Photocathodic protection of amorphous and nanorod zinc oxide thin-film coatings on stainless steel AISI 304 fabricated by spray pyrolysis and hydrothermal technique

Abstract

The photoelectrochemical cathodic characteristics of amorphous and nanorod ZnO thin-film coatings on stainless steel AISI 304 (SS304) substrates were investigated. An amorphous ZnO thin film was coated on a SS304 substrate at 200 °C using the spray pyrolysis technique. A nanorod ZnO thin film was coated on a SS304 substrate by spraying seed layers at 400 °C and growing the nanorod structures with a hydrothermal process. The results from x-ray diffractometry and energy dispersive x-ray spectroscopy confirmed amorphous and nanorod ZnO coatings. The ZnO-coated SS304 samples were subjected to the potentiostatic polarization testing under dark and UV conditions, with the ZnO-coated sides of SS304 immersed in a 0.1 M Na2S + 0.2 M NaOH solution and the opposite bare sides in a 3 wt% NaCl solution. For both amorphous and nanorod ZnO coatings, the current densities (icorr) were increased while the voltages (Vcorr) were decreased with the UV light exposure. The photoelecrochemical cathodic mechanism has been explained with the photogenerated electron-hole pairs in ZnO, the hole consumption by the S/S−2 redox reaction, and the electron transfer to SS304. The microstructure analyses showed that the pitted areas resulting from the polarization test under UV light were clearly smaller than those from the polarization test under dark condition.