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°.

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.

Erosion and abrasion are both wear processes in which a particle that strikes the surface removes material, either by impact (erosion) or contact (abrasion). These wear processes can cause damages to components, which can be protected by coatings to reduce the damage occurrence. Thermally sprayed coatings are considered candidates for a protective system against abrasion and erosion. The HVOF spray process is one of the most used thermal spray processes due to the ability of producing dense coatings, with good values of hardness and toughness. Among the materials used in thermally sprayed coatings, WC-Co based coatings are often used, as it offers a combination of high hardness, toughness and adherence, which can provide a good wear resistance. In this work, the influence of different HVOF process parameters, specifically the type of fuel used, on the residual stresses and properties of these coatings was studied. It was noted that coatings deposited by HVOF with kerosene liquid fuel, presented lower porosity, compared with coatings deposited by gas fuel. It was also observed that the coatings with lower porosity provide a better abrasion resistance, meanwhile the erosion was controlled by toughness for 30° and higher hardness for 60° of impact angle.

To evaluate the corrosion resistance of various coatings applied by the flame spraying method, and to compare their protective properties in sweet- and sea-water samples with seven types of coatings were made, one of them is paint, used to coat ship’s bottom. SEM, Neutral spray salt, according to ASTM B117, erosion-corrosion (E-C) tests in sweet- and sea-water were carried out. The test results showed that coatings based on zinc alloys and polymer are much more effective than standard epoxy coatings and can be compared with expensive zinc-filled paints, because the latter require frequent repairs and repainting. Moreover, the developed coatings are supposed to protect against fouling. At the moment, the fouling tests are continuing.

In the current work, Ni-20Cr coatings have been developed for potential use in harsh environments of power plant boilers. A pre-synthesized Ni-20Cr nanocrystalline powder was deposited on T22 boiler steel using cold-spray process. The high temperature oxidation behavior of the coating was investigated under cyclic conditions at 900° C for 50 cycles, so as to understand the kinetics of oxidation. Moreover, high temperature erosion-corrosion (E-C) behaviour of the coating was ascertained under cyclic conditions in an actual boiler at 740 ± 10°C for 1500 hours. The oxidized and eroded-corroded samples were characterized using X-ray Diffraction (XRD), Scanning Electron Microscopy/Energy Dispersive Spectroscopy (SEM/EDS) analyses. The microhardness, oxidation and E-C data for the developed coating was compared with an earlier reported cold-spray Ni-20Cr coating, which was developed by using a commercially available micron-sized Ni-20Cr powder. The results showed that the developed coating was found to have 33% high microhardness in comparison with the microstructured Ni-20Cr coating. The oxidation and E-C rates of the steel were found to decrease significantly after the application of the developed coating by 89% and 68% respectively. Moreover the nanostructured coating outperformed the corresponding micro-structured Ni- 20Cr coating with regard to high temperature oxidation and E-C resistance to boiler steel by a significant fraction. The investigated coating was found to have oxidation protective oxides such as Cr 2 O 3 and NiO in its oxide scale and was found to be spallation-free.

Rare earth has been widely used in materials manufacturing to improve hardness and toughness. In this paper, conventional, nanostructured and CeO2 modified WC-12Co powders were sprayed by using HVOF spraying technology. Erosion-corrosion behavior and interaction role of erosion and corrosion of the three coatings in 3.5wt% NaCl solution were investigated. In situ observation method was employed to analyze failure mechanism. The results showed that CeO2 modified WC-12Co coatings possessed the best erosion-corrosion resistance, which is the worst for the conventional WC-12Co coating. The addition of rare earth CeO2 into conventional WC-12Co coating can improve the erosion-corrosion behavior. Results suggested that the erosion-corrosion mechanism in the three coatings was dominated by corrosion-enhanced erosion. However, the extent of corrosion enhanced erosion was different.

Fast streaming fluid media causes cavitation-erosion in pumps, ship-propellers and rudders. To avoid severe damages, materials with high resistance against plastic deformation and a high fatigue strength should be used. Bronzes fulfill these criteria, but as-cast bulk parts are rather costly. A promising alternative is cold-spray deposition of dense and oxide-free coatings onto exposed surfaces. To achieve high quality bronze coatings by cold-spraying, parameter optimization has to tackle the high hardness of the feedstock powder materials. Additionally, practical limits due to nozzle clogging have to be considered, which may occur at gas temperatures above 700 °C. The present study investigates possible solutions by systematic process parameter and feedstock material optimization, including variation of bronze compositions. Thus, dense coating microstructures and - in consequence - high hardness and good cavitation resistance were obtained. Cold-spray coatings reach up to 8 times better cavitation resistance as compared to conventional ship-building steel (GL-A).

