SURFACE PASSIVATION OF NI/CR ALLOY AT ROOM-TEMPERATURE

Room temperature oxidation of Ni-23at%Cr alloy has been studied. Chromium is preferentially oxidized on Ni/Cr alloy surface after 1 to 2 L of oxygen exposure. Reaction of Ni with oxygen occurs more slowly than the Cr/O2 reaction, but much more rapidly than for pure Ni. Chromium oxide (Cr2O3) is formed on both oxidized pure Cr and Ni/Cr alloys. It is found that substantial amount of hydroxyl species exists on the surfaces of oxidized pure Ni and Ni/Cr alloy. The angle-resolved Ni(2p) and O(1s) spectra suggest that NiO and adsorbed hydroxyl exist on the pure Ni surface, while Ni hydroxide mainly exists on the Ni/Cr alloy surface. The oxide on the Ni/Cr alloy has a bilayer structure consisting of a mixture of Ni hydroxide and Cr oxide at the outer monolayer, and a mixture of Cr oxide, small amounts of NiO and possibly some metallic Ni over the next four monolayers. In addition, a Cr depleted metallic layer exists below the oxide layer. As compared to the three monolayers NiO formed on pure Ni, the formation of Ni hydroxide is limited to the first monolayer of oxide layer on Ni/Cr, demonstrating the increased passivation ability of Ni upon adding Cr. © 1990 Elsevier Science Publishers B.V. (North-Holland).