The effects of platinum metal additions on the properties of coatings used to protect gas turbine components have been considered here previously, most recently in a brief alerting item published last July (1). Further additions to the literature have since appeared.

One of the keynote papers presented at the European Colloquium on the role of active elements in the oxidation behaviour of high temperature metals and alloys, organised by the Commission of the European Communities and held at the Institute of Advanced Materials, Petten, considered the effects of platinum on the oxidation and hot corrosion of coatings. A group of scientists from the University of Pittsburgh presented the results of their investigation of diffusion aluminide coatings on nickel base superalloys (2). Such coatings can be degraded by several different processes, and their aims were to identify the types of degradation that could be retarded by the addition of platinum to the coating, and then to establish the mechanisms by which this advantage was obtained.

They concluded that the addition of platinum to diffusion aluminide coatings improved the resistance of these coatings to the main types of corrosion encountered in high temperature service, namely cyclic oxidation, high temperature hot corrosion and low temperature hot corrosion. In the case of the last of these the addition of platinum was not as effective as for the other two, but it still gave an improvement. For effective protection, the surface of the aluminide coating had to be predominantly platinum and aluminium, and the platinum serves to promote the selective oxidation of the aluminium.

The results of a study undertaken at Solar Turbines Incorporated to evaluate the relative corrosion resistance of a range of commercially available diffusion and overlay coatings, typical of those used to protect the base metal superalloys from which gas turbine components are made, have been reported (3). The evaluation of two nickel base (MAR-M421 and IN-738LC and one cobalt base (FS-414) superalloys was carried out after furnace testing with synthetic sea salt deposits at 704 and 899°C, temperatures regarded as crucial for low temperature and high temperature hot corrosion, respectively.

The work indicated that a silicon aluminide and platinum-chromium aluminide afforded the best protection. Recommendations for coating selection to provide optimised corrosion resistance under various conditions are tabulated.