Electrochemical responses to degradation of the surface layer nano-mechanical properties of stainless steels under cavitation

Nano-indentation technology was used to measure quantitatively the surface layer nano-mechanical properties (the nano-hardness, Hnano, and the nano-elastic modulus, Enano) for stainless steels before and after cavitation corrosion testing. The nano-mechanical properties of the surface layers were comprehensively defined as (H/E)nano, which is a dimensionless function. The relationship of the cumulative mass loss to the surface layer nano-mechanical property (H/E)nano and the corresponding AFM images of the corroded surfaces were studied. The electrochemical responses to the degradation of the surface layer nano-mechanical properties of stainless steels without cavitation and under cavitation were also investigated. It was found that degradation of the surface layer nano-mechanical properties had a significant influence on the cavitation corrosion of stainless steels and that there was an (H/E)nano threshold, above which the cumulative mass loss decreased slowly. This threshold was found to be independent of the stainless steel type. The electrochemical corrosion processes were varied, resulting in accelerated electrochemical corrosion as a result of degradation of the surface layer nano-mechanical properties. A significant peak in the low frequency regime was observed in the Bode figure. When degradation of the surface layer nano-mechanical properties was below the (H/E)nano threshold, the phase angle in the Bode figure was approximately constant. The presence of the specific peak in the Bode figure was used as a marker for the occurrence of significant cavitation corrosion as a result of degradation in the surface layer nano-mechanical properties.