| Abstract: | Effect of low silicium content and some surface treatment on the oxidation of carbon steels in carbon dioxide atmospheres at medium temperatures Carbon or low alloyed steels have been corrosion tested, for a great length of time (thousands of hours) between 375 and 450°C, in pressurized carbon dioxide (28 ou 40 bars) containing in particular: carbon monoxide (1 to 2% vol.) and water vapour (50 to 100 Ppm vol.). These steels have been investigated under several surface conditions: as received, sand-blasted, etched in a sulfophosphoric bath, etched and then passivated in a bath mainly composed of sodium bichromate, etched and passivated in a phosphate bath. The first passivating treatment induces the formation of a deposit, mostly made of chromium compound, on the steel surface. Samples which were treated in that way are much more rapidly oxidized than those prepared in the second bath or than the as-received samples. On the other hand, under given test conditions, the absence of silicon (< 0.02 %) in carbon steel corresponds to an accelerated oxidation. It should be said more precisely that the oxidation kinetic, soon become hear in that case, but keep rather slow when a similar steel contains at least 0.20% silicon. Both effects are cumulative: very high corrosion levels result of an unfavorable surface treatment applied to a steel without silicon. It has been shown, through microprobe examinations, that silicon traces are concentrated at the steeloxide interface during the oxidation process; it thus forms a “border” which rather prevents the diffusion of ions. As for the chromium traces, they tend to create disorders on the oxide scale. This scale breaks up into two layers and the development of the oxidation process is depending on the stability of the internal layer. In fact, the chromium induce a disordered growth of that internal layer, to the detriment of the external one, which either cracks or even disappears. |
