Grain-Boundary Chemistry Effects on the Intergranular Stress Corrosion of Iron Alloys in Sulfate and Nitrate Solutions

Abstract

Abstract Intergranular stress corrosion measurements have been made on Fe alloys in 1 M sodium sulfate at electrochemical potentials of +750 to +1250 +1250 mVSCE and in 55% calcium nitrate at 6CPC and +750 +750 mVSCE. Tests were conducted on Fe with variable grain-boundary S and P concentrations ranging from 0.04 to 0.23 monolayers of S and 0 to 0.31 monolayers of P. Compact tension samples were used to measure the subcritical crack growth behavior. The calcium nitrate solutions were considerably more effective in causing crack growth of Fe than was the sodium sulfate solution. Crack growth thresholds, KISCC, of 6 MPam and 3 MPam were observed for Fe with 0.23 monolayers of S and 0.31 monolayers of P, respectively, when tested in calcium nitrate and 13 MPam when tested in sodium sulfate. A clear correlation was observed between the total S+P grain-boundary concentration and KISCC and crack velocity. The crack growth rates were about a factor of 10 faster in nitrate than sulfate. In general, the crack grow...