Some mechanistic aspects of the environmental sensitive cracking of an 18Ni maraging steel

PhD Thesisstress assisted cracking of high strength steels in aqueous environments is regarded by many to occur by a hydrogen embrittlement mechanism no matter to what potential the bulk surface of the material is electrochemically polarised. Some data is presented in this work for a-high strength 18Ni maraging steel in aqueous chloride environments and interpreted in terms. of a dual mechanism of cracking according to the applied potential relative to the free corrosion potential. Several experimental techniques were employed; including the use of smooth and pre-cracked stress corrosion specimens, to assess the cracking mechanisms operative under given conditions with mixed success. The electrochemical hydrogen permeation technique whilst found to be useful for the determination of hydrogen diffusivities during cathodic charging was found to. be. unsuitable for a mechanistic experimental approach based on the observation of hydrogen permeating through the steel during anodic polarisation. However, the detection of acoustic emissions accompanying environmental cracking of pre-cracked specimens of the steel and the influence on the stress corrosion cracking susceptibility of chemical poisons or catalysts of the hydrogen evolution reaction suggested that a dual cracking mechanism was operative