The pitting corrosion resistance of surface-modified 316L austenitic stainless steel and N08367 (a ''superaustenitic'' stainless steel) were evaluated in 0.6 M NaCl solutions and compared to untreated samples of the same materials. The surface modification process used to treat the surfaces was a low-temperature carburization technology termed ''low-temperature colossal supersaturation'' (LTCSS). The process typically produces surface carbon concentrations of~15 at. pct without the formation of carbides. The pitting potential of the LTCSS-treated 316L stainless steel in the NaCl solution substantially increased compared to untreated 316L stainless steel, while the pitting behavior of the LTCSS-treated N08367 was unchanged compared to the untreated alloy.
The uptake of chloride ͑Cl − ͒ by the passive oxide film on 316L austenitic stainless steel polarized at potentials below ͑less positive than͒ the pitting potential in 0.6 M NaCl solutions was studied using X-ray photoelectron spectroscopy ͑XPS͒ and compared to our previous work on aluminum. The XPS spectra for the stainless steel showed that chloride was not adsorbed or present in the passive oxide film. This is quite different from the case of aluminum, where XPS data for the presence of chloride on the surface and incorporated into the oxide film are unambiguous. These differences in the adsorption and incorporation of chloride for these two metals will be discussed in the context of the mechanisms of passive film breakdown.
Case-hardening'' of the Ni-base superalloy IN718 has been achieved by low-temperature gas-phase carburization. After carburization under optimum conditions, the hardened surface layer (the ''case'') has about twice the hardness of the core (HV of %800) and contains %12 at pct carbon in interstitial solid solution. This causes a lattice parameter expansion of %1 pct perpendicular to the surface and, because of the mechanical constraint provided by the noncarburized core below, develops a large biaxial surface compressive residual stress (%1.9 GPa) parallel to the surface. Microstructural studies and X-ray diffractometry reveal no carbide precipitates in the case. In agreement with this observation, low-temperature carburization does not compromise the ductility and actually improves the crevice corrosion resistance of the alloy.
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