Discussion on the relationship between passivity and reactivity of metals

International audienceThe relationship between passivity of metals and their reactivity is discussed through different examples in nitric acid. By passivity, it is meant the ability of the metal to protect itself from oxidation. Passivity was characterized by in-situ electrochemical methods (linear voltammetry LV, electrochemical impedance spectroscopy EIS) and by ex-situ X-ray photoelectron spectroscopy XPS. By reactivity, it is meant the ability of the medium to reduce itself on the surface of the metal. Reactivity was characterized by electrochemical methods (LV, EIS).The first system concerns ferritic-martensitic steels in nitric acid with different chromium contents [1]. It was shown that passivity of these steels is governed by the chromium content in the oxide layer, which directly depends on the chromium content in the steel. A model was proposed to establish the relationship between the chromium content in the oxide layer and in the steel. Moreover it was shown that the kinetics of the nitric acid reduction slows down when the chromium content in the steel increases. In a second example, we studied the corrosion behavior of zirconium in nitric acid [2]. For that purpose, oxide layers with different thicknesses were electrogenerated on zirconium. It was shown that the thickness of the oxide layer governs both the oxidation rate of zirconium (through a high field mechanism) and the reduction rate of nitric acid (through the dielectric properties of the oxide). The last system concerns the comparison of the corrosion behavior in nitric acid of stainless steels with and without silicon. Contrary to the first two examples, it was concluded that the presence of silicon in the steel worsens passivity of the stainless steel but also decreases the reduction kinetics of nitric acid. From these examples, it can be concluded that there is no bijective relationship between passivity and reactivity of metals. In most of the cases, a passive metal appears to be less reactive. But in some others, it is not true