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.

Erosion-corrosion is a severe problem observed in the coal fired thermal power plant boilers which lead to premature failure of boiler tubes. Thermal spray coatings have been applied successfully to check the erosion-corrosion of boiler tubes. In the present research work NiCrTiCRe coating powders were successfully deposited on T22 boiler steel by two different coating processes i.e. high velocity oxy-fuel (HVOF) and cold spray process. The performance of the coatings in actual power plant boiler were investigated and compared. The uncoated and coated T22 boiler steels were subjected the superheater zone of the coal fired boiler for a total of 15 consequent cycles. The thickness loss data and weight change analysis were used to establish kinetics of the erosion-corrosion. X-ray diffraction, surface field emission scanning electron microscope/energy dispersive spectroscopy (FE-SEM/EDS) techniques were used in the present work for the analysis. The results of thickness loss data indicated that the cold sprayed coating performed better in thermal power plant boiler environment.

One of the promising thermal barrier coatings (TBC) options for use above 1250 °C has been La 2 Ce 2 O 7 (LC). This work explored the role of dual layered ceramic coatings in the top layer of the TBC system that has been prepared using atmospheric plasma spraying (APS). Above the NiCrAlY bond coat, 8 mol.% yttria stabilized zirconia (8YSZ) coating has been deposited with optimized APS parameters. Over the top layer (8YSZ), another layer that comprises composite with LC and 8 wt.% of 8YSZ (spray dried) has been deposited. Investigations into the hot-corrosion behavior of 8YSZ-LC based TBC subjected to Na 2 SO 4 +V 2 O 5 salt at 950 °C for 4 hours. A porous layer made mostly of LaVO 4 , CeO 2 , CeO 1.66 and YVO 4 was developed on the LC+8wt.% YSZ layer after being subjected to a hot corrosion test in Na 2 SO 4 +V 2 O 5 salt. Dissociation of LC and 8YSZ leads to the formation of new phases, such as CeO 1.66 , CeO 2 , LaVO 4 and YVO 4 as the corrosion by-products in the extreme environment. The findings indicated that delamination has occurred due to the phase transformation, cavities and cracks in the 8YSZ-LC based TBCs. The molten salt's hot corrosion mechanisms of the 8YSZ-LC based TBC are discussed in detail. Further, the potential use of 8YSZ-LC based dual coatings and scope for the future work have been derived from the current study.

The US Navy has adopted High-Pressure Cold Spray (HPCS) as a repair technique for corroded and worn components in their fleet of aircrafts, ships, and submarines. HPCS repairs are not only used for depositing corrosion and wear resistant coatings but is being successfully used for dimensional restoration in metal parts and components. By utilizing HPCS, the Navy ensures the longevity and reliability of critical components, even in harsh environments. Whether safeguarding against corrosion or restoring worn parts, HPCS is playing an increasingly critical role in maintaining operational readiness for the US Navy and other DOD agencies, as well as the commercial industry.

Bioceramics deposition on medical devices is a widespread area of research for biomedical industries since such deposits can induce excellent chemical and biological properties to the devices. Thermal spraying has been a popular choice for developing coatings to enhance the mechanical, chemical, and biological responses of medical devices. However, the high heat involved during the thermal spraying of bioceramics limits their functionality. In this context, low processing temperature in cold spraying is believed to protect the bioceramics from degradation. However, depositing bioceramics using cold spray and achieving good mechanical properties are still challenging tasks because of their poor ductility. Therefore, bio metal matrix composites with reinforced bioceramics produced using cold spray are expected to give good mechanical, chemical, and biological properties. The present work presents deposition and microstructural characterization of titanium/hydroxyapatite (Ti/HA) and titanium/baghdadite (Ti/BAG) composite coatings by cold spraying. Furthermore, the comparative corrosion response of these coatings under a simulated body fluid environment is reported. The effect of laser remelting on microstructure and corrosion behavior is also discussed.

