This paper focuses on the influence and role of galvanic interactions in the corrosion behaviour of thermally-sprayed coated components. Coatings, of different chemistry and applied by various processes (including HVOF) to substrates of carbon steel or stainless steel, have been utilised to facilitate study of galvanic corrosion phenomena both between coating and substrate and also within the coating itself. The experiments have involved the measurement of galvanic currents between separate specimens and also the microscopical examination of galvanic interactions on single specimens. Galvanic corrosion effects, on both a macroscale or microscale, have been observed and the implications of these for coating and coating/substrate integrity are discussed.

Thermal spray coatings are widely used for erosion resistance, but the relationship between the microstructure of the coatings and their erosion resistance is not well understood. In this paper the performance of several commonly used coatings at ambient and elevated temperatures is reviewed in light of the coatings' structure and compared with a new coating. Two high temperature industrial applications, solid particle erosion in steam turbines and alumina-based erosion have been chosen to illustrate the significance of a coating's structure on its performance.

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

Thermal spray coatings are very effective in combating wear and corrosion in many applications. New thermal spray processes and coating compositions continue to be developed with concomitant improvements in the performance of the coatings and their use in new applications. Nonetheless, the thermal spray coatings are not without competition from other coating and overlay processes and materials. This brief review considers the microstructures and the wear and corrosion resistance of a number of alternative coatings to thermal spray coatings, including physical vapor deposition, chemical vapor deposition, electroplating, autocatalytic, and laser cladding.

Most of the early applications of thermal spray coatings were focused towards providing a remedy to excessive wear degradation. However, as the introduction of such coatings into wider industrial sections increases there is also exposure to other potential degradation processes - aqueous corrosion is one such process. The complex microstructures in cermet coatings have been shown to translate to complex modes of corrosion attack. In this paper an electrochemical test methodology to probe the local/micro aspects of corrosion initiation and propagation will be described. A new electrochemical cell has been devised in which the corrosion can be followed `live` and in `real time`. The surface is subjected to in-situ imaging by Atomic Force Microscopy (AFM). This paper reports some of the exciting findings which have enabled the sequence of corrosion events in cermet coatings to be defined. The study has revealed that a precursor to coating degradation is the attack of the Co-base matrix. This sets up an environment which can catalyse WC dissolution. The practical and fundamental importance of the results will be discussed.

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.

The HVOF sprayed WC-CoCr coatings are widely spread due to their excellent resistance against wear and corrosion. These coatings are one of the most suitable alternatives for hard chromium in many applications. Within the research project, the most suitable hard chromium alternative for hydraulic devices in aircraft is being developed and tested. This application is highly demanding not only on the functional properties of applied coatings but also on the surface quality. Grinding and polishing of the coating are not sufficient, to achieve the necessary surface properties. This study aims to optimize the superfinishing process of HVOF sprayed WC-CoCr coating. The achieved surface quality is primarily measured using profilometry. With optimized surface preparation, the tested parts for aircraft hydraulic parts are treated and tested for leakage of operating fluids and high cyclic lifespan.

Thermal sprayed coatings produced from ultrafine- and nano- and near-nano grained powders of tungsten carbide- 10 wt.% cobalt-4 wt.% chromium (WC-10Co-4Cr) are reported to provide improved properties as compared to conventional powders. These materials show great potential for applications in the aerospace, oil & gas, power, and many other industries. A study is proposed to investigate the influence of WC grain size on HVOF coating properties. Thermal spray coatings will be produced from powders consisting of grains of WC from micron- to near-nano in size in a Co-Cr matrix. The Hall-Petch relationship cites the strengthening of materials by reducing the average crystallite (grain) size. An examination of consolidated forms will be performed using the same powders used in thermal spray in the spark plasma sintering (SPS) consolidation. The mechanical properties of thermal spray coatings have been reported to relate to those of bulk materials. Improvements observed in the HVOF spray coatings will be compared to those of bulk samples.

This paper describes an inner diameter HVOF spraying technique and associated tests methods. In the experiments, WC-CoCr was applied to an internal diameter of 150 mm and evaluated using a dedicated pin-on-disk wear test and standard axial fatigue tests. The results are presented and discussed along with the potential for further development of the inner diameter HVOF spraying technique.

