Hot-corrosion mechanism of Ni-Cr coatings at 650 deg C under different simulated corrosion conditions

• Published in: Journal of China University of Mining and Technology Vol. 18; no. 3; pp. 444 - 448
• Main Authors: Ou, Xue-mei, Sun, Zhi, Sun, Min, Zou, Duan-lian
• Format: Journal Article
• Language: English
• Published: 01.09.2008
• ISSN: 1006-1266
• DOI: 10.1016/S1006-1266(08)60092-9

The oxidation and lower temperature hot corrosion (LTHC) processes occurring on the surface of Ni-Cr coatings produced by high velocity arc spray (HVAS) were studied. Several different conditions were studied under simulated boiler conditions at 650 deg C. The protection effect of an Al coating deposited by HVAS onto the Ni-Cr coating was also investigated. Microscope, X-ray diffraction and corrosion rate curves have been used to analyze corrosion mechanisms. The experimental results show that: 1) The oxidation rates are almost superposed in both air (no SO3) and in simulated coal-fired gas (containing SO3) as long as no salt was present on the surface. These rate curves show a logarithmic relationship. 2) When the surface is coated with salt (75%K2SO3 + 25%Na2SO3) the rate curve for LTHC of the Ni-Cr coated surface shows a parabolic shape in the simulated coal-fire flue gas. In air only the oxidation reaction takes place, the second type of LTHC was not seen. And 3) the Al over coating on the Ni-Cr enhances resistance to LTHC because an inter-metallic compound, Al3Ni2, forms at the Al/Ni-Cr interface and because of the increase in coating thickness.