The aim of this study is to investigate the role of zirconium (Zr) in HVOF coatings in order to formulate a high-temperature oxidation-resistant composition. Ni-20Cr coatings with and without 1% Zr additions were deposited on 347H boiler steel by HVOF spraying and their oxidation behavior was assessed at a temperature of 900 °C under cyclic conditions. Test specimens were also subjected to actual boiler environments to ascertain their erosion-corrosion behavior. Weight change measurements were used to determine high-temperature oxidation kinetics as well as erosion-corrosion resistance. The specimens were examined after each thermal cycle to observe physical changes and spallation in their oxide scales. It was found that the addition of Zr in the Ni-20Cr coating reduced the oxidation rate but had little effect on erosion-corrosion resistance.

This study evaluates Fe-Si intermetallic powders as an alternative to powders currently used to coat furnace walls in pulverized coal fired boilers. The developed powder mainly consists of Fe 2 Si, which has a relatively low melting point among iron silicides. The powders were deposited on CrMo steel substrates by HVOF and atmospheric plasma spraying and the resulting coatings were subjected to corrosion and erosion testing. Under conditions simulating the operating environment in a low NO X boiler, the HVOF sprayed Fe-Si coatings exhibited sulfidation resistance nearly equal to that of Cr-Ni layers, and in high-temperature erosion tests, the APS intermetallic coatings with boron additions were found to be more erosion resistant than conventional Cr 3 C 2 -NiCr coatings.

This paper describes an investigation of the influence of impingement angle of a solid/liquid jet upon the erosion corrosion behaviour of a tungsten carbide-cobalt-chromium thermal sprayed coating. The coating type investigated was a nominal WC-10%Co-4%Cr material, HVOF-sprayed onto a stainless steel substrate. This coating was subjected to a submerged impinging jet at 12 m/s of 3.5% NaCl solution containing various concentrations of suspended sand particles at 18°C. The angles of impingement employed were 30, 45, 60, 75 and 90 degrees and the measured total weight losses exhibited a significant influence of impingement angle with reduced material losses at more oblique angles of jet impact. The implementation of cathodic protection to the specimens enabled the pure mechanical component of the overall erosion-corrosion damage to be determined and this was also found to be dependent upon the angle of impingement of the liquid/solid stream. In contrast, the pure corrosion component (determined from in-situ electrochemical monitoring) exhibited no systematic trends with impingement angle. The findings are discussed in terms of the detailed erosion-corrosion mechanisms and implications for operational durability of cermet coatings

Sintered protective coatings are a proven technical solution against corrosion and erosion of tubes in biomass- and waste-fired boilers. The main field of application are tubes like superheater tubes or heat exchanger tubes in fluidized-bed installations corrosion endangered by the flue-gas stream. These sintered solutions complete other thermally sprayed coatings in those critical cases, where gas-dense overlays are needed due to corrosive damage. In those cases, where oxidation and erosion dominates, thermal spraying without sintering is remaining as primary solution. The development of Twin coatings out of sintered and non-sintered thermally sprayed alloys is showing a new and interesting alternative for tubes exposed to erosive-corrosive stresses under high temperature conditions.

This paper describes results from investigations of tungsten carbide (WC) coatings with high alloy binders. Experimental powders were manufactured by agglomeration/sintering using pre-alloyed metal particles together with WC particles. Coatings were sprayed by the HVOF process and investigated with respect to microstructure, hardness, erosion and erosion-corrosion. For comparison coatings from commercial powder materials were sprayed and investigated. This and previous work has demonstrated the importance of having a corrosion resistant binder. A corrosion resistant binder is of major importance for constructions where maintenance is difficult for instance in constructions for oil and gas production from wells at large depths in the North Sea. Improved coating quality will lead to increased life time for coated components, improved safety conditions, fewer shut-downs and large reductions in maintenance costs.

The significance of biofuels and the other chlorine-containing fuels in energy production is in strong increase. Serious erosion-corrosion problems in boilers combusting fuels with high chlorine-content have been detected frequently. A series of erosion-corrosion and corrosion tests were performed on thermal sprayed coatings and coating precursors in chlorine-containing environments in order to evaluate possibilities to utilize thermal sprayed coatings for erosion-corrosion protection in boilers combusting chlorine-containing fuels. A series of hot erosion and erosion-corrosion tests were performed on thermal sprayed coatings at elevated temperatures with and without chlorine. Carbide-containing HVOF coatings performed well in hot erosion tests, but they were completely destroyed in the presence of chlorine due to rapid oxidation of carbides. Metallic HVOF coatings with high chromium content performed well in both conditions. Iron-based arc-sprayed coatings with unhomogeneous microstructure suffered more hot erosion and erosion-corrosion damages than metallic HVOF coatings. The E-C (erosion-corrosion) resistance of carbide-containing coatings in the presence of chlorides was worse than expected. A series of oxidation tests were performed on various carbides in order to elucidate the effect of chlorine on high temperature oxidation behavior of carbides. TGA and isothermal oxidation tests proved that gaseous chlorine-containing species and also solid chlorides have a detrimental effect on oxidation resistance of tested carbides.

