Stress-corrosion cracking of CMn steels in methanol-ammonia environments—I. Effects of environmental and mechanical variables

Abstract

Stress-corrosion cracking of a low-strength CMn steel in anhydrous methanol-ammonia solutions has been studied by carrying out slow strain rate tests and electrochemical measurements. Cracking occurs at the corrosion potential only in the presence of both oxygen and carbon dioxide; under anodic polarization oxygen is not required. Addition of 3000 ppm water prevents cracking. Nitrogen contamination increases the severity of cracking. Stress-corrosion in this system is associated with the formation of a protective film which is not an oxide and may be an iron nitride. Cracks initiate under static load conditions and as soon as yielding occurs in dynamic tests. The cracking is transgranular and cleavage-like. Crack propagation is associated with an anodic process and probably involves microcleavage; the observed crack growth rates cannot be sustained by a slip-dissolution process. Hydrogen embrittlement can be ruled out as the cracking mechanism. The observed stress-corrosion is essentially identical to cracking of CMn steels in anhydrous liquid ammonia.