Qualitative Mechanism of Stress Corrosion Cracking Of Austenitic Stainless Steels

Test results support the electrochemical mechanism of stress corrosion crack initiation in austenitic stainless steel. After a period of initiation, during which partial passivation occurred, local anodes corroded along certain narrow planes, resulting in cracking and ultimate failure. Crack lengths gradually increased with time, after the onset of stress corrosion crack propagation, and no cracking took place during the initiation stage. Unlike a brittle fracture, which propagates very rapidly, the cracks extended slowly and gradually through austenitic stainless steels and a magnesium-base alloy, without any steps of instantaneous advance. Crack propagation in mild steel, placed in hot nitrate solutions, took place in alternating steps of rapid and slow penetration. The rapid steps were not instantaneous. The stress corrosion cracking of aluminum-4 percent copper and aluminum-7 percent magnesium alloys, in sodium chloridesodium bicarbonate solutions, consisted of alternating stages of electrochemical corrosion and sudden physical failure. Study methods used include the automatic recording of electrochemical potential change and extension of specimens, and the acoustic recording of sound generated during cracking. 3.5.8