Evaluation of microstructure and electrochemical behavior of dual-layer NiCrAlY/mullite plasma sprayed coating on high silicon cast iron alloy

Protective single-layer mullite and dual-layer NiCrAlY/mullite coatings were deposited on high silicon cast iron by plasma spraying process. The microstructure analysis showed that NiCrAlY layer with a thickness of 90 19 pm as bond coat and the nano-structured mullite layer with a thickness of 163 +/- 23 gm as a top coat formed homogeneously on the surface of high silicon cast iron. The X-ray diffraction (XRD) results indicate amorphous phases as a result of rapid cooling of molten particles. The microhardness of mullite layer in dual-layer NiCrAlY/mullite coating was significantly higher than bond coat and substrate. In addition, the corrosion behavior of the samples was comparatively studied using electrochemical tests. Polarization and EIS results showed that the uncoated sample has not suitable corrosion resistance in 3.5% NaCl solution due to the formation of micro galvanic cells between graphite flakes and metal. However, its corrosion resistance was improved by the surface modification process. In the case of the coated samples, the dual-layer NiCrAlY/mullite coated sample exhibited better corrosion resistance due to the existence of a dense corrosion resistance bond coat and stronger bonding strength. Finally, the corrosion mechanisms for the mullite single-layer, NiCrAlY/mullite dual-layer, and uncoated samples were proposed.