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

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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’sapp) of ozone-treated stainless steel particles were situated at points corresponding to more acidic pH values compared with the pzc’sapp of oxygen-treated particles. The degree of the shift in the pzcapp 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. Cr6+ species were generated in the passive film on stainless steel particles through ozone oxidation. The generated Cr6+ species were readily dissolved into aqueous solution, whereas the amount of dissolved Cr6+ species decreased markedly by heating again at 200°C under atmospheric pressure. It was demonstrated that the situation of pzcapp 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.

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