This paper describes and discusses aspects of the erosion-corrosion behaviour of a WC-Co-Cr, HVOF sprayed coating when subjected to an impinging jet of an aqueous solution of 3.5% NaCl containing solid particles at 18°C. Although pure mechanical erosion has been found to contribute to a large part of material degradation, a considerable amount of material loss can be attributed to the direct and indirect (synergy) effect of corrosion. Aspects of the influence of time, solids loading and impingement angle have been investigated and the mechanisms of erosion-corrosion are discussed.

Due to the extreme conditions experienced in gas turbine engines, especially aero-derivative type engines, internal components need to be protected against adverse effects in order to maintain component integrity and engine performance between overhauls. Among these adverse conditions are wear in the form of friction and fretting, erosion and various forms of corrosion. This paper focuses on fretting wear. To reduce coating costs, shorten the dwell time, and provide additional options for the repair of industrial gas turbine components, it presents a comprehensive study initiated by Rolls-Royce to determine the feasibility of the Sulzer Metco HVOF process as an alternative to the evaluate the D-Gun process. Based on the results of this study, it was concluded that tungsten carbide and chromium carbide sprayed with the Diamond Jet Hybrid can be used successfully as coating alternatives to the D-Gun. Paper includes a German-language abstract.

This paper compares three thermally sprayed coatings with different carbide types in a NiCr matrix (25% by volume) with regard to their erosion, corrosion, and wear resistance. The three types of carbide are WC, chromium carbide, and TiC. The coatings were sprayed with the Sultzer-Metco Diamond-Jet 2600 equipment. All coatings were produced with the same spray parameters. If the sand particles hit the surface at high angles of impact, the WC coating is better than the other two coatings in terms of erosion/corrosion resistance. Micro-abrasive wear tests also showed the WC coating was superior (slightly better than the TiC coating). The corrosion properties of the coatings were almost the same. Paper includes a German-language abstract.

The elevated temperature erosion resistance of experimental amorphous thermal spray coatings was determined in a laboratory elevated temperature erosion tester. Test conditions attempted to simulate the erosion conditions found at the combustor waterwall tubes in fossil fuel fired boilers. Erosion tests were conducted on four experimental amorphous thermal spray coatings, using the bed ash retrieved from an operating coal fired boiler. An experimental arcspray process was used to spray coatings. These results were compared with erosion test results of two common structural materials, two commercially available arc-sprayed coatings, and a proprietary HVOF coating. Test results indicated that the Duocor coating had the highest erosion resistance among the four experimental coatings, it showed equal resistance to the HVOF coating (DS-200). Compared to AISI 1018 steel, both Duocor and DS-200 coatings reduced material wastage by 26-fold. Other test results indicated that the XJ-16, 60T and Armacor M coatings had equal erosion resistance reducing material wastage approximately 7-fold, while Armacor CW reduced by 10-fold. Only slightly better than the unprotected 1018 steel, the X-20 coating performed poorly on erosion tests. The high erosion resistance of Duocor and DS-200 coatings was attributed to their high densities and fine splat structures.

Different WC-based coatings have been sprayed by the HVOF process and characterised by different methods. The influence of different powder characteristics on the wear and corrosion performance was investigated. Powder characteristics investigated include: i) average WC particle size, ii) matrix composition and iii) powder grain size distribution. The results show that the coating properties are affected by the different powder characteristics. Small WC particles were found to be beneficial as regards the erosion and erosion-corrosion resistance. An increase of the Cr and Mo content increased the corrosion and erosion-corrosion resistance. Powders with narrow grain size distributions gave coatings of higher quality than powders with wider grain size distributions.

In corrosive media the wear resistance of ceramic-metallic coatings is dependent on the corrosion resistance of the metal matrix. Other factors that will affect the coating deterioration are the corrosivity of the medium and any galvanic interaction from the surrounding material. This paper presents results from a study where different types of WC(Co/Cr/Mo/Ni) powders have been sprayed by HVOF, Diamond Jet 2600 Hybrid equipment. The properties of the sprayed coatings have been verified by metallographic studies and by erosion-corrosion testing both under corrosive and non-corrosive conditions. The results clearly demonstrate the importance of having a metal matrix at least as corrosion resistance as the surrounding materials. When wear exposed components in pipe systems, pumps or valves are coated with a WC type coating, the corrosion resistance of the metal matrix should be compatible to the material of the rest of the system. This is especially important when the surrounding materials are corrosion resistant alloys as stainless steels, where the coatings otherwise will act as an anode.

WC-Co-Cr powders with different WC particle size have been sprayed by the HVOF process. At constant spraying conditions the powders give coatings of different quality. The deposition efficiency during spraying of powders containing large WC particles was found to be low compared to powders with finer WC grains. In addition the amounts of porosity and cracks were different. The coatings have been characterised by different methods. Erosion and erosion-corrosion tests showed that the WC particle size also influence the wear resistance of the coatings. Small WC particle size was found to be beneficial. Chemical composition of the matrix was also found to be decisive for the coating properties. An increase of the chromium content improved the erosion-corrosion resistance.