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Nickel–chromium coatings have been plasma sprayed onto a stainless steel surface in order to protect it from the high temperature and erosive conditions encountered in power plant boilers. Microstructure, porosity, and microhardness of the coatings have been studied. High temperature oxidation behaviour was studied in an atmosphere similar to service conditions used in power plant, and thermal fatigue tests were performed in an experimental combustion chamber. Finally, adhesion between the steel substrate and NiCr coating was evaluated by means of tensile tests. The results obtained are discussed with particular reference to the evolution of microstructure owing to thermal effects and coating deposition method.

This study deals with the behaviour of a nickel based alloy, NiCrMoAlFe, which was thermally sprayed, using flame, plasma, high velocity oxyfuel (HVOF), and high frequency pulse detonation (HFPD) methods, onto a stainless steel substrate. This alloy is used as a coating for heat transfer and structural elements in high temperature regions of boilers, such as superheater and reheater tubes. The microstructure, porosity, oxide content, and microhardness of the various coatings were determined. Thermal fatigue tests, under an atmosphere similar to that of power plant service conditions, were conducted in an experimental combustion chamber and, finally, the adhesion between the substrate and the coating layer was evaluated by means of tensile tests. The results obtained are discussed, with special attention being paid to the specific characteristics of the various spraying procedures.

CoNiCrAlY powders were thermal sprayed using the high frequency pulse detonation (HFPD) method onto AISI 310 austenitic stainless steel samples. The cyclic oxidation behaviour of these coatings at 800 and 1000uC in simulated gas turbine and standard atmosphere environments was experimentally determined. Austenitic AISI 310 was used as a reference. Composition and morphology of surface phase were evaluated using X-ray diffraction and scanning electron microscope techniques. Oxidised HFPD CoNiCrAlY coatings have always been characterised by the formation of a continuous and highly protective alumina layer. In contrast, the protection of the chromium oxide layer generated on the austenitic AISI 310 under the same operating conditions is much less effective and a degraded region near to the oxidised layer has been observed at the highest temperatures. Moreover, cracks appear in the chromium oxide layer when it is submitted to high thermal gradients, as a consequence of thermal stresses.

CrNiAlTi, NiCrBSi and WC-Ni coatings have been thermal and plasma sprayed projected over a stainless steel surface in order to protect it against heat and erosion actions encountered in power plant boilers. Their microstructure, porosity and microhardness have been measured. High temperature oxidation under an atmosphere similar to service conditions in power plants and thermal fatigue tests have also been performed in our experimental combustion chamber and, finally, the adhesion between the substrate and the coating layer has been evaluated by means of tensile tests. The obtained results have been discussed paying especial attention to the microstructural materials evolution due to thermal effects and coating projection methods.