Effect of cooling rate on corrosion properties of 3CR12 type alloys

Abstract

The effects of various cooling rates on the general, pitting, and atmospheric corrosion resistancss of 3CR12, a 12%Cr steel, were investigated. Alloys of type 3CR12, obtained from both laboratory and plant melts, heated to 1000°C and water quenched, air cooled, cooled at 16 K min⁻¹, or furnace cooled at 0·5 or 0·1 K min⁻¹ were tested. TME and microstructural investigation showed that the microstructure changed from a dual phased martensite–ferrite structure to a chromium carbide rich ferritic structure with decreasing cooling rate. In 0·05M H₂SO₄ the general corrosion rates were seen to decrease with decreasing cooling rate, while pitting tests conducted in sea water and a 0·025M NaCI plus 0·025M Na₂SO₄ solution showed reduced pitting corrosion resistance as the cooling rate decreased. The reason for this trend was attributed to the presence of chromium carbides of the type Cr₂₃C₆ which depleted the adjacent matrix of chromium thus rendering it susceptible to pitting attack; this was confirmed by the SEM examination which showed preferential pitting around chromium rich carbides. The effect of cooling rate on the atmospheric corrosion resistance of these alloys was more qualitative, but showed that an intermediate cooling rate (air cooling) yielded the best overall corrosion resistance.