Effect of Heat Treatment and Carbon on Stress Corrosion Cracking of Iron and Mild Steel in Sodium Phosphate Solution

Stress corrosion cracking of ARMCO iron and Fe-C materials with up to 0.1 Wt% C was examined at 25 C in 1 M NaH₂PO₄ of pH 4.0 at a potential of −0.1 V saturated calomel electrode (SCE) in constant strain rate tests. Heat treatment of the materials involved annealing, quenching from 800 C, and aging at 150 C. Effect of carbon on anodic dissolution in the absence of stress was examined on Fe-C materials containing up to 1.3% C, quenched from 950 C. It was found that small amounts of carbon promoted intergranular cracking, whereas large amounts facilitated transgranular cracking, but hindered the formation of intergranular cracks. Scanning electron microscope examinations suggested an occasional formation of corrosion tunnels along crack planes. Carbon decreased anodic dissolution of iron at potentials in the active region, but it enhanced iron dissolution in the passive region. It was suggested that both intergranular and transgranular cracking were largely associated with the deterioration of a passivating film by carbon.