Passivation of steel and stainless steels in alkaline solutions simulating concrete

The presence of the protective oxide film, known as passive film, is the basis for the durability of reinforced concrete structures. The formation of the passive film in the alkaline environment of concrete occurs spontaneously; the composition and the structure of the metallic substrate as well as the pH and the composition of the pore solution of concrete influence its growth. In this article we first critically review the results from electrochemical and surface analytical investigations carried out over the last twenty years and then we discuss the current knowledge on the mechanism of its growth and protection ability. It was found that the passive film becomes more stable with prolonged immersion (ageing), this is reflected in a decreasing passive current density, an increasing open circuit potential and in the decrease of the content of Fe(II) acting as defects in the passive film. Compiling the available data the decrease in passive current density can be related to the decrease in the Fe(II) concentration in the passive film. Similar trends were found for FeCr alloys and stainless steels. The stability of the passive film against chloride-induced localized corrosion is briefly discussed. Ageing of the passive film was found to increase the pitting potential while alloying with chromium and molybdenum increases the pitting resistance of stainless steels. Special emphasis is here given to the composition of the alloy-interface beneath the passive film and its enrichment in nickel.