Caustic stress corrosion cracking of a spheroidal graphite cast iron: characterisation of ex-service component

Abstract

This paper discusses observations of features suggesting grain boundary embrittlement ahead of stress corrosion crack tips in ex-service cast iron components exposed to strong caustic environments during Bayers process for alumina processing. The cracks and the neighbouring areas in the ex-service specimens were examined using conventional metallography, SEM, the extraction replica technique in the TEM and Auger electron spectroscopy. In all cases, the cracking was initiated at the surface of the steel exposed to the corrosive environment and it appeared that crack nucleation may have been aided by local stress concentrations and/or zones of local residual stress concentration. The fracture mode was almost exclusively intergranular and the crack path followed ferrite grain boundaries. There was clear evidence of a local zone of grain boundary embrittlement extending ahead of the tip of the major cracks examined. The phenomenon was established by investigating the fresh fracture surfaces produced by extending pre-existing cracks under impact loading at liquid nitrogen temperature. Auger electron spectroscopy of the fresh intergranular fracture surface failed to reveal any evidence of local elemental grain boundary segregation that might account for the observed embrittlement. In the absence of evidence of any other embrittling species on the exposed intergranular facets, there arises the possibility of hydrogen being involved in the embrittlement. The paper discusses hydrogen assisted intergranular cracking, as observed in the case of similar materials, to be the possible mechanism.