In our previous work, the potential of the suspension-HVOF spraying (S-HVOF) to produce dense-structured WC-12Co coatings has been shown. This contribution proposes a comparative study of the corrosion properties of the S-HVOF WC-12Co coatings and conventional sprayed HVOF coatings. The corrosion properties were evaluated at room temperature in NaCl electrolytes with different pH values and in a pH neutral 0.5 M Na 2 SO 4 solution. By varying the pH value, the corrosion mechanism of the cemented carbide coatings should be assessed more precisely, since the two components, WC and Co, show strongly different pH dependencies. The electrochemical properties of the sprayed coatings were investigated using open circuit potential measurements, linear sweep voltammetry and potentiodynamic polarization methods. Before and after corrosion tests, microstructural evaluations of the coatings were performed. Moreover, element analyses of the eluates have been performed to determine soluble corrosion products. The S-HVOF coatings show a similarly good corrosion resistance as the conventional HVOF WC-Co coatings. Generally, the coating properties, i.e. microstructure and phase compositions, as well as the electrolyte significantly influence the corrosion performance of the sprayed coatings.

Since the plasma sprayed coatings always present a limited interlamellar bonding, it is difficult for a plasma sprayed coating to be applied in corrosion environment without any post-spray treatment. In this study, a NiCr powder alloyed with boron was employed to fabricate fully dense corrosionresistant coating by plasma spraying through in-situ deoxidation effect of boron. As reported previously, plasma sprayed Ni 20 Cr 4 B coating presents fully dense microstructure with few isolated pores. Due to the oxide-free state of the inflight particles by the deoxidation effect of boron, the splats were effectively bonded upon impact so that the inter-splat boundaries were indiscernible. A long-term immersion corrosion test in NaCl solution was conducted for 80 days to confirm that the plasma sprayed Ni 20 Cr 4 B coating presents the superior resistance against the corrosion, which was comparable to the flame spray-fused NiCrBSi coating. Furthermore, the cross-sectional microstructure of the Ni 20 Cr 4 B coated Al alloy samples after 80 days immersion revealed that the plasma sprayed Ni 20 Cr 4 B coating was dense enough to completely block the penetration of corrosive substance in such an aqueous corrosion environment.

Despite their light weight, 2.3 times lighter than Al, polymers are limited to application with low thermal, wear, and abrasion demands. The enhancement of the functional surfaces of the polymers using thermal spraying techniques is a challenging task due to the thermal degradation of polymers, the low wettability, and the disparate atomic properties. The twin-wire arc spraying (TWAS) process comprises two contradictory features. Almost all spraying particles are in a molten state on the one hand, and on the other hand, the spray plume has the lowest heat output among the different thermal spraying techniques. Therefore, it is a promising spraying technique for the required surface improvement. The surface of the 3D-printed parts was metalized using two successive layers. The first layer is a TWAS coating made of low-melting ZnAl 4 to avoid thermal degradation and provide a bond coat. The topcoat is also applied using a TWAS process and was made out of Ni-WC-Co as cored wires. The top hard coating has improved the wear resistance of the polymers by 14.6 times. The erosion of the coated and uncoated specimens was determined using a low-pressure cold gas spray gun. Ni-WC-Co coating led to more than five times higher erosion resistance.

Stainless austenitic steels like the 316L (1.4404) are widely applied in various applications and were also used for surface protection using thermal spraying. The reason for this is the easy processability and the high corrosion resistance. Stainless austenitic steels typically contain the following alloying elements: The formation of an austenitic microstructure is achieved by nickel (Ni). The addition of chromium (Cr) lead to good corrosion resistance due to formation of an oxide layer. For resistance against pitting corrosion, molybdenum (Mo) can be added. Also, stainless austenites usually exhibit very low carbon and nitrogen contents to prevent chromium carbides and nitrides which reduces the corrosion resistance. However, both alloying elements cannot be classified as being detrimental in stainless austenites in general. In contrast high nitrogen contents can also be used to improve the chemical properties, especially the resistance against pitting corrosion. Finally, carbon and nitrogen lead to an increase in hardness of the thermal sprayed layer. Based on this knowledge, a high-strength austenite for thermal spraying was developed. The new high strength austenite was processed by HVAF spraying with different particle distributions and parameter variations. Resulting coatings were investigated regarding the microstructure, elemental composition, hardness and corrosion properties in comparison to the standard coating material 316L.

