Review of the corrosion of nickel-based alloys and stainless steels in strongly oxidizing pressurized high-temperature solutions at subcritical and supercritical temperatures

The corrosion phenomena of nickel-based Alloy 625 (UNS N06625), which in the present content is representative of other nickel-based alloys and stainless steels in strongly oxidizing solutions of different acids, salts, and bases, was reviewed. Acids include hydrofluoric acid (HF), hydrochloric acid (HCl), hydrobromic acid (HBr), sutfuric acid (H2SO4), nitric acid (HNO3), and phosphoric acid (H3PO4). Salts and bases are sodium chloride (NaCl), sodium hydrogen sulfate (NaHSO4), sodium sufate (Na2SO4), and sodium hydroxide (NaOH). Concentrations of these species are typically similar to0.1 mol/kg and that of oxygen > similar to0.5 mol/kg (16,000 ppm). Temperatures (up to 500degreesC) and pressures (up to similar to40 MPa) reach into the supercritical region of water. In acidic solutions, Cr(III) Oxides Or hydroxides are the thermodynamically stable compounds and thus form the oxide layer at temperatures up to similar to250degreesC to 300degreesC. Under these conditions, serious general corrosion is riot observed, but anion-induced localized attack can occur. This is the case in solutions of HCl and HBr, where pitting with penetration rates up to 1, 000 At m in 100 h is observed. At higher temperatures, the transpassive conversion of solid Cr(111) compounds toward soluble Cr(VI) species is favored. At the same time, other components of the alloys-nickel, iron, molybdenum, cobalt, or niobium-can not form a stable, protective layer since all of their oxides are also soluble in the acidic solution. The corrosion morphology of this transpassive dissolution is a uniformly attacked surface. Corrosion rates are up to similar to500 p m in 100 h and depend strongly on the solubility of the corrosion products (not the oxides). Corrosion is negligibly low in solutions Of HF and H3PO4. Surprisingly, an upper limit of severe corrosion is observed. This can be explained by the sharp drop of density, ionic product, and dielectric constant of water when increasing the temperature at constant pressure. At higher pressures, the drop shifts toward higher temperatures, Dissociation constants of acids present in the solution show the same behavior; they are extremely low in low-density, high-temperature solutions, leading to neutral conditions. The upper temperature limit of severe corrosion can be related to that drop in physical properties. The low corrosion in the low-density-and thus neutral-supercritical solutions can be explained by the stability of Ni(II) oxide, which forms the protective oxide layer. Chromium and molybdenum are still dissolved by forming their hexavalent acids, H2CrO4 or H2MoO4, respectively. In H3PO4 solution, additional corrosion mechanisms may occur. In alkaline solutions, no significant general corrosion occurs at subcritical temperatures. However, supercritical solutions of NaOH and oxygen are highly corrosive because of the formation of an oxidizing melt with a high solvency for salts. Attributing observed corrosion mechanisms on solubilities of the protecting oxides and the other corrosion products, the corrosion behavior of Alloy 625 can be generalized to other nickel-based alloys and stainless steels.