Hot corrosion at high temperatures is a major reason of premature degradation of boiler components in coal-fired thermal power plants, leading to reduced efficiency and increased maintenance costs. Conventional boiler steels exhibit poor resistance in aggressive environments containing molten sulfates, chlorides, and oxidizing gases at temperatures ranging from 560 to 980 °C. This review critically evaluates recent advances in thermal spray coatings for improving the hot corrosion resistance of boiler steels under such severe conditions. Particular emphasis is placed on coating materials, deposition techniques, microstructural characteristics, and degradation mechanisms associated with plasma spray, HVOF, D-Gun, and cold spray techniques. The main degradation processes including oxidation, sulfidation, chlorination, molten-salt attack, and oxide-scale spallation are examined in detail. Reported studies show that coatings like NiCrAlY, Cr3C2–NiCr, and WC–Co/Cr, Al₂O₃-based and composite coating systems significantly increase corrosion resistance through the development of dense Cr2O3 and Al2O3-rich protective oxide layers. The review further identifies major research gaps related to long-term cyclic exposure, biomass-fired boiler environments, and multilayer coating durability. Present limitations associated with oxide-scale instability, coating porosity, and long-term service degradation is also highlighted. Emerging developments in nanostructured and functionally graded coatings are also discussed for future high-temperature applications. The study provides a concise understanding for selecting suitable coating systems for advanced boiler protection.
Gd2Zr2O7 (GZ) and La2Zr2O7 (LZ) thermal barrier coatings were deposited by electron beam physical vapor deposition (EB-PVD) on cold gas dynamic sprayed CoNiCrAlY bond coats over Inconel 718 substrates. The systems were exposed to a vermiculite-derived silicate deposit at 1250 °C for nominal holding times of 4–32 h and examined by SEM, EDS, and XRD. Both systems exhibited intercolumnar infiltration, reduced measured porosity, increasing local hardness, oxide growth, and eventual loss of the continuous ceramic layer. Pronounced fragmentation was observed in the 16 h GZ specimen, and no continuous GZ topcoat was observed at the 20 h condition. A continuous LZ layer remained identifiable at 20 h, whereas the continuous layer was absent at 24 h. These are post-test observations from different specimens, not exact failure times. Diffraction features assigned to silicate phases and residual zirconates were interpreted together with the microstructural observations; residual zirconate peaks do not demonstrate preservation of a continuous coating. The examined LZ specimens retained a continuous ceramic layer to a later sampled exposure condition than the GZ specimens. This qualitative comparison is limited by one specimen per coating–time condition, discrete 4 h sampling and the absence of deposit-free controls, which prevents separation of thermal aging from deposit-assisted degradation.
Laser-clad Co-based coatings were fabricated on 304 stainless steel substrates and subjected to cyclic oxidation in air at 600, 700, and 800 °C for 240 cycles (240 h total elapsed time, 200 h cumulative hot time) to investigate their high-temperature oxidation behavior. The oxidation resistance of the coatings progressively deteriorated with increasing temperature. At 600 °C, the coating exhibited the lowest mass gain of 0.10 mg/cm2 (absolute mass change 0.14 mg, close to the balance resolution), with the formation of CoO, Cr₂O₃, and CoCr₂O₄ spinel, together with the Co₇Fe₃ intermetallic phase originating from the coating. Increasing the oxidation temperature to 700 °C promoted the development of a relatively thin oxide scale with a similar phase constitution. At 800 °C, a thick Cr₂O₃-rich oxide scale formed, while the porous CoO layer underwent severe spallation. Based on the observed phase evolution, a plausible oxidation sequence is proposed, in which Co and Cr are preferentially oxidized to form CoO and Cr₂O₃, followed by the formation of CoCr₂O₄ spinel through solid-state reactions between the constituent oxides. Meanwhile, Fe reacts with Co to form the Co₇Fe₃ intermetallic phase. The formation of a relatively dense Cr₂O₃ layer, together with the CoCr₂O₄ spinel, may contribute to the oxidation resistance of the coating, whereas the severe spallation of the porous CoO scale at 800 °C promotes further oxidation. These findings provide a basis for understanding the temperature-dependent cyclic oxidation behavior and oxide-scale evolution of laser-clad Co-based coatings in air.
The effects of martensite transformation on the oxidation resistance and corrosion resistance of zirconium alloy cladding tubes have been systematically investigated. The oxidation film growth behavior, element distribution, and corrosion electrochemical behavior were analyzed by oxidation weight gain tests, optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and electrochemical workstation, respectively. The results reveal that the high-temperature oxidation kinetics of the four temperature-transformed samples all follow super-linear oxidation kinetics law. Considering the weight-gain behavior together with the oxide-scale thickness and morphology, the 1050 and 1150 °C-treated specimens exhibit relatively similar oxidation behavior, whereas the 1250 °C-treated specimen shows more pronounced oxide-scale growth and degradation under the present experimental conditions. The three-layer oxidation structures consisted of ZrO2 layer, α-Zr(O) layer, and prior-β layer are formed after the oxidation treatment is applied. The interface between α-Zr(O) and prior-β layers exhibits wavy undulation. Cracks were observed within the oxide scale, although their specific contribution to oxygen transport during high-temperature oxidation cannot be directly established from the present post-quench observations. The corrosion resistance of the alloy shows initially increases and subsequently decreases with the increase of transformation temperature in the environment of 1 wt
MCrAlY overlays are limited by a depleting aluminium reservoir: β-NiAl consumption, spinel formation, interdiffusion, expansion mismatch. Alumina-forming MAX phases are proposed as alternatives, but on the wrong member of the family: almost every quantitative comparison against MCrAlY concerns Cr2AlC, while the Ti–Al–C literature is dominated by synthesis. The extrapolation is unsafe: the competing oxide in Ti-containing MAX phases is non-protective TiO2, not Cr2O3, and Ti3AlC2 carries a third less aluminium and more TiO2 than Ti2AlC. Purity is decisive: at 1000 °C, Cr2AlC of 64 and 82 wt
