The mechanisms of breakaway oxidation of three mild steels in high-pressure CO2 at 500�C

Abstract

The mechanisms of breakaway oxidation of a rimming steel, a low-alloy steel (1 Cr-0.5 Mo), and a free-cutting steel (En1A) have been studied in high-pressure CO2 at 500°C. Average compressive stresses up to 170–280 ton in−2. in the scale have been derived from foil elongation and creep data. Carbon contents of up to 6 and 15% of the weight gain have been found in protective and breakaway scale and are larger than previously reported.1 Most of the carbon is deposited near the scale-metal interface, showing that CO2 diffuses through the scale. Oxidation in CO2-tritiated water mixtures gives a maximum tritium content in the metal at 250–500 h, which declines thereafter. Treatment with some sulfur compounds before oxidation, or the presence of sulfur in the metal, reduces the rates of protective and breakaway oxidation in wet CO2 and carbon transfer to the metal, but not to the scale. It is proposed that breakaway is initiated by an effect of hydrogen such as the accumulation of hydrogenous gases at the scale-metal interface under pressures sufficient to rupture the inner scale. Carbon deposition may assist initiation, and is probably the main factor in propagating breakaway oxidation.