Transpassive Dissolution of Alloy 625, Chromium, Nickel, and Molybdenum in High-Temperature Solutions Containing Hydrochloric Acid and Oxygen

Abstract

Coupons of nickel, molybdenum, chromium, and the nickelbased Alloy 625 (UNS 06625) were corroded in strongly oxidizing hydrochloric acid (HCl) solutions at 350°C and a pressure (p) of 24 MPa, with reaction times between 0.75 h and 50 h. For Alloy 625, the effect of surface roughness also was investigated. Nickel and molybdenum showed strong material loss after only 5 h of reaction as a result of the instability of the solid oxides formed under experimental conditions. The attack on chromium started at the grain boundaries. At longer reaction times, thick, spalling oxide layers formed on the surface. The attack on Alloy 625 also started at the grain boundaries and at inclusions leading to the formation of small pits. On polished surfaces, the growth of these pits occurred faster than on nonpolished surfaces, but fewer pits grew. Corrosion products formed at the surface consisted of oxygen and chromium. On isolated spots, nickel-and chlorine-containing products also were found.

Corrosion Properties of Candidate Materials in Supercritical Water Oxidation Process

“…However, the acidic solution would lead to NiO instability in high temperature (58,63). The most mass reduction and the most severe morphological change were both observed under subcritical condition (29,30,45,64,65). In contrast to 304L SS, 316L SS and Inconel 690, the oxide on Inconel 625 is too thin to measure in deaerated supercritical water over the temperature range 400 °C to 550 °C (36).…”

“…The higher Cr concentration would make lower penetration rate of alloys. The chromium oxide presented the lowest solubility among the oxides of chromium, iron and nickel (53,58,65,90,91,93). The Cr showed the most stable state among Fe, Cr and Mo in solution with 5 mass% NaCl at temperature of 300~500 °C (33).…”

Behaviour of Ni-based coating alloys in supercritical water

“…CANDU SCWR operating region[2] Schematic of proposed Japanese SCWR[9] Phase diagram of water [15] Physical properties of water vs. temperature at 24 MPa. Dielectric constants of typical organic solvents at room temperature are also indicated[16] Density as a function of temperature at various pressures[19] 27 Ionic product of high-temperature water at different temperatures versus pressure[5] Stability Island" of a protective oxide and principle mechanisms of dissolution[5] Stability Islands of chromium and nickel[5,29] 8 SEM image of oxide products after exposure to SCW at 500°C (a) T91 in 25ppb DO SCW for 505hr (b) T91 in 2ppm DO SCW for 503hr[30] Cross-section image and EDS of HCM12A exposed to 600°C SCW with 25ppb DO for 1026h[31] 10 Cross-section of oxide formed on Alloy 800H exposed to 500 °C SCW with 25 ppb dissolved oxygen concentration for 505h[31] 11 SEM morphology of D9 alloy sample after exposure to 2000 ppb DO SCW at 500 °C for 503 h [35]. (A typical spallation for austenitic alloys) SEM cross-section image of NiCrAlY coating Specified areas of NiCrAlY sample 49 Schematic diagram of the SCW test system 50 Naming of various reactor components 51 Purge, pump and injection system for filling the reactor with water while keeping impurities out 52 Sample holding tree with samples 53 Water added (normalized volume) vs. density calculated from temperature and pressure readout Pressure and temperature vs. time for stage 1 of testing button samples 59 Pressure and temperature vs. time for stage 2 of testing button sample 59 10 Pressure and temperature vs. time for stage 1 of testing NiCrAlY coated samples 61 11 Pressure and temperature vs. time for stage 2 of testing NiCrAlY coated samples 61 12 Image of SEM facility in Carleton University 62 Sample surface conditions after Stage 1 (500h)-run of button 63 Sample surface conditions after Stage 2 (940h)-run of button 64 3 3-D bar chart showing the weight change (Z-Axis) as a function of surface preparation (X-Axis) and sample type (Y-Axis) during Stage 1 of testing 66 D bar chart showing the weight change (Z-Axis) as a function of surface preparation (X-Axis) and sample type (Y-Axis) during Stage 2 of testing 66 D bar chart showing the cumulative weight change (Z-Axis) as a function of surface preparation (X-Axis) and sample type (Y-Axis) 67 SEM images (Left: Low Mag., Right: High Mag.)…”

Passive and transpassive films formed on a nickel alloy in ammoniacal solution

“…[9,10] To examine the surface characteristics of alloy 625 after transpassive corrosion a potential of 0.59 V was chosen for potentiostatic testing, which is well within the transpassive range but below the oxygen evolution potential (∼0.65 V) at pH 9.8.…”