Corrosion Resistance of Chromium-Coated Ferritic-Martensitic Steel in Supercritical Water

Corrosion behaviour characterisation of 316L stainless steel and Inconel 625 in supercritical water containing hydrochloric acid and high oxygen

“…In addition to forming polyhedral crystals, the outer oxide scale of Inconel 625 also has nodular oxide particles, as shown in Figure 7. As discussed in Section 3.2, nodular oxide is attributed to the vaporisation of chromium in high concentration oxygen environment (see reactions 5-6) [14,17,28 32]. However, a process similar to reactions 5-6 leads to a downward trend of chromium, and the chromium in point ‘5’ is less than half of that in point ‘6’ (in Figure 8).…”

Oxidation Processes and Involved Chemical Reactions of Corrosion-Resistant Alloys in Supercritical Water

“…At the initial stage of oxidation, dissolved oxygen diffuses to the metal interior through short circuit paths (e.g., micropores and grain boundaries), and then preferentially reacts with Cr to form Cr-rich oxides (mainly Cr 2 O 3 ) at the initial metal/H 2 O interface through reaction . ,,− This can be attributed to the higher oxygen affinity of Cr than that of Fe and Ni. − As a result, a continuous compact inner layer is formed due to the high lattice diffusion coefficient of Cr ion in Cr 2 O 3 and the high diffusion rate of O along grain boundaries . In addition, Cr 2 O 3 , CrO 3 , and Cr 2 O 7 2– may also be produced via reactions – ,,,,− in the processes above.…”

“…Although the alloy with a high Cr content tends to form a dense, continuous, and protective Cr 2 O 3 layer than does the alloy with a low Cr content, the optimum Cr content is approximately 20 wt % commonly. , However, Cr 2 O 3 can evaporate in the forms of CrO 3 , CrO 2 (OH), and especially CrO 2 (OH) 2 in SCW with a high oxygen content . More specifically, Cr 2 O 3 and CrO 3 can react with water and oxygen to form an oxide hydroxide (i.e., CrO 2 (OH) 2 (chromium­(VI)) by reactions ,,,,,,,− and . CrO 2 (OH) 2 is an active volatile species and probably formed in the metal wastage process in high-temperature and high-pressure water on the oxide/substrate interface .…”