Tungsten carbide-12 wt.% cobalt (WC-12Co) coatings and chromium plating are used to provide wear resistant surfaces in gas turbine applications. These treatments provide surfaces with hardnesses greater than 60 Rockwell C. In addition, a surface finish better than 8 microinches RMS is required for optimum performance. To achieve this surface finish, diamond grinding is required. The diamond grinding step adds considerable cost to the product and economical benefits could be achieved if more conventional grinding techniques were incorporated. A program was initiated to develop an alternative thermal spray coating, with a target hardness lower than 60 Rockwell C, but high enough to provide the wear resistance required. Spray development was conducted on five commercially available materials using the Diamond Jet 2600 high velocity oxy-fuel process. Laboratory evaluation included coating microstructure, macro- and microhardness, bond strength, salt spray corrosion, and cyclic compression tests.

Fiber-reinforced polymer composites are an important class of structural materials, offering high strength-to-weight ratios and high rigidities. For many applications, however, their wear resistance is less than desirable. Wear-resistant thermal spray coatings have the potential to improve the surface properties of fiber-reinforced polymer composites, although some require the application of a bond coat to achieve sufficient adhesion. The present study was conducted to find acceptable bond coat materials and compare their performance. Materials such as polyamides, polyimides, polyether-ether-ketone, or simply aluminum or nickel were found to be suitable bond coats for many composite substrates.

Surface quality lifetime and wear resistance of protective coatings can be improved by decreasing carbide grain size from submicron to nanoscale. In this study, experimental WC-CoCr powders were manufactured via novel powder manufacturing approach using water-soluble raw materials. Produced powders were sprayed with the High-Velocity Air-Fuel (HVAF) spray process to control the particle temperature and to avoid in-flight decomposition of the nanocarbides. As a result, dense and wear resistant coatings with nanosized carbides were produced. Reference coatings were sprayed using commercial sub-micron WC-CoCr powder to compare the properties of the experimental coatings to the current state-of-the-art. Phase composition and microstructural characterization of the coatings were carried out with X-ray diffraction and electron microscopy, respectively. Mechanical properties were studied by using microhardness tester, as well as rubber wheel abrasion and cavitation erosion wear tests. The wear surfaces were characterized after the abrasion and cavitation erosion tests to understand the effect of nano-carbides on degradation mechanisms. Coatings with the nanosized carbides in the structure showed excellent mechanical properties in wear testing, and even outperformed reference coatings in cavitation erosion test. Based on the obtained results, these novel nano-carbide coatings are promising alternatives for demanding applications in which better surface quality lifetime is vital.

This study investigates the tribological properties of two HVOF-sprayed tungsten carbide coatings, each with a different content of cobalt, which were subjected to electrical discharge machining (EDM). It was found that the electrospark deposition of WC8Co over the HVOF-sprayed WC12Co and WC17Co coatingsresults in the formation of a homogeneous layer of tungsten carbide with a low cobalt content. The EDM layer is well bonded with the HVOF-sprayed coating as confirmed by the gradual change in the content of the elements in the coating and layer. The friction coefficients obtained for the two coatings with the EDM layer are very similar as their composition, microstructure, hardness, and surface roughness are approximately the same.

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.

Fireside corrosion and erosion of heat exchanger tubes is a serious problem. One of the methods to combat this is by applying corrosion and erosion resistant coatings. Nickel– chromium alloys have already been used as coatings to deal with oxidation environments at high temperature. The wear resistance of these coatings can be improved by adding different hard precipitates such as carbides of refractory metals and cemented carbides. In the present study, various compositions comprising of Ni-Cr with 15, 35, 60 and 100% wt% WC/Co were made using thermal spray grade powders. These were then coated on steel substrate by the HVOF method. After detailed characterization of the coatings, the performance of the coatings at high temperature was studied by exposing the coated samples in the temperature range of 600-700°C. Ni-alloy coatings with moderately (15%) added WC/Co showed better oxidation resistance than coatings with high percentage of WC/Co and also than the pure Ni-alloy coating in high temperature range of 600-800°C. From the characterization, it was found that Ni-Cr alloy with 15% WC/Co gave the optimum results.

