Coatings cobalt-base alloys

The compositions of the alloys evaluated in Phase I are summarized in Table III. These alloys represent most classes of high-temperature iron-, nickel-, and cobalt-base alloys that could be considered for coal gasification service. Pack aluminized and chromized coatings on AISI 310 and IN-800 were also evaluated in the test program. 

Gas turbine blades are essentially nickel-base or cobalt-base alloys containing substantial amounts of chromium, several percent aluminum, and a few hundredths percent yttrium. Their susceptibility to hot corrosion and sulfidation has already been discussed. Applied coatings of aluminum or of aluminum-chromium-yttrium increase resistance to attack. 

In conclusion, the Quinn s model can not be easily adapted to analyse the depassivation -repassivation process determining the wear laws observed in tribocorrosion conditions. However, the main idea of this model, namely that wear proceeds by a succession of growth and delamination of an oxide layer, can be retained. It was at the origin of the tribocorrosion model presented to explain the the wear of a cobalt-based alloy coating applied on the... 

Weld hardfacing coatings, for example, high-carbon iron-chromium alloys, tool steels, nickel-chromium-boron alloys, cobalt-base alloys, and austenitic manganese steels... 

Amato, L.E., L6pez, D.A., Galliano, P.G., and Cerd, S.M. (2005) Electrochemical characterization of sol-gel hybrid coatings in cobalt-based alloys for orthopaedic implants. Mater. Lett,... 

Table 4-15 lists base materials Elliott has tested. This list, which is continually being expanded, includes low alloy steels, high alloy iron base, nickel base, cobalt base materials, and odiers. Table 4-16 shows some of the coatings Elliott has tested. The list indicates die supplier, coating designation, and major components of the coating composition. 

In recent years, the spraying process has been adapted for hard facing, using the chromium-nickel-boron alloys which have become known as Colmonoy. More recently still, the cobalt-base Stellite alloys have also been used. These materials in powder form are sprayed on to the surface in the usual way. The deposit is afterwards heat treated by a torch, so that fusion takes place. The process is often known as spray-welding. Such coatings are primarily used for hard facing under wear conditions, but as the Anal surface is nickel-chromium or cobalt-chromium they exhibit very high anticorrosive properties. 

Yttrium is also used in other areas of metallurgy notably as a component of certain nickel-base and cobalt-base superalloys of the NiCrAlY and CoCrAlY type.(3) These alloys possess excellent corrosion and oxidation resistance, properties that have attracted the attention of the aero-engine industry where they are used as protective coatings on turbine blades. The alloys, when applied by vapour deposition, form an oxide coating that exhibits remarkable adhesion, a property attributed largely to the yttrium component acting to prevent the formation of voids at the oxide/substrate interface.(4)... 

The process is used to apply abrasion protective coats of cobalt and nickel-based alloys. Tungsten carbides (there is a choice of square or round carbides) are used to enhance abrasion protection on the screw flights, they are not used on the drive faces as the roughness which appears there during their use impedes the transportation of the ceramic material (Fig. 5). 

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