"Improvement of Corrosion Resistance of Thermal-Sprayed Stainless Steel Coating by Addition of Some Deoxidizing Elements"

A modified stainless steel coating, named as M-SUS here, was prepared by APS and HVOF methods and compared with a conventional stainless steel (JIS: SUS316L). Anodic polarization tests using H2SO4, NaCl, and HCl solutions, neutral salt spray test, and exposure test in actual tank for HCl storage were employed for the evaluation of corrosion resistance. Structure of the coatings was investigated by use of optical microscope, scanning electron microscope, electron probe micro-analyzer, and transmission electron microscope. It was found that the coating M-SUS exhibited a remarkably superior corrosion resistance by all tests mentioned above, compared with the conventional ones. Although both of the coatings compose of gamma-austenite and delta-ferrite phases, the coating M-SUS reveals much less oxide of amorphous and/or nano-crystalline structure with chromium and more delta-phase enriched with silicon and molybdenum. Another exposure test using a mixed acid of 25%HNO3 and 75%HCl yielded that the delta-phase was not etched out but the gamma-phase vanished, that is, the delta-phase of M-SUS exhibits a strong anti-corrosion property. It is considered that the superior corrosion resistance of coating M-SUS is attributed to the extensive formation of anti-corrosive delta-phase and inhibition of chromium depletion resulting from oxide formation.