Austenitic stainless steels passivity oxidation states

As discussed in Section 19.2.4, stainless steels are best employed under fully aerated or oxidizing conditions, which favor the passive state. Whether used in handling chemicals or exposed to the atmosphere, the alloy surface should always be kept clean and free of surface contamination. Otherwise, crevice corrosion may cause pitting and localized rusting. Austenitic stainless steels cooled too slowly through the sensitizing temperature zones tend to rust in the atmosphere. 

Passivation is generally believed to take place by the rapid formation of surface-adsorbed hydrated complexes of metals, which are sufficiently stable on the alloy surface that further reaction with water enables the formation of a hydroxide phase that, in turn, r idly deprotonates to form an insoluble surface oxide film. Failure in any of these stages would lead to continued active dissolution. The passivation potential is critical to this process, in part because it governs the oxidation state of the metal, which in turn governs its solubility. In addition, the electric field strength has to be sufficient to cause deprotonation of the surface hydroxide phase in order to enable the oxide barrier film to become estabilished. No evidence has been found by surface studies that passivity of austenitic stainless steels is possible by formation of a simple hydroxide film. It is peihaps surprising that the passive film formed on austenitic stainless steels does not always contain each of the alloying elements added to stabilize the austenitic phase, even when such additions appear to improve the chemical stability of the steel. Ni exemplifies this behavior. 

One of the most effective elements added to austenitic stainless steel, and for that matter even ferritic stainless steel, in order to improve pitting resistance is Mo [8]. Molybdenum, however, is a highly versatile element, existing in the passive film in a number of oxidation states. In the case of the hexavalent state it has been observed in both the cationic and anionic states, namely as molybdenum trioxide and ferrous molybdate. It has most commonly been reported to exist in the quadrivalent state as molybdenum dioxide and oxyhydroxide. 

C A state of surface dealloying with no continuous oxide, e.g., gold alloys in many aqueous solutions [75,99]. Dealloying may also occm within localized corrosion sites in passive alloys such as austenitic stainless steels. 

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