This study demonstrates a two-step laser cladding process for copper substrates in which cold spraying is used as a powder preplacing method to overcome problems associated with the high laser reflectivity of copper as well as the effects of high-temperature oxidation. In the first step of the process, Inconel powders are cold sprayed onto pure copper, producing a layer with a thickness of about 250 μm and a porosity of 0.88%. This is followed by a 3.5 kW laser remelting treatment using a 1030 nm laser with a spot size of 2.5 mm. Examination and testing of the as-sprayed and remelted layers show how the laser treatment improves coating microstructure, hardness, density, and metallurgical bonding.

The high strength nickel alloys, and in particular Inconel type alloys, are extensively used in several applications, such as aeronautics and petroleum industry, thanks to the combination of their high mechanical properties and their thermal and chemical resistance. In particular Inconel 625 is already used in oil pipelines and pipelines of large thermal plants, and the possibility to replace high cost bulk Inconel parts with Inconel coated steel parts is of great interest. On this context the first topic to allow the use of coated parts instead of bulk Inconel is the capability to provide high corrosion and thermal resistance. The aim of this study is to investigate the capability of Coldspray in the deposition of high strength materials, such as hard nickel alloys, for corrosion protection, and to compare the corrosion behaviour of Coldspray coatings with commercial HVOF deposited coatings. Inconel 625 coatings were deposited by using CGT Kinetic3000 deposition system with nitrogen as carrier gas on AISI316L flat substrates. The coating thickness ranges between 0.3 and 1.0 mm. Different feedstock materials were used and the effect of powder size distribution on the growth capability, as well on coating microstructure and porosity, were evaluated. The corrosion behaviour of Coldspray coatings were studied by electrochemical potentiondynamic analysis and compared with the behaviour of commercially available coatings deposited by HVOF that could be considered as a high-quality benchmark. In particular, the effects of the different coating microstructures due to the different deposition processes were related with the corrosion resistance. Further development and key features are finally outlined in order to candidate the Coldspray as promising technology for the deposition of high-strength nickel alloys.

It is well known that residual stresses in plasma-sprayed coatings play a prominent role on coating-substrate adhesion in particular. This is all the more prominent because the coating is thick and adhesion intricate. The control and measurements of residual stresses in plasma-sprayed coatings onto organic-based substrates result in an issue of very high concern, even though very little (not to say nil) was published in this specific area. In this work, thick coating of a polyurethane resin with Inconel 625 was achieved by plasma spraying coupled with Atmosphere and Temperature Control (ATC), i.e. using cryogenic cooling to limit thermal degradation of the substrate when spraying. Residual stresses were determined by X-ray diffraction at the coating surface due to low X-ray penetration in nickel. In addition, residual stress in-depth profiles were obtained using the incremental hole drilling method. The investigation mainly focused on the measurement of residual stresses. In this paper, residual stresses were studied as a function of plasma spraying conditions. Results are discussed in the light of the coating microstructure. Residual stress measurements resulted in optimizing and controlling coating deformation and adhesion which are crucial for applications.

Ni base overlay coatings are being used to protect metallic engineering components in extreme conditions and actually traditional thermal spray deposition technologies such as Air Plasma Spraying (APS) and High Velocity Oxy-Fuel (HVOF) are mainly used to deposit these materials. However, Coldspray is receiving increased attention during the last years because of the lower spraying temperature required to deposit metallic coatings avoiding oxidation and reducing the coating porosity and the amount of residual stresses. The adhesion to the substrate and the growth mechanism of coldspray deposits are based on plastic deformation of impinging particles, so, in the case of high strength materials such as for example Ni alloys, it could be a lack in plastic deformation leading to insufficient compactness of the coating, barrier properties and high temperature resistance. Further improvements in the coatings performances could be attained by post-deposition thermal treatments to enhance coating adhesion and barrier properties. In this sense, the aim of this study is to explore a two-step way to produce high performances Inconel 625 alloy coatings by coldspray deposition followed by a laser glazing treatment. Coldspray Inconel 625 alloy coatings has been deposited onto AISI304 steel substrates. Laser glazing is performed using high power diode laser (HPDL) ROFIN-SINAR 13DS; the local thermal treatment on the coating surface induce microstructural changes which could modify and improve the coating compactness and performances. Coating morphology and microstructure has been evaluated and reported both before and after laser consolidation as a function of different laser conditions.