Cobalt containing carbide powders such as WC-Co and WC-Co-Cr for thermal spraying exist in numerous modifications varying in chemistry, carbide size, and production method. They are widely used for wear, erosion and corrosion protection in many industrial fields. However, for decades it has been well-known from the hard metal industry that WC and Co containing hard metals in breathable dust form can provoke severe lung diseases if inhaled. Recent examinations have proven that this toxicity can be significantly reduced if the Co is pre-alloyed by Fe. In thermal spraying employees are also dealing with Co containing carbides, for example in powder and coating production. Therefore, in order to reduce the hazards for health and the environment, a new class of agglomerated and sintered carbide powders using Fe based binder materials have been developed and investigated. In the present study the powders were HVOF sprayed in order to examine the influence of their different composition and morphology on the microstructure and the properties of the coatings in comparison to standard materials. The experiments comprise microstructural examinations, wear and corrosion tests.

HVOF sprayed tungsten carbide (WC) thermal spray coatings are finding increasing acceptance for replacement of electroplated chromium on aircraft landing gear parts. In order to replace chrome plating by an HVOF WC coating, the latter should exhibit wear and fatigue characteristics at least as good as those of chrome plating. Sliding wear performance and fatigue life of tungsten carbide coated parts depend on morphology and phase composition of the coating which in turn depend on spray parameters such as powder characteristics, powder feed rate, gas flow rates, and spray distance. A Design of Experiments (DOE) approach for a Jet Kote™ HVOF spray system was used to identify optimal spray parameters for WC-Co and WC-Co-Cr coatings based on best sliding wear and best fatigue characteristics. Best sliding wear was defined as minimum block loss in the standard ASTM-G77 wear test in which a rotating WC coated ring is tested against a stationary Al-Ni-Bz block. Best fatigue characteristics were defined as compressive residual stresses in the coating in the range 250 to 450 MPa. Spray parameters found to strongly affect wear were powder feed rate, oxygen flow rate, powder size, and a powder type-powder size interaction. Spray parameters strongly affecting residual stresses include powder type, hydrogen flow rate, and powder feed rate. Some spray parameters were found to be orthogonal with respect to wear behavior and residual stress. This means some parameters can be adjusted to maximize one performance criterion (wear or residual stress) without adversely affecting the other.

Environmental protection and worker safety measures against hard chrome plating are increasing and leading many industries to adopt alternatives. HVOF thermal spray technology for applications of tungsten carbide based coatings have proved to be more environmentally friendly and effective than hard chrome. In this study, the results of wear tests for WC-12%Co sprayed coatings deposited onto AFNOR 25CD4 low carbon steel are compared to those for hard chrome. It was shown that WC-12%Co coatings exhibit the adequate tribological properties compared to those of electrodeposited hard chrome.

Thermal sprayed coatings produced from ultrafine, near-nano and nano grained powders provide improved properties as compared to conventional (micron size) powders. These ultrafine, near-nano and nano grained materials show significant potential for applications in the aerospace, energy, oil & gas and a great many other industries. A study was conducted to investigate the influence of grain size on the microstructures formed and mechanical properties of conventional, ultrafine, near-nano and nano size WC materials. Powders and coatings as well as consolidated forms of tungsten-carbide-10% cobalt- 4% chromium (WC-10Co-4Cr) and tungsten-carbide- 12% cobalt (WC-12Co) materials are examined. Thermal spray coatings are produced of carbides of several different grain sizes using high velocity oxygen-fuel (HVOF) thermal spray processing. Spark Plasma Sintering (SPS) is performed to provide consolidated forms of WC-10Co-4Cr materials. An examination of the thermal sprayed coatings is conducted using microstructural analysis and mechanical property testing. A brief examination of the wear and bend performance of a near-nano, and nano-enhanced material will be compared to a conventional material (micron sized).

HVOF sprayed coatings are widely used to improve the service life of machines components. Influence of spraying parameters and different kinds of materials on coatings properties have been investigated in many works. Our investigations concern properties of HVOF sprayed two tungsten carbide coatings after applying electrospark deposition process (ESD). After finishing microstructure of sprayed coatings were changed by applying tungsten carbide electrode. Changes of microstructure and properties of the sprayed and post ESD process coatings were analysed with a scanning microscope JOEL JSM-5400. The element distribution was analysed with a microprobe ISIS 300 Oxford Instruments whereas phase composition was analysed by diffractometer D8 Advance (BRUKER). The roughness of coatings was measured with Talysurf-4, whereas Matuzawa MMT-X3A was used to study changes of their hardness. Ball on disc tester and dry abrasive rubber wheel tester were applied to estimate coefficient of friction and wear resistance of sprayed coatings before and after EDM process.