A. Azimi Dastgerdi, U. Bellè, A. Brenna, M. Ormellese, M. Pedeferri, F. Bolzoni
Politecnico di Milano,
Italy
Keywords: stainless steel, pitting corrosion, crevice corrosion, potentiodynamic polarization test, design of experiment
Summary:
Stainless steels usually have a good corrosion resistance because of the protective passive film that forms on the surface, however they are prone to localized corrosion (pitting and crevice) due to the breakdown of passive film. Two important forms of localized corrosion are pitting and crevice corrosion that accounts for failures in industrial applications: these phenomena occur in the presence of oxygen and chlorides over a critical threshold. To evaluate corrosion behavior of stainless steels in chloride containing environment the Pedeferri’s diagram, is proposed. The diagram aims to define corrosion and passivity (perfect and imperfect conditions, as defined by Pourbaix) as a function of the potential and chloride content. The Pedeferri diagram was initially proposed to study the cathodic protection and prevention of carbon steel in concrete that can suffer localized corrosion in presence of chlorides, similarly to stainless steel. It is well known that metallurgical and environmental parameters affect localized corrosion behavior of stainless steel. In particular, aim of this paper is the study of the effect of chloride content, solution pH and temperature (environmental factors) on localized corrosion of stainless steel grade AISI 304L (UNS S30400). Potentiodynamic polarization tests were carried out in different solution; these electrochemical tests are currently used to characterize localized corrosion. The effect of the parameters was investigated by a nontraditional experimental approach based on design of experiment (DOE) method, which aims to study the effect of a fixed number of variables by an optimization of experimental conditions [8]. The method allows identifying the best experimental combination of parameters for testing and results in a polynomial regression. DOE benefits different modes of design which can develop first or second order models. Response surface methodology (RSM) is one of the most important modes of DOE that can provide the possibility of having quadratic models. Data for regression analysis were obtained from potentiodynamic polarization tests on AISI 304L stainless steel specimens in the experimental condition (pH, temperature, chloride content) defined by DOE method. The elaboration of the results showed that the second order model (quadratic) is sufficiently simple for describing the interaction of the parameters in localized corrosion of AISI 304 stainless steel. The results revealed strong influence of pH, temperature and chloride concentration on breakdown potential as well as the interaction between Chloride and pH through second order equation generated by DOE. The Pedeferri’s diagram was finally determined for some fixed conditions, for example pH 6, 20°C.