Effect of the potential on the film composition and the stress corrosion cracking of mild steel in ammonium nitrate solutions
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2018-01-01
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Abstract
The relation between the film composition and the susceptibility of mild steel (0.17% C) to stress corrosion cracking (SCC) in ammonium nitrate solutions at different potentials has been studied. The susceptibility to SCC is evaluated by constant slow strain rate tests under potentiostatic conditions (-0.5 V - 1.5 V, SCE). The composition of the surface film is identified by Mossbauer spectroscopy. It is found that in the region of the highest susceptibility to SCC (0 V - 0.8 V, SCE) the film is composed mainly of fine particles of FeOOH as well as some quantity of γ-Fe2O3. However at potentials outside this region, where the resistance of the steel to SCC increases considerably, the film is composed exclusively by oxides with a spinel structure Fe3O4 and γ-Fe2O3. It is suggested that the conditions which favour the formation of a spinel structure film may lead to an increased resistance of the steel to SCC.