Effect of Carbon and Oxygen in Iron on Stress Corrosion Cracking in Nitrate Solution

Abstract

It was established previously that nitrogen or carbon in iron is necessary for susceptibility to stress corrosion cracking in boiling aqueous 60% . Present measurements show that oxygen in Fe at a concentration as high as 0.07% does not induce similar susceptibility. Pure iron‐carbon alloys containing 0.001% N or less are susceptible as quenched from 925°C if they contain more than about 0.002% C. Below this carbon content they are resistant. However, resistant alloys become susceptible when heated subsequently for 3 hr at 550°C, including zone‐refined iron containing <0.001% C and <0.0003% N. Only “floating” zone‐refined iron of still higher purity is immune as quenched or when heated at 550°C. These results are interpreted in terms of crack‐sensitive paths consisting of interstitial atoms associated with imperfections of the grain boundaries. Quenched‐in imperfections within grains transport interstitial atoms to grain boundaries which induce susceptibility of an otherwise resistant iron.