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.

In corrosive media the wear resistance of ceramic-metallic coatings depends on the corrosion resistance of the metallic binder. Other factors influencing the coating deterioration are the corrosivity of the media and any galvanic interaction from the surrounding material. The corrosion resistance of the metallic binder should be comparable to the material of the rest of the system. This is especially important when the surrounding materials are corrosion resistant alloys such as stainless steels, where the coatings otherwise will act as an anode. This paper summarises results from corrosion tests of different ceramic-metallic coatings sprayed by the HVOF process. The corrosion properties of the coatings have been compared with results from corresponding tests obtained for stainless steel and some nickel based alloys. The corrosion compatibility and risk of galvanic corrosion is discussed.

In recent years Norwegian industries together with SINTEF and NTNU have run several projects dealing with development of thermally sprayed ceramic-metallic coatings for corrosion and wear applications. The mechanisms of erosion, corrosion and combined actions have been studied. The coatings studied are of the metal-carbide / metallic binder type sprayed by the High Velocity Oxygen Fuel (HVOF) process. The influence of powder properties like carbide particle size, powder grain size distribution, chemical composition (i.e. type of carbide and composition of the metallic binder) together with the influence of spray systems and parameters have been studied. The studies have demonstrated how optimization of powder and spray parameters improve corrosion and wear behavior of the coatings. Attention has also been paid to methods used for manufacturing powders for thermal spraying. This is very important from a corrosion point of view. The manufacturing methods should make sure that metallic binders are sufficiently alloyed to achieve the necessary corrosion resistance. The results from the work done in these projects are valuable for suppliers of thermal spray powders, spray companies and end users of coatings.

Different weld overlays were welded by using different weld processes. Both Ni based alloys and Co based alloys were welded on either a c-steel (Weldox 700) or a stainless steel similar to superduplex (UNS 32760). The weld overlays were investigated and compared with respect to effect of the number of layers, chemical compositions of the surface, critical crevice corrosion temperature, microstructure, hardness, friction coefficient, adhesive and abrasive wear resistance. Wear testing was performed on the Co based alloys only. Some electrolytically deposited Ni-coatings were also investigated with respect to crevice corrosion. Both an austenitic and a superduplex stainless steel were tested as reference materials.

This paper examines WC based coatings with high-alloy binders with regard to their structure, corrosion behavior and wear. The coatings are compared to conventional coatings with respect to corrosion and wear. The coatings were made using HVOF spraying of various powders. All powders examined were produced by agglomeration/sintering, that is by agglomeration of metal particles with WC particles and subsequent sintering. Some powders have been made using pre-alloyed metal particles. The paper also examines a mixture of ceramic-metallic powder and purely metallic powder. Various methods are used to characterize the powders and coatings. 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.

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.