Hot salt corrosion test procedures and coating evaluation

Abstract

There is considerable uncertainty regarding the relevance to practical applications of the various test procedures used to assess the resistance of superalloys to hot salt corrosion. A description is given of work to evaluate the applicability of the results obtained from current test methods in comparison with the performance of components in service. The test methods used were (i) burner rigs, (ii) crucible tests with and without electrochemical acceleration and (iii) various furnace tests such as the Dean test and those using a synthetic ash. The morphology of the corrosion product obtained was compared with that formed on blades and other parts during service exposure. The results indicate that burner rig testing gives the most realistic simulation of service behaviour and that some of the other tests can be modified to be more relevant. A highly significant aspect is the influence of contaminants on corrosion behaviour, which can vary depending on test method and must therefore be understood in detail when a particular test procedure is defined. It is shown that the deposition rate of sulphate from the vapour phase is an important factor controlling corrosion rates in burner rigs and Dean tests. In burner rigs the obvious correlation with contaminant concentration was not found, and a more important parameter is believed to be the contaminant mass flux. Tests in which the flux was close to that in operating gas turbines gave the best simulation of corrosion. The corrosion resistance of coatings can be assessed using similar procedures, but more care is required in the interpretation of results since failure of a coated component may result from a single defect. A statistical approach to the interpretation of behaviour is therefore discussed.