Formation of self-organized nanoporous anodic films on Type 304 stainless steel

The formation of self-organized nanoporous anodic films on Type 304 stainless steel in fluoride-containing ethylene glycol electrolyte is reported. A key factor in the formation of the nanoporous anodic films is the water concentration in the electrolyte, which must be 0.3 mol dm− 3 or less for the present conditions of film formation, although nanoporous anodic films have been formed on iron at higher water concentrations. The films contain relatively high concentrations of iron, chromium, nickel, oxygen and fluorine species distributed throughout the film thicknesses. The fluoride species can be removed by post-annealing in air, which improves the chemical stability of the films without damaging the nanoporous morphology of the remaining oxide.

► Self-organized nanoporous anodic films were formed on stainless steel.
► The films grew by the reduction of H2O content in the ethylene glycol electrolytes.
► Post-annealing converted water-soluble oxyfluoride to insoluble oxide.