In biomass boilers, corrosion is a prevalent concern that arises at high temperatures. This is mainly because the fuels consumed in these boilers have a high alkali, chlorine, and other molten salt content that has occasionally led to material depletion, leaks, and unforeseen plant shutdowns. Applying protective coatings using thermal spray techniques is a practical answer to this issue. The current work focused on applying powders of Inconel 625 and Inconel 718 to boiler steel using a high-velocity oxy-fuel spraying method. The samples after coating deposition were subjected to the conditions of a biomass-fired boiler for 15 cycles to study the performance of the coatings in a real environment. The decrease of thickness over time was used to evaluate the erosion-corrosion process. Various characterization techniques were used to examine the as-sprayed and eroded-corroded specimens. The X-ray diffraction (XRD) technique was utilized to analyze the phases, while the surface characteristics of powders, coatings, and samples exposed to erosion-corrosion were investigated through scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS). When exposed to the actual boiler environment, the findings showed that Inconel 625-coated steel performed better than Inconel 718-coated steel.

Cavitation is a wear process in engineering systems caused by the energy release of collapsing bubbles leading to the failure of critical components such as valves, pumps, and propellers. Thermally sprayed coatings can be applied to improve the wear resistance of these components. This investigation considers a WC-NiCrBSi coating composition under cavitation wear, where the WC phase provides the strength and the NiCrBSi matrix offers corrosion resistance in seawater. Coatings were deposited on AISI 440C stainless steel discs of 32mm diameter and 8mm thickness using industrially optimized parameters for the HVOF JP5000 system. Indirect cavitation tests were conducted using a modified ASTM G32 testing procedure on coated test coupons in as-sprayed and Hot Isostatic Pressed (HIPed) conditions. Two tests were performed for each coating using natural seawater of pH 8.19 at room temperature, and averaged wear values are reported to compare the cavitation rate and cumulative mass loss of the coatings. Coating microstructural phases in the as-sprayed and HIPed conditions were identified using X-ray diffraction. The microstructure of the coating substrate system and post-cavitation test wear scars were investigated using Scanning Electron Microscopy (SEM) equipped with energy dispersive spectroscopy (EDS). This investigation provides an understanding of the corrosive-cavitation wear behavior and failure modes of coatings. The cavitation erosion rate and cumulative mass loss results showed that the as-sprayed WC-NiCrBSi coatings improve the cavitation wear resistance of the substrate.

The performance and applicability of thermal barrier coatings (TBCs) depend strongly on the top coat and bond coat interface integrity. The interlayer in TBC systems is often processed prior to top coat spraying to tailor its material properties or surface topography. Both, the bond coat spraying process and the further post-processing may significantly influence the thermally grown oxide (TGO) build-up which is crucial in terms of enhancing the TBC lifetime. In this work, NiCrAlY bond coats were sprayed by means of atmospheric plasma spraying. The as-sprayed bond coats were subjected to laser microtexturing which resulted in different bond coat topographies. Then, the samples were exposed to isothermal oxidation conditions under various oxidation dwell times to see the TGO evolution. The preliminary assessment of the oxidation mechanisms and oxide distribution was done by confocal laser scanning microscopy (CLSM). Scanning electron microscopy with energy dispersive X-Ray spectrometry (SEM/EDS) was used in order to analyze the evolution of bond coat structure and chemical composition during the high temperature oxidation.

In this paper, the development of surface oxide scale and the evolvement of spallation mechanism of Fe-21Cr-5.6Al super alloy were investigated at 1200°C and 1300°C. The oxidation kinetic curves were obtained by isothermally measuring the weight gain of alloy oxidized with various time durations. The morphologies of oxide scale and grain structures were observed by SEM/EDX, and the phase structure was analyzed by XRD. The results show that the oxidation processes follow the parabolic law and the oxidation rate is higher at 1300°C than 1200°C. Though the FeCrAl alloy shows capabilities against oxidation even at a high temperature of 1300°C, the oxidation behavior and mechanism are distinct from those at moderate temperatures (<1000°C). Different morphologies and phase structure were found in oxide scales generated at different temperatures within the same time duration. Typical buckling was observed in the super alloy when it was subjected to 1200°C. Equiaixed grains with multiple voids were found near the alloy surface. At 1300°C, a flat and thicker oxide layer was formed. The grains were stretched vertically against the alloy and presented as coarse and compact near the interface. The vertically stretching of grain was triggered by fast element transportation inside the alloy. The differences in grain morphologies among the different test temperatures demonstrated that although the super alloy followed parabolic law at both test temperatures, the oxidation processes were different due to the evolvement of grain morphologies and oxide scale structures caused by exposure to high temperature.