Corrosion behavior of YSZ and YSZ/NiCo coatings on inconel 625 exposed alkali chlorides

Alkali chloride attack on boiler pipe walls is considered the main problem of corrosion in the waste-to-energy (WTE) industry, even though uses superalloy. Electrophoretic deposited (EPD) yttria-stabilized zirconia (YSZ) coating is carried out to protect the Inconel 625 substrate. YSZ is deposited directly both on the Inconel 625 substrate and NiCo-Inconel 625. Corrosion resistance was conducted using the 3.5% NaCl electrochemical test and the hot salt corrosion test at 600°C in alkaline salt media such as NaCl, KCl, and CaCl2. The potentiodynamic polarization curve shows that the YSZ coating deposited on the substrate (single-layer) has a corrosion rate of 0.065 mm∙y‒1, lower than that deposited on NiCo coating (double-layer). The double-layer, NiO2 is formed in the NiCo layer due to the NaCl solution being trapped. Meanwhile, in hot salt corrosion at 600°C, CaCrO4 is formed as a protective oxide layer. Furthermore, in the double-layer, an imperfect oxide layer is formed causing spallation and coating failure. The corrosion rate for single-layer hot salt corrosion for 40 h is 0.310 mm∙y‒1. As a result, the corrosion resistance of the single-layer is increased by the presence of the Cr2O3 oxide layer formed during sintering.