Hexavalent chromium has been widely used in the coating industry and more specifically in gas turbine hot end component protection. UK REACH (registration, evaluation, authorization, and restriction of chemicals) have given an end date of September 2024 for the use of hexavalent chromium and as such, the industry must adapt to these regulations. Indestructible Paint LTD have developed a new aluminium diffused slurry coating, CFIPAL, that does not contain hexavalent chromium like its predecessor, IP1041. Both CFIPAL and IP1041 were deposited onto Nimonic 75 alloy and underwent metallurgical and chemical analysis which included scanning electron microscopy, energy dispersive spectroscopy, electron backscatter diffraction, hardness testing, contact angle testing, surface roughness testing and finally, salt spray corrosion testing. The results indicated that CFIPAL is a suitable alternative for hexavalent chromium-containing coatings, such as IP1041.

Plasma spraying is the most versatile coating process for depositing a wide range of materials to enhance material performance in harsh conditions. However, severe oxidation during the plasma spraying of metal coatings often results in coatings with high oxide content, limiting interlamellar bonding. Consequently, as-sprayed metal coatings offer inadequate protection against severe corrosion and wear. To address this challenge, we developed Ni-, Cu-, and Fe-based materials containing boron as a deoxidizer. This innovative approach effectively suppresses in-flight oxidation, producing oxide-free molten droplets and enabling the formation of bulk-like metal coatings with sufficient metallurgical bonding between splats. We employed a modified tensile test to evaluate the adhesive and cohesive strengths of these coatings. The Ni-based coatings exhibited adhesive strength exceeding 150 MPa on Fe-based substrates, while cohesive strength surpassed 260 MPa with a novel bond coat. Corrosion and gas penetration tests confirmed the creation of dense, bulk-like Ni-based alloy coatings, demonstrating their potential for various applications in severe service environments.

High-pressure die casting (HPDC) is a well-established manufacturing process used in the automotive sector to make high-precision components. The necessity to reduce fuel consumption increases the use of low-density components in the automotive industry. Corrosion induced by molten metal is one of many failure modes for dies, changing the die's geometry and surface roughness. All combined wear changes the dimensional precision of the manufactured parts but also the surface quality of the components. Many additive deposition methods are applied to decrease wear and recover the surface. Thermally sprayed coatings can improve the surface properties and recover the geometry of the die caused by the aluminum attack. The main objective of this work is to observe the behavior of the H13, Cr3C2-25NiCr, and WC10Co4Cr coatings deposited by HVOF and HVAF, tested against Aluminum corrosion and Die-soldering tests. After dissolution, the chromium carbide reacts with the aluminum, creating a tough intermetallic interface, and raising the extraction tensile stress. After Aluminum corrosion tests, it was observed that the WC 10Co 4Cr HVAF coating presented low adhesion to the aluminum with no observed coating failure due to the formation of intermetallic. Die soldering tests indicated that the WC 10Co 4Cr protects the substrate, resulting in lower extraction tensile stress than H13 base material and other HVOF coatings. It was possible to observe that WC 10Co 4Cr HVAF coating showed results comparable to AlCrN PVD coating.

Due to their excellent corrosion resistance, austenitic stainless steels are suitable for surface protection applications. However, the application potential is often limited by the low wear resistance. An interstitial hardening of the surface layer area can solve this problem for massive wrought alloys. Further potential for improvement lies in the transition to surface technology. For this purpose, powder feedstock of the stainless-steel grade AISI 316L was gas nitrocarburized at low temperatures. The formation of a metastable expanded austenitic phase was achieved. Subsequently, the processing was carried out by cold gas spraying. Due to the simultaneously high process kinetics and low thermal load, dense coatings were produced while maintaining the metastable state of the feedstock. When compared to solid reference systems, the scratch resistance saw a marked improvement.

