Electrochemical Conditions at the Tip of an Advancing Stress Corrosion Crack in AISI 4340 Steel

Stress corrosion cracking in AISI 4340 steel was studied utilizing an Sb/Sb2O3 Ag/AgCl probe which simultaneously measures the crack tip electropotential and the electrolyte pH of the region. Additional confirmatory measurements were made with a glass electrode and Ag/AgCl reference electrode. Specimens heat treated to a yield strength of 200 ksi were cracked under varying conditions of bulk solution pH and impressed electrochemical potential. The pH and electrochemical potential measured at an active stress corrosion crack are such that water is thermodynamically unstable at the crack tip and the conditions are favorable for hydrogen to be liberated at the crack tip. A proposed mechanism depicts the cracking process as one of degradation of the metal by hydrogen regardless of external potential or pH of the bulk corrodent.