Effects of heat treatment on the microstructure and properties of HVOF-sprayed Ni–Cr–W–Mo–B alloy coatings

Abstract

The corrosion properties of Ni–Cr–W–Mo–B alloys sprayed by high-velocity oxy-fuel (HVOF) were studied as a function of heat treatment by using both potentiodynamic polarization and immersion tests in a H 2SO 4 solution. A microhardness tester estimated their mechanical properties. The microstructural characteristics of the as-sprayed and annealed coatings at 550, 750 and 950°C were analyzed by means of optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe microanalyzer (EPMA) and transmission electron microscopy (TEM). The results showed that the corrosion resistance was improved by increasing the annealing temperature, and was closely related to the microstructural changes of the coating layer by heat treatment. As-sprayed coatings had metastable and heterogeneous phases such as amorphous, nanocrystalline and very refined grain crystals and precipitates, which induced a localized corrosion. In particular, localized corrosion occurred at unmelted particles, preferentially those that were composed of a Ni matrix and Cr, W and Mo-rich phases segregated in the boundary. These two phases appeared to act as galvanic electrodes, i.e. anode and cathode respectively. As the annealing temperature increased, the microstructure showed the following changes: a reduction in porosity (the densification by sintering or self-diffusion between splats), the decomposition and crystallization of amorphous/nanocrystalline phases, grain coarsening and the formation and growth of precipitates such as M 23C 6 and M 7C 3. In addition, the compositional difference between the matrix and boundary phases gradually disappeared, which changed the corrosion type from localized to general corrosion and thus enhanced corrosion resistance.