Wettability Recovery Behavior Governed by Desorption of Hydroxyl Species in Steel

The radiation induced surface activation (RISA) effect which occurs at the bilayer of metal and oxide due to irradiation contributes to improving the materials surface wettability and enhancing electrothermal characteristics and corrosion resistance. The purpose of the present study is to reveal the correlation between the wettability recovery behavior and the microstructural features of the adsorbed water and/or hydroxyl species on the surfaces. The wettability change due to the RISA effect was investigated in the oxidized austenitic stainless steel with various holding times under darkness after UV irradiation. The microstructure features of the adsorbed water and/or hydroxyl species on the surfaces were characterized by Raman spectroscopy and scanning probe microscopy (SPM). Results show that the contact angle of the specimen after UV irradiation parabolically increased with increasing holding time, regardless of the chemical composition within these two steels. The absorbed water layer was observed in both specimens by Raman analysis, and its intensity decreased with increasing holding time. From the force-distance curves of SPM analysis, the hydrophilic component was rapidly decreased at a holding time below 200 h, then gradually saturated when the holding time increased above 200 h from the SPM analysis. These results reveal that the wettability recovery behavior in oxidized austenitic stainless steels after UV irradiation can be ascribed to two kinds of mechanisms. One is the desorption of the hydrophilic components which are predominant when the contact angle is less than 30°, and the other is the absorption of the hydrophobic components which works mainly when the contact angle is above 30°.