Oxidation of 1% Cr, 0.5% Mo steel in carbon dioxide.

Abstract

Carbon steels do not suffice as structural materials in carbon-dioxide cooled reactors at gas temperatures above 410ºC, because of insufficient creep resistance. Low alloy steels of the Croloy variety appear to be the first alternatives as the small additions of chromium and molybdenum provide increased creep resistance. The corrosion of a 1% Cr, 0.5% Mo steel in carbon dioxide has been measured over the range of 450 to 525ºC under varying conditions of surface preparation, pressure, velocity, and impurity content of the gas. Weight changes were measured as a function of time of exposure in tests of up to 4,000 hours. It has been found that surface preparation of the specimens and pressures of the gas have little effect on the rate of oxidation of this steel in CO2. Also, presence of moisture up to 20,000 p.p.m. does not materially alter the rate of attack. Weigh-gains of specimens in pure oxygen were always found to be less than weight-gains obtained in carbon dioxide under identical conditions of temperature, pressure, and moisture content. Scaling of the oxide layer was never encountered under static or semi-static conditions. However, scaling occurred on many specimens exposed in flow gas; the extent depended on the temperature and gas velocity. Metallographic examination verified that a protective Cr2O3 film was never likely to be formed on this steel under the conditions of the tests. The major products of the reaction were Fe3O4 and another unidentified spinal. This study has shown that it is not safe to recommend this steel for use in carbon dioxide cooled reactors at temperatures above 450ºC.