Double loop electrochemical potentiokinetic reactivation test optimization in checking of duplex stainless steel intergr
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I. INTRODUCTION
AUSTENO-FERRITIC duplex stainless steels (DSS) are well known for their high resistance to all kinds of localized corrosion.[1–4] They are particularly resistant to the selective dissolution of chromium-depleted zones resulting from the precipitation of chromium carbides and intermetallic phases. This precipitation, sometimes inevitable, occurs when manufacturing conditions heat these stainless steels to temperatures ranging from 400 °C to 1050 °C.[5,6,7] During reception inspection of finished stainless steel products, standard test types such as Strauss, et al.[8–11] are typically used to evaluate the intergranular corrosion (IGC) susceptibility. These tests are difficult to perform in situ and the interpretation of the results, especially in the case of a very thin precipitation, is very difficult. These tests require a significant quantity of material for testing, which is often detrimental to the service life of structures, and can affect the integrity of the pieces during the reception inspection. Because of the need to insure nondestructive examinations, an electrochemical potentiokinetic reactivation (EPR) test was developed mainly to assess cast products.[12] Numerous studies[4,13–22] have contributed to the development of this technique over the years. The most common of the many variations of this technique used today is the
T. AMADOU, Lecturer, and H. SIDHOM, Professor, are with the Laboratoire de Mecanique, Matériaux et Procédés, 1008 Tunis, Tunisia. C. BRAHAM, Senior Lecturer, is with the Laboratoire de Microstructure et Mécanique des Matériaux, F75013 Paris, France. Contact e-mail: [email protected] Manuscript submitted October 23, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
double loop electrochemical potentiokinetic reactivation (DL-EPR) method,[14–17,23–25] which has the advantage of having only a weak dependence on surface texture.[22,23,24] Today, DL-EPR tests can be performed in situ using portable cell devices.[19] The DL-EPR test is very suitable for reception inspections of finished pieces and assembled devices.[20] It has been demonstrated in numerous cases of stainless steel (SS) or nickel alloys[4,12–26] that the high sensitivity of this technique to the electrolyte composition and to operating conditions does not affect its reliability. Nevertheless, to detect the effect of very fine precipitations using the DL-EPR test, an appropriate electrolyte composition and temperature and appropriate conditions for potential limits and scan rates are required for each alloy.[4,19,22,26] The summary of technical literature related to austeno-ferritic DSS shows that the electrolyte often consists of sulfuric acid (H2SO4) solution with the addition of NaCl or KSCN as depassivators (Table I). However, hydrochloric acid (HCl) is used as the depassivator in the case of DL-EPR tests for super austenitic SS.[19] The potential scan rate remains between 0.5 and 5 mV/s and the chosen temperature is often the room temperature or close to 30 °C.[4,21,25,27–29] The first objective of t
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