| Abstract: | The Formation of Surface Films and the Corrosion Resistance of the Silicon Containing Austenitic Steel X 2 CrNiSi 18 15 in Nitric Acids The oxide layers consist of two parts: the lower one is mainly Croxide, the upper one is SiO2. ESCA and AAS measurements were made in order to study the formation of the surface layer on the austenitic steel X2 CrNiSi 18 15 in nitric acids. A film, which is chromium oxide, is formed first. On top of this film a second film consisting of SiO2 grows. The elements nickel and manganese are not found in the formation of the oxide films. Immediately below the oxide layer the steel is enrichened with chromium and depleted of iron. In order to find the in some cases very small corrosion rates (5 · 10?5 mm · a?1) in a reproducible manner, the amounts of iron, chromium and nickel which had been dissolved were measured by means of the AAS method as a function of time. For stationary samples apparent activation energies of 65.2 kJ/Mol (azeotropic nitric acid) and 37.5 kJ/Mol (highly concentrated nitric acid), respectively, were found. These data confirm the assumption that the corrosion rate is determined by reactions at the phase boundaries. No appreciable influence of the flow velocity on the corrosion rate was detected. |
