Transpassive dissolution of nickel-base alloys and stainless steels in oxygen- and chloride containing high-temperature water

The corrosion behaviour of different nickel base alloys and stainless steels (2.4605 [alloy 59; NiCr23Mo16Al], 2.4633 [alloy 602 CA; NiCr25FeAlY], 2.4819 [alloy C-276; NiMo16Cr15W], 2.4856 [alloy 625; NiCr22Mo9Nb], 2.4606 [alloy 686; NiCr20Mo16], 2.4646 [alloy 214; NiCr16AlFe] and 1.4401 [UNS S 31600; X5CrNiMo17122]) was investigated in oxygen and chloride containing high-temperature water (temperatures up to 600 degrees C; pressures up to 38 MPa; oxygen concentration 0.48 mol/kg; chloride concentrations up to 0.1 mol/kg). All alloys show a similar corrosion behaviour, depending on temperature. At temperatures below about 150 degrees C, only slight intergranular corrosion was observed. At higher temperatures (between about 150 and 300 degrees C) pitting was detected. Most of the original surface in this temperature range remained unattacked. At higher temperatures, morphology of pitting changed towards shallow pitting and the whole surface is penetrated. The high general corrosion observed in these areas can be attributed to transpassive dissolution of the alloys' protecting chromium oxide layers with following dissolution of the alloy. At supercritical temperatures, corrosion decreased drastically, and only transpassive intergranular corrosion was detected. The observed decrease of ion-induced corrosion phenomena can be attributed to the change of physical and chemical properties of water (solubility of ions). Corrosion in neutral and alkaline solution was significantly less. Both pitting and transpassive dissolution shifted towards higher temperatures or were not detected respectively.