Vol. 43 (2007), Issue 6, p. 110-111

ELECTRICAL PROCESSES IN ENGINEERING AND CHEMISTRY

Statistical Modeling of Electrochemical Reactivation Conditions for Detecting Sensitization to IGC of Austenitic Stainless Steel Type 316L

Kriaa A., Hamdi N., Jbali K., Sidhom H.

Abstract

УДК 620.197

 

The aim of this study was to determine optimum conditions of the DL EPR test in order to evaluate the highest values of the degree of sensitization (DOS) of a forged austenitic stainless steel type 316L, evaluated by the ratio Ir/Ia or Qr/Qa in %. The criteria of sensitization to the IGC corresponds to Ir/Ia >1% and Qr/Qa >1%. A model using a full factorial design has been established and the selected factors were the sweep rate (dE/dt), sulphuric acid concentration (H2SO4), ammonium thiocyanate concentration (NH4SCN) and temperature in °C. A first order model is obtained by using 24 full factorial design. An experimental test carried out using a factorial design 24 indicated that all the factors and their interactions have a positive effect on the response Ir/Ia. Furthermore, the highest value detected of Ir/Ia was found to be 65,6 %. On the other hand, a microstructural study based on optical microscope and Scanning Transmission Electron Microscope (STEM), have been carried out for annealed and aged samples (550°C-80 000 h). The profiles of chromium, nickel and molybdenum concentration established by X-ray microanalysis in STEM, confirm a decrease of content of these elements leading to the formation of Cr-depleted zones responsible of the sensitization to the IGC.

 

С использованием метода планирования эксперимента определены оптимальные условия (скорость изменения потенциала, концентрация серной кислоты, концентрация родамида, аммония, температура) применения метода электрохимического потенциала – динамической реактивации (EPR) определения склонности аустенитной нержавеющей стали к межкристаллитной коррозии, основанного на определении отношения тока реактивации Ia к максимальному анодному току Ia (Qr/Qa) при потенциодинамической регистрации поляризации (от коррозионного потенциала до максимального анодного тока Ia и обратно до коррозионного потенциала). Показана корреляция наблюдаемых значений Ir/Ia с результатами микроструктурных исследований и локального химического анализа.

 

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