Development of high quality thermal spraying process by shielding control

The shielding controlled plasma spraying process was investigated to improve corrosion resistance of metal surfaces. In this process, a shielding nozzle that covered just only spraying area was attached in front of the tip of a commercial plasma spray gun nozzle, and the environment surrounding the plasma jet was controlled by nitrogen flow. As the oxygen concentration in the shielding nozzle was maintained as low as 0.5%, the metal oxide contents in volume of CoNiCrAlY coating and the porosity of the coating reduced to 0.2% and 0.3% respectively under optimal spray particle size. The corrosion potential of CoNiCrAlY coating sprayed by this process in an acid solution including chloride ions stayed about -150 mV for 1000 hours, and no rust was observed during this test. On the other hand, that of the coating sprayed by atmospheric plasma spraying process changed from about -300 mV to about -500mV for 1000 hours, and the rust came to the surface of the coating after 10 hours. Therefore the developed shielding controlled plasma spraying process is concluded to improve the corrosion of the metal.