Evaluation of the service life of protective coatings

A serious problem in predicting the service life of protective coatings is the absence of a reliable method for evaluating their corrosion resistance. At present the main criteria used to assess qualitative corrosion characteristics at high temperature are: (a) the weight index of corrosion (weight gain or weight loss); (b) the measured thickness of the remaining coating layer; (c) the number of heat cycles up to failure; (d) the length of time to full failure of the coating. None of these methods gives a very precise estimation and this leads to inaccuracy in predicting the service life of the articles in use. In order to evaluate the heat resistance of protective coatings under laboratory conditions a method has been developed as follows. (i) Corrosion resistance tests are carried out under conditions which closely simulate those found in actual operation. (ii) Coated specimens are selected periodically for testing: the concentrations of the elements in the coating, which give it its corrosion resistance, phase composition and structural state, are measured by the method of micro-X-ray spectral analysis. (iii) The results obtained are plotted on a graph. The protective properties of the coating are considered to be exhausted when the coating elements reach their concentrations in the substrate. This method has been tested on many types of metallic coatings, prepared by diffusion, plasma, electron beam, magnetron etc. processes, and was used to determine their service life. A U.S.S.R. State Specification has been formulated on the basis of these results.