Statistical modeling of electrochemical reactivation conditions for detecting sensitization to IGC of austenitic stainless steel type 316L

Abstract

The aim of this study was to determine the 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 I r / I a or Q r / Q a in %. The criteria of sensitization to the IGC corresponds to I r / I a > 1% and Q r / Q a > 1%. A model using a full factorial design has been established, and the selected factors were the sweep rate ( dE / dt ), sulphuric acid concentration (H 2 SO 4 ), ammonium thiocyanate concentration (NH 4 SCN), and temperature in ° C. A first order model is obtained by using a 2 4 full factorial design. An experimental test carried out using a 2 4 factorial design indicated that all the factors and their interactions have a positive effect on the response I r / I a . Furthermore, the highest value detected of I r / I a was found to be 65.6%. On the other hand, a microstructural study based on an optical microscope and a scanning transmission electron microscope (STEM) has been carried out for annealed and aged samples (550 ° C-80000 h). The profiles of the chromium, nickel, and molybdenum concentration established by X-ray microanalysis in STEM confirm a decrease of the content of these elements leading to the formation of Cr-depleted zones responsible for the sensitization to the IGC.

New High-Resolution Solution for Measuring Degree of Sensitization of Duplex Stainless Steel 2205 Using Double-Loop Electrochemical Potentiodynamic Reactivation Technique

“…The EPR is a quantitative method that indicates degree of sensitization (DOS) based on electrochemical parameters obtained from the potentiodynamic polarization curve. [10][11][12][13][14][15][16] The EPR test performed in 2.5 M sulfuric acid (H 2 SO 4 ) solution was first introduced in 1969 by Cihal, et al, 17 and then in 1977, Clarke, et al, 18 proposed a single-loop EPR method executed in 0.5 M H 2 SO 4 + 0.01 M potassium thiocyanate (KSCN) solution, which required very fine surface polish (1 µm) and time-consuming measurement of grain size. Finally, Akashi, et al, 19 developed a double-loop EPR (DL-EPR) method in 1980, and the advantage of this method lies in its independence of both the inclusion contents of the material and its surface finish.…”