Effect of Superficially Applied MnO₂ and Al₂O₃ Oxide Inhibitors in Enhancing High-Temperature Corrosion of T22 Boiler Steel

Hot corrosion is a severe form of industrial corrosion which occurs at high temperatures under the influence of oxidizing gases and can be prominently linked to the formation of a molten salt/ash layer over any metallic substrate. Throughout the years, several types of inhibitors and coatings have successfully been employed to reduce its devastating effects to a certain extent. In this study, two synthesized metal oxides namely Al₂O₃ and MnO₂ were used as corrosion inhibitors and XRD, FTIR analysis were carried out to assess their structural properties. Further, TGA analysis for Al₂O₃ and MnO₂ was carried out to determine thermal stability characteristics. The as-synthesized materials were further deposited as inhibitor coatings (Al₂O₃ with MnO₂ bond coating, MnO₂ coating and Al₂O₃ + 50% MnO₂ coating) on T22 boiler steel specimens. All the specimens (bare T22 and coated T22 steel) were investigated for hot corrosion studies in Na₂SO₄-60%V₂O₅ environment at the temperature of 900 °C for 50 consecutive cycles. Every cycle involved a 1-h heating step in furnace followed by 20-min cooling at room temperature. Weight gain data were collected using a digital balance. XRD and (SEM–EDS) analysis were carried out to characterize the samples after exposure to hot corrosion environment. A better resistance to hot corrosion was observed for all the different types of coatings, with Al₂O₃ + 50% MnO₂ coating representing maximum resistance. A high concentration of protective oxides such as Al₂O₃ and Cr₂O₃ present on the surface and their interaction to form dense layers on the coated samples explains the enhanced hot corrosion inhibition.