The Corrosion of Cu–Al Binary Alloys in H2/H2S/H2O Atmospheres at 400–900°C

Abstract

The corrosion behavior of pure Cu and of three Cu–Al alloys containing 1, 5, and 10 wt.% Al was studied at 400–900°C in a H2/H2S/H2O gas mixture. Both Cu–1Al and Cu–5Al alloys had the single-phase structure of α-Cu, while Cu–10Al was the intermetallic compound Cu3Al. In general, the corrosion behavior of all the alloys followed the parabolic rate law, and the corrosion rate constants generally increased with increasing temperature but decreased with increasing Al content. The scale formed on pure Cu was an exclusive single layer of Cu2S, while the scales formed on Cu–Al alloys were heterophasic and duplex, consisting of an outer layer of Cu2S and an inner layer of Cu2S and CuAlS2. X-ray diffraction results showed no evidence of oxides and the amount of CuAlS2 increased with increasing Al content. The formation of Cu2S and CuAlS2 on higher Al-content alloys resulted in a subsurface phase transformation from α-Cu (for Cu–5Al) or from Cu3Al (for Cu–10Al) to Cu3Al + Cu9Al4. The formation of CuAlS2 in the inner layer of Cu–Al alloys was responsible for the reduction of corrosion rates, as compared to those of pure Cu.