Effect of Some Environmental and Stainless Steel Metallurgical Variables on the Values of Degree of Sensitization Measured by the Double Loop-Electrochemical Potentiokinetic Reactivation Test Pablo M. Altamirano,* Mariano A. Kappes, ‡, * , ** , *** and Martín A. Rodríguez* , ** , *** The double loop-electrochemical potentiokinetic reactivation (DL-EPR) method estimates the degree of sensitization in stainless steels with i r /i a , the ratio of peak current densities during reactivation (reverse) and activation (forward) scans. Beyond sensitization level, other metallurgical variables, like inclusion content and cold work, or testing environment variables, like solution deaeration and solution aging, could potentially affect i r /i a . Austenitic stainless steel Types AISI 304 (UNS S30400) and AISI 303 (UNS S30300) were tested to assess the effect of those secondary variables on i r /i a . AISI 303 stainless steel had similar chromium and nickel contents to AISI 304, but higher contents of sulfur, resulting in a higher volume fraction of inclusions. i r /i a increased with inclusion content and decreased with cold work in AISI 304 stainless steel. In all cases, the DL-EPR test could discriminate solubilized from thermally aged specimens because confidence intervals of respective samples never overlapped. The testing solution was 0.5 mol/L H 2 SO 4 + 0.01 mol/L KSCN, (sulfuric acid and potassium thiocyanate solution) and a common practice is to freshly prepare it just before the test. Measurements are usually performed under deaerated conditions. However, deployment of the technique, especially in the field, could be facilitated if naturally aerated solutions prepared in advance in a laboratory can be used. The oxygen content in solution and solution aging did not have a statistically significant effect on i r /i a . KEY WORDS: corrosion tests, degree of sensitization, intergranular stress corrosion cracking, stainless steels, sensitization INTRODUCTION W hen stainless steels are subjected to temperatures in the range of 500°C to 800°C, the precipitation of chromium-rich carbides occurs preferentially at grain boundaries (GB). 1-2 In practice, the undesired exposure to this temperature range can happen due to excursions in process variables or during fabrication, for example, in welded compo- nents. The precipitation of carbides at GB causes chromium- depleted zones in areas adjacent to GB. When Cr concentration falls below 12 wt% (weight percent), the formation of an adequate passivating layer is impeded, promoting localized at- tack. This phenomenon is known as sensitization. A sensitized microstructure has lower resistance to chloride stress corrosion cracking (SCC) 1 and can experience intergranular SCC in high- temperature water, polythionic acids, 3 and fluoride solutions. 4 There are different tests to evaluate the sensitization of austenitic stainless steels. 5-8 The practices established in the ASTM A262 standard 6 involve etching stainless steels under specific conditions followed by microscopic observation or hot acid immersion tests. They require material removal from the part or component under study (except for method A in ASTM A262, which might be adapted for in situ, nondestructive evaluation), so they are destructive tests. Alternatively, there are electrochemical potentiokinetic reactivation (EPR) tests, 5 standardized by ASTM G108 as the single-loop (SL-EPR) 7 and by ISO 12732 as the double-loop (DL-EPR) 8 variants. Although the SL-EPR and DL-EPR methods were developed for nondestructive field measurements, 5,9 in most cases they are performed in a laboratory on small specimens cut from components. For in- stance, the cells and sample holder drawings in the respective standards 7-8 are intended for small specimens cut from com- ponents and are not suitable for field deployment of the methods. Recently, an interlaboratory testing study successfully assessed the repeatability and reproducibility of the DL-EPR test for measuring the degree of sensitization (DOS). 10 DOS is a measure of the extent of chromium depletion along the grain boundaries due to M 23 C 6 carbide precipitation. 2 DOS depends on the proportion of GB with chromium deficiency, the width of these areas, and the minimum chromium level in these zones. 2 Compared to the SL-EPR variant, the DL-EPR method offers several advantages, including superior reproducibility, less sensitivity to variations in scan rate, and solution concentration, Submitted for publication: August 4, 2022. Revised and accepted: September 12, 2022. Preprint available online: September 12, 2022, https://doi.org/10.5006/4177. ‡ Corresponding author. E-mail: kappes@cnea.gov.ar. * Instituto Sabato, UNSAM/CNEA, Av. Gral. Paz 1499, San Martín, Buenos Aires, Argentina B1650KNA. ** National Commission of Atomic Energy of Argentina, Av. Gral. Paz 1499, San Martín, Buenos Aires, Argentina B1650KNA. *** National Scientific and Technical Research Council, Godoy Cruz 2290, Autonomous City of Buenos Aires, Argentina C1425FQB. SCIENCE SECTION CORROSIONJOURNAL.ORG ISSN 0010-9312 (print), 1938-159X (online) © 2022 AMPP. Reproduction or redistribution of this article in any form is prohibited without express permission from the publisher. NOVEMBER 2022 • Vol. 78 • Issue 11 1067 Downloaded from http://meridian.allenpress.com/corrosion/article-pdf/78/11/1067/3137821/4177.pdf by China Metallurgical Standardization Research Institute user on 04 November 2022