Self-organized honeycomb-like nanoporous oxide layer for corrosion protection of type 304 stainless steel in an artificial seawater medium

Abstract A new passivation technology for growing a self-organized nanoporous oxide layer with a honeycomb morphology on Type 304 stainless steel is proposed. FESEM, cross-sectional EDX and XRD are employed to evaluate the morphology, composition and crystal structure of the developed anodic film. Cross-sectional EDX analysis reveals that the nanoporous oxide layer contains a significant amount of fluoride species, which becomes negligible after successive annealing in air at 773 K for 2 h. XRD indicates that the amorphous nanoporous oxide layer is converted to a crystalline magnetite phase (Fe3O4) after annealing. Electrochemical analysis such as potentiodynamic polarization and electrochemical impedance spectroscopy reveals stainless steel with a thermally treated nanoporous oxide layer has improved corrosion resistance in an artificial seawater medium. FESEM analyses are employed to evaluate the stability and microscopic changes in the morphologies of the nanoporous oxide layer after prolonged immersion in the artificial seawater medium. The key factors and corrosion inhibition mechanism by the nanoporous oxide layer are also discussed.