An important problem for the petrochemical industry is the behaviour of materials in aggressive environments, when hydrogen sulphide, carbon dioxide and sand, which contribute to corrosion erosion of the surface, are present. Generally, the use of hard materials such as thermal sprayed tungsten carbide and chromium carbide reduces this problem. Cemented carbides are quite suitable for this purpose: they are composite materials of pure carbides with binder metal alloys of low melting point and high ductility; the selection of the binder metals depends mostly on its ability to wet the surface of the carbide particles to ensure secure coating adhesion. Among the cemented carbides, namely tungsten carbide cobalt-chromium based (WC/CoCr) is considered as the standard for application to ball valve bodies and seats in the petrochemical field, while chromium carbide nickel-chromium based (Cr 3 C 2 /NiCr) is suitable for particular applications. Inconel 625 is also used in this field and usually applied by welding. This paper addresses the characterization of corrosion behavior of HVOF coated samples of WC/CoCr, Cr 3 C 2 /NiCr and Inconel 625 in aggressive environments, and in particular ferric chloride test according to standard ASTM G48-92 and H 2 S/CO 2 test based on NACE standards has been carried out. According to the test results, WC/CoCr based coatings show the best behavior both in terms of corrosion, thus confirming to be very versatile and useful for the application in petrochemical field.

Arc spraying has always been the most cost-effective way of thermal spraying metal alloys but oxide content and degradation of the alloy by loss of particular alloying elements have limited quality of the coatings. In this paper a process is described which greatly reduces degradation and improves coating density and oxide content. Corrosion behaviour both in aqueous and high temperature environments is markedly improved. For aqueous applications coatings of Inconel 625 were tested in a potentio-dynamic cell and by salt spray testing to evaluate both the inherent properties and the permeability of the coating and significantly improved behaviour was found in both cases. For high temperature corrosion, samples of FeCrAl were tested in air and in a sulphidising environment with and without thermal cycling. Coatings of NiCrAl and NiCrTi were also examined. The coating types and test regimes were aimed at specific practical applications such as fireside corrosion in boilers and waste incinerators, hot oxidation of flare stack burners etc. In the aqueous situation valves and process vessels are being examined as candidate applications for Inconel 625 coatings.

A number of studies have examined the influence of process parameters on the microstructure and corrosion resistance of metallic coatings produced by gas-fuel HVOF spraying, but much less is known about the coatings produced by liquid-fuel HVOF processes. The aim of this study is to investigate the corrosion behavior and microstructure of nickel and cobalt alloys deposited by both methods and show how it compares with that of various superalloys. Paper includes a German-language abstract.

The paper describes the some differences of laser coating (laser cladding or laser spraying) process in comparison to thermal spraying. Laser coating is a novel coating process, which produces coatings with high density, metallurgical bonding and low heat input to the substrate. Laser coating types and coating properties are reviewed and compared with thermally sprayed coatings. Typical application areas of laser coatings are presented.

Inconel alloy coatings have been obtained by means of Cold Gas Spray technology with a high efficiency. Alumina has been sprayed simultaneously achieving an homogeneous distribution along the deposit with a good cohesion between particles. Furthermore, the alumina particles were found to improve the spraying feasibility. A proper optimization of the spraying conditions has been carried out in order to obtain the lowest porosity as possible and good embedment of the alumina. Then, the mechanical properties as well as tribological and oxidation behavior have been characterized. Alumina behaves as reinforcement leading to an amelioration of the abrasive and friction performance. Regarding the oxidation and corrosion behavior, the coatings proved to withstand reasonably well up to 800°C and salt fog tests (5% NaCl solution at 35 °C) indicate durability up to 1000 h.

High velocity oxygen-fuel (HVOF) thermal spray processes are used in applications requiring the highest density and adhesion strength, which is not achievable in most other thermal spray processes. Like other thermal spray processes, however, a normal HVOF process is not able to apply fine powders less than 10 µm via a conventional powder feeder. The advantages of using smaller and even nano-sized particles in a HVOF process include uniform coating, less defective microstructure, higher cohesion and adhesion, full density, lower internal stress and higher deposition efficiency. A new process has been developed to realize HVOF forming of fine-grained alloy layers by using liquid precursors containing fine metallic particles. Process investigations have shown the benefits for making single and duplex layered coatings with full density and high bond strength attributing to the very high kinetic energy of particles striking on the substrate surface and the better melting of the small particles. One of the targeted applications is for the water walls of a fossil-fired boiler that operate in a high temperature and corrosive environment. The new coating system is based on material selection, structure design, process innovation and diagnostics, microstructure, and property evaluation. It is promising to provide better protection of the boilers against various types of degradations like corrosion, oxidation, erosion and interfacial failure.

