Oxidation behavior of stainless steels 304 and 316 under the Venus atmospheric surface conditions

In general support of planetary exploration missions, the chemical, structural and microstructural behavior of stainless steels 304 and 316 was probed after exposure to a gas mixture with temperature, pressure, and composition mimicking the Venus lower atmosphere. Exposures were carried out in the Glenn Extreme Environments Rig (GEER) chamber with the Venusian gas mixture (96.5% CO2, 3.5% N2, 30 ppm H2O, 150 ppm SO2, 28 ppm CO, 15 ppm OCS, 3 ppm H2S, 0.5 ppm HCl and 5 ppb HF) at 9.2 × 106 Pa and 740 K for a duration of 10, 21, and 42 days. Stainless steel 304 and 316 samples were characterized before and after the experiment by gravimetric analysis, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and cross-section electron microscopy analysis. All samples exposed to the Venusian atmosphere formed double-layered scales in which the outer layer is mainly magnetite (Fe3O4) and the inner layer is mainly spinel (Fe3-xCrxO4). Nickel sulfide and oxide formed at the interface of stainless steel 304. Nickel sulfide also formed in the inner and outer layer of the stainless steel 316 scale. Chromium carbide was only detected by Auger analysis at the interface of the unpolished side of the stainless steel 304 coupon sample. Stainless steel 304 with its smoother surface (780.62 nm total roughness on the polished side and 3.57 μm on the unpolished side) and slightly higher chromium and slightly lower nickel content, exhibited slower kinetics of oxidation (3.1 ± 0.5 × 10−3 mg cm−2 day−1) than stainless steel 316 (0.05 ± 0.07 mg cm−2 day−1) with its rougher surface (4.78 μm total roughness on both sides). The slightly higher molybdenum content in stainless steel 316 was also responsible for increasing its kinetics of oxidation because of the formation of molybdenum sulfide. Our thermodynamic calculations of the phase assemblage containing the Venus gas mixture and the stainless steel samples predicted hematite Cr2O3(s), Fe2O3(s), CrS(l), Cr2S3(l), NiS(l), FeS(l), CrO(l), H2(g), HCl(g), HF(g), NH3(g) and H2S(g) as the thermodynamically stable phases formed at the Venus atmospheric conditions.