Influence of Surface Oxide Characteristics and Speciation on Corrosion, Electrochemical Properties and Metal Release of Atomized 316L Stainless Steel Powders
Journal article, 2012
Surface oxide characteristics of powder particles are important to consider for any toxicological risk assessment based on in-vitro or in-vivo tests. This study focuses on a multi-analytical approach (X-ray photoelectron spectroscopy, Auger electron spectroscopy, scanning-and transmission electron microscopy, and different electrochemical techniques) for in-depth characterization of surface oxides of inert-gas-atomized (GA) AISI 316L stainless steel powder, compared with massive sheet and a water-atomized (WA) 316L powder. Implications of differences in surface oxide phases and their surface distribution on corrosion, electrochemical properties and metal release are systematically discussed. Cr was enriched in an inner surface layer for both GA powders, with Mn and S enriched in the outermost surface oxide. The surface oxide was 2-5 nm thick for both GA powder size fractions, amorphous for the GA powder sized <4 mu m and partially crystalline for the powder sized <45 mu m. A strong ennoblement, i.e. positive shift in open circuit potential, of up to 800 mV, depending on solution, was observed for the GA powders. This ennoblement was induced by catalytic oxygen reduction properties of tri-or tetravalent Mn-oxides, not present on the massive sheet or WA powder. In contrast to the predominant presence of a trivalent Cr-oxide in the surface oxide of the GA powder particles, the WA<45 mu m powder revealed oxidized Cr, most probably present in its hexavalent state (not chromate), within a silicate-rich surface oxide. This study clearly shows that the surface oxide composition and speciation of differently sized GA and WA powders are unique (strongly connected to the atomization process) and of large importance for their pitting corrosion and metal release properties. For the GA<45 mu m powder, Mn-rich oxide nanoparticles were proposed to account for its higher pitting corrosion susceptibility, a more stable surface ennoblement, and a shift of the MnO2 oxidation/reduction peaks in the cyclic voltammogram, compared with the GA particles sized <4 mu m. The thermodynamically unstable ferritic structure of the small sized particle fraction (GA <4 mu m), despite an austenitic composition, revealed a higher pitting corrosion susceptibility and higher nickel release compared with the austenitic particle fraction of the GA <45 mu m powder.