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

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.

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.