Stress corrosion cracking of pressure vessel steels in high-temperature caustic aluminate solutions
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I.
INTRODUCTION
L O W alloy pressure vessel steels are the common structural materials for welded reaction vessels (e.g., digesters, precipitators, and evaporators) in the Bayer process for extraction of alumina from hydrated oxide ores (e.g., bauxite). These low alloy reaction vessels are in contact with hightemperature concentrated caustic alnminate solutions and frequently suffer from stress corrosion cracking (SCC) during service.[1,2] Although SCC of steels in simple NaOH solutions has been the subject of numerous studies, f3,41little work has been done in caustic aluminate solutions at 92 oc.t5,61 To purify alumina from lower quality ores, the extracting temperature has been elevated. However, there are no data on SCC susceptibility of steels used in the alumina industry at higher temperatures. The objective of the present work is to study the SCC behavior of low alloy pressure vessel steels with different sulfur contents in an imitative Bayer process at 200 ~ to 300 ~ Over the past few years, a number of research works [7,8,9] have shown that nonmetallic sulfide inclusions can cause environmentally assisted cracking to occur in high-temperature water related primarily to Boiling Water Reactor (BWR) and Pressurized Water Reactor (PWR) environments. The effect of MnS inclusions on caustic SCC property is also discussed in this article to give recommendations for improving SCC resistance of materials used in the alumina industry. II.
EXPERIMENTAL P R O C E D U R E
Studies were conducted on three kinds of low alloy steels: 16MnR, A48CPR, and rare-earth-treated 16MnRE. The pressure vessel quality rolling steel plates used in this work were 50-mm thick and were annealed at 650 ~ The chemical compositions and mechanical properties of these steels were similar (Tables I and II), although the sulfur
SU'E LIU, Research Associate, ZIYONG ZHU and WEI KE, Professors, and HUI GUAN, Graduate Student, are with the State Key Laboratory of Corrosion and Protection, Institute of Corrosion and Protection of Metals, Academia Sinica, Shenyang 110015, China. Manuscript submitted June 3, 1994. METALLURGICALAND MATERIALSTRANSACTIONSA
contents were very different. The cylindrical tensile specimens were 24 mm in gage length and 5 mm in diameter with threads at each end to fit tensile grips. Two kinds of tensile test pieces were sectioned from the steel plates parallel (L specimen) and perpendicular (T specimen) to the rolling direction to investigate specimen orientation effects. All specimens were polished with a 1000 grit emery paper and then cleaned with alcohol and acetone before testing. The test environment imitated the industrial Bayer process, and the temperature was varied from 200 ~ to 300 ~ The initial molal (M) concentration (Table III) in imitative Bayer solutions (IBS) was 7.42M NaOH, 1.32M A1203 9 3H20, and some impurities: carbonate, sulfate, and chloride. The test solution was prepared from distilled water and analytical grade chemicals. The resulting concentrations of anions are based on stoichiometric formatio
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