Mechanism of Stress Corrosion Crack Initiation of Cu-Containing 18Cr-8Ni Stainless Steel in Sulfuric Acid Solution

Abstract

Crack initiation mechanism of 18Cr-8Ni stainless steel containing Cu in H2SO4 solution was studied in terms of activation energy for the initiation. The initiation was detected by photo-microscopical observation of specimen surface tested in the temperature range 50 to 90°C at stresses from 75 to 300 MPa. The apparent activation energy was either about 17 or 86 kJ/mol depending on the stress level and the temperature. The discussion lead to the conclusion that the initiation stage is a series of a mechanical slip process of the crystal planes and a dissolution process of resultant emergent steps.KEY WORDS: activation energy, initiation mechanism, stainless steel, stress corrosion cracking, sulfuric acid solution boundaries (A); (4) corrosion grooves parallel to wire axis (m);

Effect of corrosion product layer on SCC susceptibility of copper containing type 304 stainless steel in 1 M H2SO4

“…In the crack initiation stage, the surface corrosion product, being porous, could expose the base metal at the various sites of interiors of grains, twin boundaries, grain boundaries, edges of corrosion grooves parallel to stress, small corrosion-patches, or micropits under stress [7], which act as anodic sites for crack initiation. The base metal exposed under stress acts as an anode and the surface film as the cathode leading to further crack growth, as mentioned earlier.…”

“…The environment cell is encased in a furnace for testing at temperatures above room temperature. The details of the experimental set-up can be found elsewhere [7]. The SCC tests were conducted at an applied initial stress of 216 MPa, which is 1.2 times the proof stress.…”

“…The surface corrosion product formed during testing showed the presence of Cu [3,7], which was suggested to be protective against general corrosion and also a prerequisite for the SCC initiation and/or propagation. This was supported by the SCC resistance exhibited by the Cu containing alloys, when H 2 S gas was led into the SCC system to hinder the deposition of metallic Cu into the surface film by forming the sulfides of Cu which otherwise was susceptible to SCC [4] though the SCC system might be perturbed by the H 2 S gas introduced [9].…”

“…Earlier studies on Cu additions in the Cr-Ni-Mo austenitic stainless steels [3][4][5] and the type 304 stainless steels [6][7][8] indicate the susceptibility of both the alloys to SCC in hot H 2 SO 4 environments without chlorides at open circuit potential (OCP) and potentials close to it. The surface corrosion product formed during testing showed the presence of Cu [3,7], which was suggested to be protective against general corrosion and also a prerequisite for the SCC initiation and/or propagation.…”