Investigation of the photocathodic protection of a transparent ZnO coating on an AISI type 304 stainless steel in a 3% NaCl solution

The coating of a suitable n-type oxide semiconductor on a metal substrate can provide a photocathodic protection for which the coating protects the metal as a non-sacrificial photoanode under light illumination. In this work, the photocathodic protective characteristics of a transparent ZnO coating on an AISI type 304 stainless steel (SS304), in a 3% NaCl solution, was investigated. The ZnO film was coated on a SS304 substrate by spray pyrolysis using a precursor solution of zinc acetate dihydrate and methanol. The substrate temperatures were varied at 200 and 400 °C during the coating processes. The ZnO coating at 200 °C (200ZnO) exhibited dense nano-powdery films with non-crystalline structures, while the ZnO coating at 400 °C (400ZnO) showed loosely-packed needle-like nano-granular deposits with polycrystalline structures. The photocathodic characteristics of the coatings were analyzed by the potentiodynamic linear polarization and open circuit potential (OCP) measurements under dark and UV illumination. Both 200ZnO and 400ZnO coatings exhibited photocathodic characteristics with lower corrosion potentials and higher current densities under UV illumination. In the anodic polarization region, 200ZnO coatings exhibited passivity similar to bare SS304. Yet, the microstructure of 200ZnO coatings after the polarization test showed traces of partial dissolution of ZnO. 400ZnO provided no passivity and poor corrosion resistance as the coating completely dissolved and the exposed SS304 was uniformly corroded under both dark and UV conditions.