Nano-crystalline coating to improve cyclic oxidation resistance of 304 stainless steel

Cyclic oxidation of nano-crystalline 304SS coating was compared with that of conventional, micro-crystalline 304SS in both dry and wet (10% H2O) air at 900 °C. The nanocrystalline alloys were prepared by direct current magnetron sputtering coating. Weight change kinetics were measured by continuous thermogravimetry. For micro-crystalline alloy in dry air, a small initial weight gain was followed by gradually increasing spallation. Addition of water vapour to air produced immediate spallation, and led to significant breakaway oxidation. Microstructural analyses revealed the formation of iron-rich oxides on the surface. The addition of water vapour also accelerated the development of internal oxidation in the form of fine spinel precipitates dispersed in the chromium depleted matrix. In contrast, nano-crystalline 304SS, underwent relatively rapid oxidation in the very early cycles of reaction, but then the reaction slowed down. No spallation was apparent in either gas. The weight gain was higher in wet air than in dry air. In both gases a dense and continuous chromium oxide layer was formed at the surface, together with other oxides distributed inside the coating layer. The effects of cyclic reaction, grain size and water vapour on the resistance of 304SS to breakaway oxidation are discussed.