Passivation and Breakdown of Passivity of Stainless Steel Constituents Chromium, Iron-Chromium, Iron-Chromium-Molybdenum, Nickel (Passivering en Afbraak van de Passieve Laag van de Bestanddelen van Roestvast Staal Chroom, Ijzer-Chroom, Ijzer-Chroom-Molybdeen, Nikkel)

摘要

Part of the results of a study on the passivation behavior of some of the components of stainless steel (Cr, Ni) and of some of their mutual alloys (FeCr, FeCrMo) has been presented in the thesis. The corrosion behavior of stainless steels is principally determined by Cr. It transfers its excellent corrosion properties to the stainless steel. Hence the thesis begins with the electrochemical characterization of the passivation of Cr. The electrochemical experiments were all performed in 0.5 M H2SO4. In the transition of passivity to transpassivity and vice versa, intermediates like Cr(IV) are involved. The existence of ions with different oxidation states in the (trans)passive film is also suggested by the results of so-called back-pulse experiments. The corrosion behavior of Cr was further investigated by means of ellipsometry. The active-passive transition is governed by the relation between the passivation rate of Cr, the dissolution rate of Cr and Fe from the alloy and the stability of Fe(II) ions in the chromium oxide matrix which is being formed. Passivation begins with the formation of chromium oxide which leads to a current maximum.