Corrosion problems in coal-fired boiler superheater and reheater tubes: steam-side oxidation and exfoliation. Development of a chromate-conversion treatment. Final report

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Abstract
This report describes the continued work on steamside scale exfoliation begun under Task 2 and presented in the steamside corrosion report of October 1980. The effort recorded in this report (Task 3) centers on the prevention of scale exfoliation by means of the chromate treatment. Evaluation of scaled tubes, first chemically cleaned by three established techniques and then chromate treated and tested in steam, reveals in-service performance equal to that previously obtained from new tube surfaces also chemically cleaned and chromate treated. In addition, chromate treatment reduces the oxide growth rate of 2-1/4%Cr-1%Mo steel to one-third that of untreated steel surfaces. Exfoliation resulting from cool-down cycles is also prevented by the chromate treatment. The improved scale stability produced by the chromate treatment is attributed to the formation of an Fe-Cr spinel phase on the treated surface, and to the suppression of wustite (FeO) at the metal/oxide interface, observed after 1150/sup 0/F (620/sup 0/C) steam exposure. The scale exfoliated from chromate-treated 304 stainless steel is also reduced; and such dissimilar metal welds as T-22 steel through 304 stainless steel also demonstrate full compatibility with the chromate-treatment process.

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