Both as bulk material and coatings, cemented carbides currently occupy very well-established market niches and exhibit a promising future thanks to the development of compositions and manufacturing parameters. Direct comparisons of the properties of both are found only very rarely in the literature, very likely because the fields of application are complementary to each other but keep mostly separated. The current work is intended to evaluate similarities and differences in terms of microstructure and properties for two submicron WC-12 wt.%Co coatings obtained by High Velocity Air Fuel (HVAF) and Cold Gas Spray (CGS), together with a conventional sintered part. Microstructural features are discussed according to the inherent characteristics of each processing method. This covers a wide range in terms of the mechanical and thermal stresses acting on the material. While in CGS, the impacting particles do not melt, but experience extremely high plastic strain rates, the cobalt matrix is fully molten in the conventional sintering process, allowing time enough for diffusion processes. HVAF is to be placed in between, since the deposition process is characterized by a moderate heat input, leading to partial and/or full melting of cobalt, followed by rapid cooling. The microstructure and phases of the deposited coatings and bulk are characterized by using Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD). Electron Backscattered Diffraction (EBSD) investigations enable local phase distribution of Co and WC in the samples. The hardness of the alloy processed by the three different routes is investigated as well. Additionally, electrochemical corrosion measurements in NaCl media are presented to evaluate the facility for electrolyte penetration and how the degradation of the material is affected by its inherent microstructure.

Biofouling has been persisting as a worldwide problem due to the difficulties in finding efficient environment-friendly antifouling coatings for long-term applications. Developing novel coatings with desired antifouling properties has been one of the research goals for surface coating community. Recently hydrogel coating was proposed to serve as antifouling layer, for it offers the advantages of the ease of incorporating green biocides, and resisting attachment of microorganisms by its soft surface. Yet poor adhesion of the hydrogel on steel surfaces is a big concern. In this study, porous matrix aluminum coatings were fabricated by cored wire arc spray, and the sizes of the pores in the aluminum (Al) coatings were controlled by altering the size of the cored powder of sodium chloride. Silicone hydrogel was further deposited on the porous coating. The hydrogel penetrated into the open pores of the porous Al coatings, and the porous Al structure significantly enhanced the adhesion of the hydrogel. In addition, hydrogel coating exhibited very encouraging antifouling properties.

High-velocity oxyfuel (HVOF) sprayed coatings of Cr3C2-NiCr containing solid lubricants such as nickel cladded graphite and hexagonal boron nitride were successfully developed and characterised with the aim of optimizing their friction and wear behaviour. HVOF technology was used for the integration of solid lubricants to achieve strong cohesion between particles while minimizing thermal decomposition. Coating microstructure and composition were measured and correlated to the results of tribological and corrosion tests. The integration of the solid lubricant greatly reduced friction and wear volume at room temperature, but the lubricating effect was highly dependent on atmosphere and temperature. Cr3C2-NiCr with hBN, however, tends to exhibit more stable wear resistance over a wider temperature range and can be used at temperatures beyond 450 °C.

In this study, two coatings, one produced by HVOF spraying, the other by physical vapor deposition (PVD), were applied on a nickel superalloy substrate in order to compare their hot corrosion behavior. The coating samples were initially characterized by OM and SEM-EDS analysis, then a mixture of V 2 O 5 , Na 2 SO 4 , and NaCl was deposited on the surface and the samples were to 900 °C for 35 h. The results show that the PVD CrN coating provided better corrosion protection than HVOF-sprayed CrC-NiCrAlY.

