisms of plastic deformation of alloys with increasing nickel content is considered to be a dominant factor in the TGSCC of ferritic stainless steels. With Ni contents above 4%, the increase of resistance to SCC by forming duplex phase is interpreted as the results of the selective deformation of austenite and crack arresting due to enhanced attack in the ferrite phase assisted by the cathodic action of austenite phase.