Residual stress can be developed in most thermally sprayed coatings due to the momentum of molten particles during impact, and heat transfer during solidification of the splats. Another reason for residual stress built-up in thermally sprayed coatings is due to splat curl-up during solidification and the differences in thermal expansion coefficients between the coating and the substrate. However, in the cold spraying process, it is believed that the main reason for residual stress formation is plastic deformation during impact and flattening of solid particles. Residual stresses can drastically influence coating quality and reduce its service time. In this study, residual stress is measured for two well-known nickel based super alloys (Inconel 625 and Inconel 718) deposited on 7074 aluminum alloy substrates by the cold spraying technique. Residual stress in Inconel 625 was found to be highly tensile on the surface and compressive on the subsurfaces. After heat treatment the residual stress was relieved and was compressive in nature. Whereas for Inconel 718, residual stress was compressive on the surface and tensile on the subsurfaces in the as-sprayed condition. After heat treatment, the residual stress was compressive with increased magnitude. The heat treatment at 800°C made the residual stress more compressive. The porosities of both Inconel 625 and Inconel 718 were reduced after heat treatment.

In the present study, surface engineering related to simultaneously thermally sprayed and laser melted corrosion resistant coatings has been carried out. A 6 kW high power diode laser or a Nd:YAG laser and a flame spray gun were used to construct the laser assisted thermal spraying system. The main aims of the study were to find out the optimal processing parameters and materials to create dense corrosion resistant coatings on steels. The corrosion resistance of the manufactured coatings was measured and the microstructure characterized using optical and scanning electron microscopy.

Thermal spray processes are widely used to protect materials and components against wear, corrosion and oxidation. Despite the use of the latest developments of thermal spraying, such as HVOF and plasma spraying, these coatings may in certain operation conditions show inadequate performance, e.g. due to insufficient bond strength and/or mechanical properties and corrosion resistance inferior to those of corresponding bulk materials. The main cause for a low bond strength in thermal sprayed coatings is the low process temperature, which results only in mechanical bonding. Mechanical and corrosion properties typically inferior to wrought materials are caused by the chemical and structural inhomogeneity of the thermal sprayed coating material. In order to overcome the drawbacks of sprayed structures and to markedly improve the coating properties, laser remelting of sprayed coating was studied in the present work. The coating material was nickel based superalloy Inconel 625, which contains chromium and molybdenum as the main alloying agents. The coating was prepared by high-velocity oxy-fuel spraying onto mild steel substrates. High power continuous wave Nd-YAG laser equipped with large beam optics was used to remelt the HVOF sprayed coating using different levels of power and scanning speed. The coatings as-sprayed and after laser remelting were characterized by optical and electron microscopy. Laser remelting resulted in full homogenization of the sprayed structure. This strongly influenced positively the performance of the laser remelted coatings in adhesion, wet corrosion and high temperature oxidations test. The properties of the laser remelted coatings were compared directly with the properties of as-sprayed HVOF coatings, and with PTA overlay coatings and wrought Inconel 625.

One of the economical and fast solutions for failure against erosive wear in oil and gas industries is the deposition of cermets using HVOF thermal spray. Recently, especially with the new development of bimodal feedstock powders, the composition percentages of the mixed powders have played a key factor in the final coating performance. In the present study, a design of experiment (DOE) software was implemented to study the influence of different powder percentages on the coating performance. The coating mechanical properties and its performance were investigated via dry solid particle erosion tests, hardness measurement and SEM respectively. The results showed that both the hardness and erosion resistance of the coating increases as the composition percentage of the nanostructured WC-12Co increased due to the strong adhesion of WC nano size grains at the substrate/coating interface as a result of improved mechanical interlocking.

Structured surfaces are needed for improving the heat transfer in a lot of industrial processes. The paper deals with the production and the characterization of coatings with rough and porous surface. Structured coatings of inconel or copper are deposited on copper tubes by means of radio-frequency vacuum plasma spraying. The microstructure as well as the surface roughness of the coatings are investigated. Boiling experiments are carried out on the coated tubes to measure their heat transfer coefficient. The results show that the coated tubes exhibit improved heat transfer values in comparison to smooth tubes. The enhancement ratio can reach more than 20 for inconel coatings.

Cold spraying (CS) of high strength materials, e.g., Inconel 625 is still challenging due to the limited material deformability and thus high critical velocities. Further fine tuning and optimization of cold spray process parameters is required, to reach higher particle impact velocities as well as temperatures, while avoiding nozzle clogging. Only then, sufficiently high amounts of well-bonded particle-substrate and particle-particle interfaces can be achieved, assuring high cohesive strength and minimum amounts of porosities. In this study, Inconel 625 powder was cold sprayed on carbon steel substrates using N 2 as propellant gas under different refined spray parameter sets and powder sizes for a systematic evaluation. Coating microstructure, porosity, electrical conductivity, hardness, cohesive strength and residual stress were characterized in as-sprayed condition. Increasing the process gas temperature or pressure leads to low coating porosity of less than 1 % and higher electrical conductivity. The as-sprayed coatings show microstructures with highly deformed particles and well bonded internal boundaries. X-ray diffraction reveals that powder and deposits are present as γ- solid-solution phase without any precipitations. By work hardening and peening effects, the deposits show high microhardness and compressive residual stresses. With close to bulk material properties, the optimized deposits should fulfill criteria for industrial applications.