This study investigates the synergistic effects of cavitation and corrosion on Cr 3 C 2 -25NiCr coatings with different levels of porosity. The coatings are deposited by HVOF spraying and evaluated based on SEM analysis, Vickers microhardness, potentiodynamic polarization measurements, and cavitation erosion tests in various environments under ultrasonic vibration. The results show that higher porosity reduces both cavitation and corrosion resistance, as expected. However, the samples did not show significant alteration of their cavitation properties in NaCl, probably because of the high corrosion resistance of the different phases in the coating. The influence of HVOF fuel-oxygen ratio and total gas flow on coating porosity, as well as phase morphology, is also discussed.

This paper provides an update on the state of cold spray corrosion mitigation and repair as it applies to equipment operated by the U.S. Navy. It also presents several application scenarios in which cold-sprayed Al 6061 and NiCr-CrC can improve preventative maintenance and dimensional restoration procedures currently used on A36 steel and CuNi structures.

In this study, NbC coatings, 250 µm thick, were deposited by low-velocity flame spraying on stainless steel substrates and were laser remelted in a controlled argon atmosphere. Isolated passes transverse of the coatings were performed at different focal lengths at speeds of 10, 15, and 20 mm/min. Using the selected laser parameters, layers were recast with eight passes at 10% superposition. Erosion-corrosion tests were performed and coating surfaces and cross-sections were characterized via SEM, EDS, and XRD analysis. Modified surfaces of dense, 800-µm thick coatings with no defects and excellent metallurgical bonding with the substrate were obtained. It was found that dilution of the coating with the substrate formed a gradient of chemical composition and mechanical properties and that erosive-corrosive wear resistance was highest for an erodent impact angle of 90°.

Different surface protection technologies were investigated in a waste-wood fired fluidized bed boiler. This biomass fuel environment is more aggressive than those firing virgin wood due to the elevated presence of sodium, potassium, lead, and zinc, leading to the deposit of alkali metal chlorides in conjunction with ash on boiler tube surfaces. As laboratory tests are seldom representative of the complex firing, chemistry, temperature, and local heat flux encountered in actual operating conditions, five different commercial, near commercial, and development coatings were applied to a 1 m length of plain carbon steel tubing used in the furnace walls. The coatings were fully characterized and measured prior to installation and after exposure. Iron and nickel-based weld overlays, two high velocity thermal spray coatings, and a laser-clad nanosteel coating were tested. After exposure, the tube was extracted from the boiler and corrosion scales and material losses were evaluated in comparison to unprotected tube material.

Due to the nonsymmetric distribution of the particle plume in conventional plasma spraying, significant influence of the gun scanning pattern can appear in the structure of the coatings obtained. In this study, three scanning patterns are used to deposit YSZ powder by means of air plasma spraying. Cross-sections of the coatings are examined and interfacial fracture toughness and hot corrosion tests are conducted. Improvements in coating adhesion and corrosion resistance were obtained by modifying the scanning pattern of the gun to decrease the possibility of horizontal weak bonding between spray passes.

This study evaluates the hot corrosion behavior of NiCoCrAlY, NiCoCrAlYHfSi, NiCoCrAlTaReY, and CoCrAlYTaCSi coatings on 1.4923 stainless steel, applied by high-pressure HVOF spraying. All coatings were cycled in in an environment of Na 2 SO 4 and 82% Fe 2 (SO 4 ) 3 at 690 °C. Each cycle consisted of 1 h of heating in a silicon carbide tube furnace followed by 20 min of cooling. Weight change measurements were performed after each cycle to track corrosion kinetics, and SEM and EDS analysis were employed to analyze the corrosion mechanism. CoCrAlYTaCSi showed the best corrosion resistance of the coatings tested.

This work investigates the high-temperature oxidation kinetics of CoCrAlSiY coatings with different Si concentrations. Hot-corrosion resistance is determined at 800 and 900 °C via hot salt coating, thermal shock resistance is measured at 1050 °C, and the oxidation and corrosion products are analyzed through mineralogical and micro analysis. The results show that Si promotes the formation of an Al 2 O 3 film that improves oxidation and corrosion resistance, but excessive amounts reduce thermal shock resistance.