Thermally sprayed Al 2 O 3 -TiO 2 ceramic coatings provide exceptional hardness and corrosion and wear resistance, but the high velocities at which they are applied result in an inherently porous structure that requires some type of remediation. This study evaluates the effectiveness of ultrasonic aluminum phosphate sealing treatments on plasma sprayed Al 2 O 3 -40TiO 2 ceramic coatings. The sealants were applied with and without ultrasonication (20-40 kHz) and were assessed using SEM/EDX analysis, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS). Test data indicate that optimum sealing, corresponding to the highest values of corrosion protection and erosion resistance, are achieved under ultrasonication at 30 kHz for 5 hours.

A typical structure of thermal spray coatings consisted of molten particles, semi-molten particles, oxides, pores and cracks. These factors caused the porosity of sprayed coatings, leading to a great influence on the coating properties, especially their wear-corrosion resistance. In this study, a post-spray sealing treatment of Cr3C2-NiCr/Al2O3-TiO2 plasma sprayed coatings was carried out, then their corrosion properties were evaluated, before and after the treatment. For sealing process, aluminum phosphate (APP) containing aluminum oxide (Al2O3) nanoparticles (~10 nm) was used. The permeability of APP into the sprayed coating was analyzed by scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS). The treatment efficiency for porosity and corrosion resistance of sprayed coatings were evaluated by electrochemical measurements, such as the potentiodynamic polarization and electrochemical impedance spectroscopy. In addition, the wear-corrosion resistance of the sealed coating was examined in 3.5 wt.% NaCl circulation solution containing 0.25 wt.% SiO2 particles. The obtained results showed that APP penetrated deeply through the sprayed coating. The incorporation of Al2O3 nanoparticles into APP sealant enhanced the treatment efficiency of porosity for sprayed coating. The effect of the post-treatment on corrosion protection of the sprayed coating has been discussed.

High-entropy alloys (HEAs) represent an innovative development approach for new alloy systems. These materials have been found to yield promising properties, such as high strength in combination with sufficient ductility as well as high wear and corrosion resistance. Especially for alloys with a body-centered cubic (bcc) structure, advantageous surface properties have been revealed. However, typical HEA systems contain high contents of expensive or scarce elements. Consequently, applying them as coatings where their use is limited to the surface represents an exciting pathway enabling economical exploitation of their superior properties. Nevertheless, processing conditions strongly influence the resulting microstructure and phase formation, which in turn has a considerable effect on the functional properties of HEAs. In the presented study, microstructural differences between high-velocity oxygen fuel (HVOF) and high-velocity air fuel (HVAF) sprayed coatings of the alloy AlCrFeCoNi are investigated. A metastable bcc structure is formed in both coating processes. Precipitation reactions are suppressed by the rapid solidification during atomization and by the relatively low thermal input during spraying. The coating resistance to corrosive media was investigated in detail, and an improved passivation behavior was observed in the HVAF coatings.

Cold spraying is a semi-empirical method in that spray parameters must be optimized experimentally in regard to coating quality and deposition efficiency. In this work, porosity and deposition efficiency are the key parameters in the optimization of corrosion-resistant zinc coatings produced by high-pressure cold spraying. The deposition process is described along with the tests used to assess the morphology, adhesion, and anticorrosion properties of the coatings obtained.

In this study, nickel-base superalloy coatings are deposited on CrMoAl substrates by different spraying methods and the coatings are evaluated based on their microstructure, phase composition, surface and splat morphology, bond strength, and corrosion properties. In the experiments, NiCrAlY powders with a particle size range of 30-45 μm were sprayed by conventional air plasma and a new laser hybrid plasma spraying technique. The spraying parameters are presented along with test results, observations, and conclusions. The coatings produced by laser hybrid plasma spraying had an average porosity of 0.9%, a bonding strength of 117 MPa, and significantly better corrosion properties than the APS layers.