The appropriate selection of bulk materials and coatings of valve components, is an important factor for the economic success of oil and gas production activities in petrochemical field. Materials and coatings are important because particle erosion and surface wear is associated to corrosion by hydrogen sulphide during oil and gas flow. The wear of high pressure valves of gas system will lead to pollution, safety problem and cost increases. The most popular solution of these problems is the deposition of hard material like tungsten carbide or chromium carbide by thermal spray. Particularly these coatings are deposed by HVOF (High Velocity Oxygen Fuel) to obtain a very high hardness with excellent cohesion and adhesion. Tungsten carbide cobalt-chromium based coating, chromium carbide nickel-chromium coating as well as Inconel 625 are adopted actually in the specifications of the industrial petrochemical companies and their behavior and wear, erosion and corrosion properties are reported in literature. This paper addresses the study and surface analysis and characterization of alternative coatings such as NiAl and composite material WC / intermetallic compounds containing mainly Ni, Cr, Co and Mo. The best parameters to produce these coatings has been found by implementing a DOE and the obtained coatings have been systematically submitted to corrosion and functional tests based on the determination of the behaviour of the thermal spray coatings in an atmosphere of H 2 S and CO 2 [1] and to wear and erosion test according to ASTM G75-95; removed material weight and usured surface damages have been determined. Furthermore the coatings have been completely characterized before and after the tests from the point of view of the structure (porosity, coating cohesion and adhesion, hardness, wear) and of the surface properties by means of a prototype 3- dimensional stylus micro-topography surface analysis system. Their corrosion and functional behaviour have been finally compared with the behaviour of the above mentioned coatings applied at present as standard in the petrochemical sector. The results state that WC/intermetallic compound could be a good substitute of IN625 for certain kind of application where good antierosion behaviour is requested.

Hot corrosion tests have been conducted on Ni- and Cr-based laser coatings, a high-velocity oxy-fuel (HVOF) sprayed coating and various wrought alloys covered with a synthetic salt of Na 2 SO 4 -V 2 O 5 and exposed at 650°C for 1000 h in air. Coating microstructures and reaction product layers were analyzed with scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). The hot corrosion resistance of tested specimen was evaluated by measuring its mean thickness loss. Generally, wrought alloys, HVOF coating and Cr-based laser coatings suffered from selective corrosion beneath salt film, that is, distinct Cr-depleted layer was formed at alloy/salt interface. Cr-based laser coatings exhibited extended solid solubility and they transformed towards equilibrium condition. Cr-rich phases enriched further with Cr and they were prone to corrosion. Low diluted laser coatings and HVOF coating were more resistant to hot corrosion than commonly used industrial standard alloy, Nimonic 80A. Ni-based laser coating exhibited resistance equivalent to Cr-based coatings and superior to corresponding wrought alloy.

HVOF spraying process is widely used to improve component life in service due to the high bond strength of the coatings, which is a result of the high particle velocity upon impact, and consequent low coating porosity. However, many parameters can affect metallic coatings properties, especially unmelted particles and oxidation level. Flame parameters, such as calorific power, combustion ratio and temperature, are of prime importance. Moreover, the fuel gas employed in this spraying process can lead to various coating properties and deposition efficiency. The aim of this work was focused on the influence of some fuel gases, namely propane, propylene (LPG) and hydrogen, on stainless steel coating characteristics. A specific domain common for those three gases was determined in order to effectively compare those gases with the same flame parameters. Flame characteristics were computed using a simple model for all the fuel gases considered. Temperature as well as calorific power were fixed. For different substrate temperatures, obtained through a special CO 2 cooling nozzle system, richness was varied from 1.4 to 1.6. Microstructure investigation as well as oxide content and microhardness measurements were conducted. For the same kinetic torch parameters, thickness-per pass gave an idea of the deposition efficiency. In the range studied, deposits properties were quite similar for both LPG fuel gases. Hydrogen led to better characteristics in term of oxide content, although its deposition efficiency was a bit lower. A general law was established to link oxide content within the coatings to the flame parameters. A reasonable regression analysis was obtained for all the coatings sprayed. The combination of cooling efficiency (i.e. CO 2 flow rate) and flame characteristics (i.e. interaction of the particle in flight) led to a good correlation. These correlations were further verified by spraying another metallic powder, namely Inconel 625.

A study was conducted into the use of HVOF coatings on hydraulic cylinder rods in marine environments subject to both biofouling and corrosion. The study comprised a laboratory phase for candidate material selection and HVOF process optimisation. This was followed by long term field testing of the most promising candidates, which were subsequently assessed based on corrosion, adhesion, surface roughness and hydraulic sealing performance. Two candidates, nickel based WC cermet coatings, WC–10Ni5Cr and WC–18Hastelloy C, showed excellent results, and are considered as candidates for the application.