The corrosion behaviour of nickel-base alloy 625 (NiCr22Mo9Nb; 2.4856) andceria stabilized tetragonal zirconia polycrystal (Ce-TZP) against oxidizing aqueous solutions of hydrofluoric acid (HF), hydrobromic acid (HBr), and hydriodic acid (HI) at sub- and supercritical temperatures

In the present article, the corrosion of the nickel-base alloy 625 (NiCr22M o9Nb; 2.4856) in strongly oxidizing hydrothermal aqueous solutions of hydro fluoric acid (HF), hydrobromic acid (HBr), or hydriodic acid (HI), respecti vely, is investigated. The reaction conditions were temperatures between 25 and 500 degrees C, pressure of 24 MPa, acid concentrations of 0.05 mol/kg, oxygen concentrations of 0.5 mol/kg, and reaction times of 50 h (for HF an d HBr), or 3.5 h (for HI), respectively. The investigated alloy 625 tubular samples were corroded by a continously f lowing solution. The corrosion of alloy 625 caused by HF is only weak corro sion over the entire temperature range. HBr causes pitting at T > 170 degre es C. At temperatures between 320 and 380 degrees C, general corrosion due to transpassive dissolution leads to severe material loss. In supercritical solutions of low densities, corrosion is negligible. HI was oxidized almos t completely to iodine leading to a plugging of the reactors after short ti mes. Samples of ceria stabilized tetragonal zirconia polycrystal (Ce-TZP) were c orroded inside the tubes in a section where T = 500 degrees C. The only cor rosion phenomenon which was observed in all acids was intergranular corrosi on, with HF being less corrosive than HBr. Neither a weight loss nor a tetr agonal to monoclinic phase transformation could be measured. Furthermore, the corrosion behaviour of both materials was compared to that caused by other acids.