In recent studies, crack formation was observed in oxidized areas of wire-arc sprayed Zn-Al coatings. As corrosion tests show, these cracks allow electrolyte to penetrate the coating, reducing effective service lifetime. Wire-arc sprayed coatings usually exhibit tensile residual stresses with the potential to cause such cracking. To determine the extent of that potential, the stress state of Zn-Al coatings was measured and correlated with corrosion test results. Residual stress was obtained using the sin2ψ method based on XRD analysis and the results are combined with those of previous studies, forming a hypothesis for the root cause of crack formation in wire-arc sprayed Zn-Al coatings, its effects, and its control.

Ceramic coatings are often applied to metallic substrates using a bond coat to promote adhesion. The corrosion environment between the substrate and ceramic layer can be very harsh due to the absence of dissolved oxygen, high concentrations of corrosive electrolytes, and galvanic and crevice corrosion mechanisms within the bond coat itself. This study assesses the performance of several bond coats in sulfuric acid, including plasma sprayed tantalum and HVOF sprayed NiCr, Hastelloy C, and Ultimet. The bond coats were deposited on Hastelloy substrates and covered with a Cr 2 O 3 topcoat. Electrochemical polarization and open circuit potential measurements were recorded and coating microstructures were examined before and after the corrosion tests. Plasma sprayed tantalum exhibited the best corrosion resistance at room temperature, followed by HVOF sprayed Hastelloy C.

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.

The economic use of offshore wind turbines requires a reliable and long-lasting corrosion protection. Sophisticated multilayer coating systems consisting of a thermal spray coating – mainly ZnAl15, a sealer and several layers of organic coating – have been proven to provide such protection. Damages to these duplex-coatings can, however, not be prevented necessitating on-site repair. In case of severe damages, the remaining coating close to the damage is often removed and subsequently, the duplex coating is rebuilt from scratch. In the present study, two integrated coating removal and substrate pre-treatment methods are investigated. For this purpose, duplex-systems were produced, artificially damaged by milling and afterwards treated by either grit blasting or with a rotating steel-wire brush, i.e. a Monti Bristle Blaster. Afterwards, the duplex coating was re-applied in the considered area. To evaluate the influence of the pre-treatment method on the coatings’ corrosion protection potential, a 38 week-long salt spray test was used. The test revealed a pronounced influence of the pre-treatment method on the corrosion protection potential. In case of grit blasting, no substrate corrosion could be detected. The use of a Bristle Blaster, however, resulted in coating failure and some spots of red rust.

Nowadays combustion engines are the most common way to impel vehicles. Thereby losses occur, due to cooling, exhaust gas and friction. Modern engines roughly dissipate 8% of the chemical energy stored in the fuel because of friction in different tribological systems. The highest potentials for optimisation can be found in the tribological system of inner surface of combustion chamber and piston ring. Besides friction, corrosive stress of inner surface of combustion chamber increases e.g. due to the utilization of auxiliary systems such as Exhaust-Gas-Recovery. In order to save energy, reduce emissions and enhance the lifetime of combustion engines innovative coating material systems need to be developed, especially for inner surface of combustion chamber. This study focuses on the development of innovative iron based materials for combustion chamber application using Plasma Transferred Wire Arc (PTWA) and Rotating Single Wire Arc (RSW) technologies. In order to improve the wear and corrosion resistance boron and chromium are added into the feedstock material. After deposition, different honing topographies are manufactured in order to evaluate their influence on the tribological behavior. Furthermore, electro-chemical corrosion tests are conducted by using an electrolyte simulating the exhaust gas concentrate. In conclusion an optimised coating material deposited by PTWA and RSW and improved surface topographies can be combined.

Three Fe-based powder alloys, Höganäs Fe SP529, Fe SP586 and 6AB, have been deposited by HVOF and HVAF spraying onto mild steel plates. The sprayed samples were first ground and then shot peened using glass shot in order to seal the surface interconnected pores and other surface imperfections. The samples as ground and ground/glass shot peened were tested by salt spray (fog) exposition for 238 h according to ASTM B117/ISO 9227. FeSP586, HVOF and HVAF sprayed and glass shot peening samples achieved surface sealing enough to pass the test with appearance rating RA = 9 according to ISO 10289. All other samples achieved moderate to excessive pitting and/or moderate to excessive staining types of corrosion defects.

