Evaluation by the Double Loop Electrochemical Potentiokinetic Reactivation Test of Aged Ferritic Stainless Steel Intergr
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IC stainless steels are usually used in the manufacturing of steam generators of power plants. These parts are very sensitive to the heat generated by welding, during the maintenance operations, and throughout service aging. The thermal cycles specific to these conditions induce structural changes, by a diffusion mechanism, i.e., the precipitation of chromium carbides and intermetallic phases or without a diffusion mechanism, i.e., the transformation of austenite into martensite during cooling. Such changes may lead, directly or indirectly, to the formation of relatively large depleted chromium zones that are able to sensitize the steel to intergranular corrosion.[1] These phenomena have occurred in several ferritic stainless steel grades H. SIDHOM, Professor, and T. AMADOU, Lecturer, are with the Laboratoire de Me´canique, Mate´riaux et Proce´de´s, 1008 Tunis, Tunisia. C. BRAHAM, Senior Lecturer, is with the Laboratoire d’Ingenierie de Materiaux, F75013 Paris, France. Contact e-mail: [email protected] Manuscript submitted July 23, 2009. Article published online September 3, 2010 3136—VOLUME 41A, DECEMBER 2010
such as AISI 430, AISI 430 Ti, AISI 430 Nb, AISI 434, AISI 444, and AISI 446 after solution annealing (1 hour at 1366 K (1093 C) and water or air cooling) and after aging at 1033 K (760 C) for 1 hour.[2] They were also observed in the case of AISI 405 at the solution annealed state (0.5 hours at 1323 K (1050 C) and water cooling) and at aged states at temperatures varying between 773 K and 1123 K (500 C and 850 C).[3] The same phenomenon was observed for superferritic stainless steel X1Cr26Mo aged at temperatures varying from 723 K to 923 K (450 C to 650 C) for periods ranging from 2 minutes to 10 hours.[4] The frequently used methods to determine the degree of sensitization to the IGC are either destructive standard tests (ASTM A 262), i.e., Strauss[5] or Huey,[6] which provide qualitative evaluations, or nondestructive tests (ASTM G 108) of the electrochemical double loop–electrochemical potentiokinetic reactivation (DL-EPR)[2–4,7–27] type, which provide a quantitative assessment. However, the ICG assessment by the DL-EPR test is not absolute. Indeed, this evaluation is highly dependent on test operating conditions, such as the nature and the concentration of the electrolyte, the depassivator type, the potential scanning rate, and the METALLURGICAL AND MATERIALS TRANSACTIONS A
temperature of the solution. In previous works, authors have optimized and validated the test operating conditions in the case of duplex[8] and austenitic stainless steels.[9] For these materials, the reproducibility of the results, the selectivity of the attack, and the ability of the test to detect the effects of weak dechromizations were clearly confirmed. In order to continue investigating the optimal operating conditions of the DL-EPR test for a quantitative evaluation of the degree of sensitization (DOS) to the IGC of ferritic stainless steels, the experimental design method was used to determine the effects of the
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