Stress corrosion cracking of CMn steels in methanol-ammonia environments—II. Electrochemical and fractographic studies

Abstract

The mechanism of stress-corrosion cracking of CMn steel in anhydrous methanol-ammonia solutions has been investigated using detailed fractography and electrochemical measurements. SEM and TEM examination of the stress-corrosion fractures in a large-grained material show that the opposing transgranular fracture surfaces are generally matching and interlocking; however, this was not invariably the case with certain arrays of striations. One type of striation, spaced by about 2 μm, matches peak-to-peak on opposing surfaces and is shown by periodic overloading (load pulsing) to correspond to successive positions of the crack front. Load pulsing at increasing frequency caused an increase in the overall crack velocity, and a shortening of the crack jump distance. However, no loss of correspondence between the numbers of load pulses and crack front markings was found. There is thus very strong, but not entirely conclusive, evidence that the crack advance is discontinuous. Electrochemical measurements on rapidly strained electrodes and on microelectrodes inside artificial crevices show that the highest reasonably sustained anodic current density is about 200 A m −2, which is roughly one order of magnitude too low to account for the observed crack growth rate by an anodic process. Passivation of iron microelectrodes by ammonia was shown to be necessary for stress-corrosion cracking and to coincide with the oxidation of ammonia on platinum, indicating that the passive film is probably an iron nitride. The various observations described above, together with the enhancement of stress-corrosion cracking by the addition of nitrogen to the electrolyte, are believed to provide strong support for a film-induced cleavage mechanism of cracking, wherein a thin, nitrogen-hardened layer forms preferentially (due to a mass action effect) in the presence of adsorbed molecular nitrogen and subsequently triggers about 1 μm of cleavage.