The hot corrosion behaviour of HVOF sprayed MCrAIX coatings under Na2SO4 (+NaCl) salt films

The hot corrosion behaviour of two NiCoCrAlYTa and CoCrAlYSi HVOF sprayed coatings and a CoCrAlY VPS coating were investigated under laboratory conditions at 900degreesC using a synthetic gas atmosphere containing sulphur as an impurity. All the coatings tested showed good protection under Na2SO4 salt films. In the presence of NaCl in the Na2SO4 salt films, the corrosion rates of low Al containing coatings increased considerably but the NiCoCrAlYTa coating with higher Al content still revealed good performance. It is suggested that NaCl in the salt film causes premature failure of the protective scale and reduces the incubation period of corrosion in the coatings of lower At content. Furthermore, it seems that the finely dispersed Al rich oxide particles in the sprayed and heat-treated HVOF coating microstructure do not lead to internal corrosion. The experimental investigations include short-term corrosion kinetic measurements and SEM analyses.