Passivity breakdown of high purity iron and AISI 4340 steel in 0.5M NaCl solution

The anodic and cathodic electrochemical behaviour of high purity iron and of AISI 4340 high strength steel in unbuffered 0·5M NaCl solution has been studied. The results were correlated with the SCC failure mechanism of the steel. The pitting potentials of both high purity iron and AISI 4340 steel are coincident. Heat treatment affected the pitting potential of the steel very little. Failure of AISI 4340 steel under both anodic and cathodic polarization was found. SCC under anodic polarization was closely related to the pitting potential. Conditions at the metal-solution interface for pit maintenance are given.