Control of the Surface Charge and Improved Corrosion Resistance of Stainless Steel by the Combined Use of Gaseous Ozone and Heat

Abstract

The surface charge of 316 L stainless steel particles treated at various temperatures under oxygen or gaseous ozone of 0.2 to 15% (vol/vol) was determined as a function of pH by potentiometric titration. The apparent points of zero charge (pzc's app ) of ozone-treated stainless steel particles were situated at points corresponding to more acidic pH values compared with the pzc's app of oxygen-treated particles. The degree of the shift in the pzc app of stainless steel particles depended on the ozone concentration and temperature. The combined ozone/heat treatment was found to induce the surface dehydroxylation of stainless steel particles. Cr 6+ species were generated in the passive film on stainless steel particles through ozone oxidation. The generated Cr 6+ species were readily dissolved into aqueous solution, whereas the amount of dissolved Cr 6+ species decreased markedly by heating again at 200°C under atmospheric pressure. It was demonstrated that the situation of pzc app and the surface charge density of stainless steel particles could be controlled, to some extent, by the combined ozone/heat treatment followed by post-ozone heat treatment. Controlling the surface charge of stainless steel particles resulted in a lower affinity for pectin adsorption. It was also shown that the combined ozone/heat treatment resulted in the formation of relatively thick and dense passive film on stainless steel particles, thereby giving a higher resistance to surface dissolution in aqueous solution than oxygen-treated particles.