Apparent Inversion of the Effect of Alloyed Molybdenum for Corrosion of Ordinary and Enhanced 316L Stainless Steel in Su
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Apparent Inversion of the Effect of Alloyed Molybdenum for Corrosion of Ordinary and Enhanced 316L Stainless Steel in Sulfuric Acid Gloria Kwong1, Anatolie Carcea and Roger C. Newman2 1
Ontario Power Generation (OPG) 22 St. Clair Avenue East, Toronto, Ontario, Canada M4T 2S3
2
University of Toronto Dept of Chemical Engineering & Applied Chemistry 200 College Street, Toronto, Ontario, Canada M5S 3E5
ABSTRACT An aging assessment of the OPG waste resin storage system predicted the potential for premature failure of the carbon steel resin liners. Consequently, resin liners made of 316L stainless steel with a minimum content of 2.5% molybdenum were selected to replace the carbon steel liners. The 2.5% Mo 316L stainless steel was specified to enhance pitting resistance in the spent resin environment. With the additional Mo, one would expect that a brief electrochemical corrosion test will reveal the superiority of such alloy over conventional 316L steel. This study reports a contrary experience. Introduction Type 316L stainless steel is a workhorse material, but some believe that it does not meet modern requirements for applications with sulfuric acids, as in OPG spent ion exchange resins. This is due to the low Mo content dictated by the economics of production: typically 2.1%. So manufacturers have introduced special grades containing no less than 2.5 or 2.7% Mo, usually with nitrogen addition. The nitrogen improves pitting resistance and stabilizes the austenite phase. One naturally expects that a brief electrochemical corrosion test will show the superiority of such a material over ordinary 316L steel. This study observed the opposite effect of Mo alloyed in ordinary and enhanced 316L stainless steel in sulfuric acid. The anodic dissolution of stainless steels in dilute acids has been studied using voltammetry and surface analysis [1-5]. Both Ni and Mo enrich on the dissolving surface, and this is the fundamental reason for the reduction in critical current density for passivation (icrit) by these alloying elements. The effect of Mo is particularly strong if expressed as the reduction in icrit per atom-percent of Mo addition. This inhibiting effect on dissolution is believed – at least by the present authors – to be the underlying reason for the beneficial effect of alloyed Mo on localized corrosion resistance in salt water [6]. Wanklyn [7] associated this with the stability of MoO2 in acid. Probably the authors of Ref 2 would not agree with such interpretations – they used to draw a line between ordinary acid corrosion and pitting, preferring to associate the latter with subtle effects on ion transport in the passive film.
Experimental Procedures
Two 316L steels with different Mo contents were obtained as ¼” thick plate. Their compositions are shown in Table 1. Specimens for electrochemical study were cut as small blocks and mounted in resin to expose one face to various concentrations of sulphuric acid. Lacquer was used to mask the edges, leaving an exposed area of 0.1- 0.2 cm2. The surface was abraded to a 1
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