WC-based thermally sprayed coatings are now widely used in a range of industries to improve the mechanical strength of the components, but also enhance the resistance of the substrate to wear and corrosion. High velocity oxygen fuel thermal spraying has been accepted as a pre-eminent technology to deposit composite coatings with high density, superior bond strengths and relatively low decarburization due to the high particle velocities and relatively low particle temperatures induced in the deposition process. Many types of tungsten carbide/metal cermet coatings have been developed by alloying with different chemical elements, in an attempt to enhance corrosion and even erosion-corrosion resistance of coatings produced by HVOF for industrial applications, for example oil and gas industries. In this paper the galvanic series and the erosion-corrosion behaviour of three types of tungsten carbide coatings and three different sealing technologies in simulated oil and gas industry environments are compared. The ultimate goal is to have an understanding of how optimum choice of coating for a given application can be made for applications in the oil and gas industry.

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

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.

Coatings obtained by thermal spray techniques are widely used in many industrial applications where wear and abrasion resistance is required. High Velocity Oxy-Fuel (HVOF) spraying represents the state of the art of such types of coatings. The high velocity of the particles achieved during the spraying process and relatively low temperatures at the point of application enhance bond strength and eliminates substrate deformation by thermal effects. These aspects make the process attractive for high value components. In this work the electrochemical behaviour of both as-sprayed and HIPed Stellite 6 coatings are compared. Hot Isostatic Pressing (HIPing) consists of simultaneous applications of high pressure and high temperature. In this paper HIPing has been used as a post-treatment process for HVOF coating. Electrochemical DC anodic polarisation corrosion experiments were conducted in seawater at 18°C, 30°C, 40°C, 50°C, 70°C and 90°C. To support the electrochemical tests, detailed microscopy was conducted after corrosion tests to determine the attack mechanisms.

In this paper, the corrosion characteristics of two HVOF-sprayed coatings made with self-fluxing NiCrSiB powders are assessed in the as-sprayed condition. One of the coatings is also investigated after post-spray treatment with polymer sealing and after vacuum furnace fusion. It was found that cracking at lamellar boundaries dominates the corrosion behavior of the layers in the as-sprayed and sealed states, while pitting corrosion resulting from the failure of hard phases plays the main role in the melted layer. Paper includes a German-language abstract.

The focus of this paper is to determine the friction and wear characteristics between conventional metal-metal contacts and cermet-metal contacts. A WC-Co based cermet, applied by the High Velocity Oxy-Fuel (HVOF) process on a carbon steel substrate has been investigated in contact with cast iron pins. That has been compared with conventional carbon steel/cast iron couples. A mineral oil and a mineral oil containing the additive zinc dialkyl dithiophosphate (ZDDP) were used as the lubricants. The formation of a wear film has been shown to vary on the metal and cermet surface. The friction and wear response and the wear film nature are discussed.

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.

Primarily thermal-spray coatings have been developed to combat excessive degradation of components due to mechanical wear. However, these coatings are increasingly being required to function in aqueous environments where corrosive attack is possible. The durability of thermal spray coatings in terms of corrosion resistance is the focus of this paper. Two coating methods were studied: high-velocity oxyfuel (HVOF) and thermal spray-fused. The HVOF coating was WC-Co-Cr and the spray-fused coatings were WC-Co based and Ni-Cr-Si-B. Samples have been exposed to aqueous environments at a range of different temperatures and of varying salinity (500ppm and 35,000ppm Total Dissolved Solids) in order to simulate freshwater and seawater environments. The detailed material loss and degradation mechanisms have been investigated using electrochemical-monitoring techniques supported by precise post-test microscopical examination using light microscopy, scanning electron and atomic force microscopy and x-ray microanalysis. The study has demonstrated that there is a strong dependence on coating durability for all the coatings as a function of temperature but salinity has a lesser effect. However there are interesting differences in the extent and detailed mechanisms of such effects between the different coatings.

Many of the state-of-the-art thermal-spray coatings (e.g. plasma, HVOF) have been developed with wear resistance as a primary aim. However, these coatings are increasingly being required to function in environments where corrosive attack is possible. This paper comprises a description of a study of the corrosion a WC-based coating containing 10%Co and 4%Cr as the metallic binder. The coating, in the form of test coupons, involving a substrate of superduplex stainless steel, has been exposed to seawater at ambient temperature (18°C) and 50°C. The corrosion behaviour and detailed corrosion mechanisms have been investigated using electrochemical monitoring techniques supported by precise post-test microscopical examination using light microscopy, scanning electron microscopy, atomic force microscopy and x-ray microanalysis. Results have shown the corrosion resistance of the coating material to be critically dependent on the temperature of the solution and that important changes in corrosion mechanisms arise as a function of the temperature.

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.