Stress corrosion cracking of high-purity carbon steel in carbonate solutions

Slow strain rate tests (SSRT) were performed on a high-purity 0.02% C steel in ammonium carbonate ([NH4]2CO3) solutions at 70 degrees C as a function of applied electrochemical potential. As in previous studies, a narrow critical potential range for fatal cracking in the active-to-passive transition of a rapidly developed polarization curve was observed. At potentials slightly active to the critical potential range, transgranular (TG) and intergranular (IG) fissuring were observed. At potentials noble to the critical range, IG fissuring was observed. Fatigue precracking had no effect on the extent or rate of stress corrosion cracking (SCC), nor did hydrogen embrittlement (HE). Results supported previous theories that IGSCC of carbon steels is the result of a very specific interaction between grain-boundary chemistry and the micromechanics of near-boundary interactions.