Crevice Corrosion - NaCl Concentration Map for Alloy 625 at Elevated Temperature
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CREVICE CORROSION - NaCI CONCENTRATION MAP FOR ALLOY 625 AT ELEVATED TEMPERATURE TOSHIAKI AMANO, YOICHI KOJIMA and SHIGEO TSUJIKAWA The University of Tokyo, Faculty of engineering, Department of Metallurgy 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113 JAPAN
ABSTRACT
The repassivation potentials, ER,CREV' S, for metal/metal-crevice of Alloy 625 were determined in 0.3-10% NaC1 solutions for temperatures up to 250 C. The ER,CREV'S were found to be the least noble at temperatures around 100 and 125 C. The ER,CREV became more noble as temperature increased; this Based on the tendency was particularly strong in diluted solutions. experimental data, a crevice corrosion map showing the critical condition in terms of temperature and NaCl concentration was presented. As for the map, a wide repassivation region was found in elevated temperatures, similar to that of commercially pure titanium, C.P.Ti.
INTRODUCTION In order to identify the most appropriate material for nuclear waste disposal packages, the long term integrity of C.P.Ti, and low alloy titanium, two strong candidate materials, was studied by using the repassivation concept [1]. The results of these previous studies were presented in crevice corrosion maps in terms of NaCI concentration and temperature [1] [2]. The map was categorized into a repassivation region, where crevice corrosion ceased to occur, and a crevice corrosion region, where crevice corrosion might occur. Furthermore, the maps for various stainless steels were also studied by the authors for temperatures up to 80 C[3]. The objective of this study is to continue the previous efforts by determining the crevice corrosion map for another strong candidate material, Alloy 625.
EXPERIMENTAL METHOD Chemical composition in mass% of used Alloy 625 was Ni:60.9, Cr:21.3, Mo:8.8, C:0.03, Si:0.24, Mn:0.27, Nb:3.62, Ti:0.19, AI:0.05. The schematic of test specimen is shown in Figure 1. A Ti alloy bolt and two nuts of the The Alloy 625 were assembled to provide the needed metal/metal-crevice. would-be crevice surfaces of the two nuts were wet-polished to #1500 SiC paper before finger-tightened onto the Ti bolt. Mat. Res. Soc. Symp. Proc. Vol. 353 ©1995 Materials Research Society
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ERCREV'S were measured in 0.3, 1, 3 and 10%NaC1 solutions for temperatures ranging from 80 to 250 C. The measurement procedure for elevated temperatures was as follows: (1) deaerate the solution with highly purified N2 for 30 minutes, (2) immerse the crevice specimen in the solution and set the potential of the specimen at OmV, (3) heat the solution to the testing temperature , (4) increase the electrode potential to a few hundred mVs above ERCREV before the specimen current, I, begins to fall, (5) upon the observation of a continuous increasing current, retain the current at the constant level, IH, by adjusting the potential, (6) after the total amount of electrical charge reaches 100 Coulomb, decrease the
potential in steps of -10mV/10min until IL is reached, (7) decrease the potential in steps of -lOmV/60min when I is less than IL. The
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