300M steel is one of most important aerial materials, which can be used for landing gear and flap & slat track. Some surface engineering technologies are needed to be adopted on its surface, because of its bad corrosion performance. WC10Co4Cr Coatings by high velocity oxygen-fuel spray processing (HVOF) is an environmental friendly method for this protection. In this paper, WC10Co4Cr coatings were prepared on 300M by optimized HVOF processing. And their corrosion performance has been estimated by neutral salt fog test, according with ASTM B117. The results indicate that the porosity gets larger and the hardness gets higher for the dissolution of bonding phases after the test. And for the optimized coatings, there are no corrosion products in the surface and interface between the coating and 300M steel, after 2000 hours ASTM B117 test. So the coatings have a good corrosion performance.

It is well known that the presence of KCl deposited on superheater tubes in biomass- and waste-fired boilers leads to a severe corrosion and premature damage. In order to protect such critical components which are routinely exposed to aggressive environments, thermal sprayings are frequently proposed as a potential solution. By virtue of the techno-commercial benefits that provides as a direct outcome of its ability to cost-effectively deposit coatings virtually free of porosity and in situ formed oxides, the high velocity air-fuel (HVAF) process offers a particularly attractive approach. In the present work, the influence of KCl on the oxidation behavior of four HVAF-sprayed Ni-based coatings (Ni21Cr, Ni5Al, Ni21Cr7Al1Y, and Ni21Cr9Mo) has been investigated. The coatings were deposited onto specimens of 16Mo3 steel, a widely used boiler tube material. High temperature corrosion tests were carried out in ambient air at 600°C, with 0.1 mg/cm2 KCl being sprayed onto the samples prior to the exposure. Uncoated substrates and an identical test environment without KCl were used as reference. SEM/EDS and XRD techniques were utilized to characterize the as-sprayed and exposed samples. The results showed that the small addition of KCl significantly accelerated damage to the coatings. It was further revealed that the alumina-forming NiAl coating was capable of forming a more protective oxide scale compared to other chromia and mixed-oxide scale forming coatings. In general, the oxidation resistance of the coatings based on the kinetic studies had the following ranking (from the best to the worst): NiAl >NiCr> NiCrAlY> NiCrMo.

In this study, HVOF sprayed NiCr alloy coatings on A213 TP347H boiler steel are evaluated by severe corrosion tests in a high-temperature molten salt environment. XRD and FE-SEM/EDS analysis results are presented and discussed and correlated with corrosion kinetics.

This study investigates the formation of ZnNi/Cr 2 O 3 -TiO 2 -CuO-SiO 2 /PTFE composite ceramic coatings by atmospheric plasma spraying and assesses their ability to improve the corrosion, friction, and wear performance of reciprocating parts. The as-sprayed coatings were examined then subjected to a series of tests to evaluate corrosion and fouling resistance. Reciprocating parts that had been coated were relatively intact after 5000 h in a realistic ocean environment. Cyclical changes in coating weights were found to be influenced by the dissolution of oxide films and the accumulation of secondary products. PTFE proved to be an effective sealing agent, reducing mass loss and porosity by approximately 30%.

In this study, the in-situ corrosion behavior of an Fe-based amorphous coating is investigated in a simulated deep sea environment (80 atm). FeMoCrYCB powder produced by gas atomization was deposited on 316L stainless steel substrates by HVOF spraying. The amorphous iron coatings exhibited greater pitting resistance than stainless steel under high hydrostatic pressures, evidenced by higher pitting potential, longer pitting incubation time, and reduced pitting growth. Passive films that formed on the amorphous coatings were also analyzed and found to be thicker, more uniform, and harder than those that developed on 316L stainless steel, indicating that the former are more difficult to break down and more resistant to Cl- ion penetration.