Oxidation of a Novel Chromium Coating with CeO₂ Dispersions

Abstract

Chromium coatings have been in widespread use to enhance the high-temperature oxidation resistance of low-alloy steels since the 1950s [Samuel and Lockington, Metal Treatment and Drop forgeing 18, 354 (1951); Sully and Brandes, Chromium, 2nd edn. Butterworths, London 1967]. In this work, a novel CeO₂-dispersed chromium coating with a fine-grain structure was manufactured on a mild-carbon steel by prior composite electrodeposition of Ni with CeO₂ particles, followed by chromizing using a conventionall halide activate, pack-cementation method. For comparison, chromizing was also performed on the carbon steel and on the one with Ni-plating pretreatment. Cyclic oxidation at 840°C for 120 h showed that among the three various samples chromized, the CeO₂-dispersed chromium coating exhibited the best oxidation resistance, while the worst for the coating on the carbon steel. The samples after oxidation were investigated using X-ray diffraction (XRD), scanning-electron microscopy equipped with energy dispersive X-ray analysis (SEM/EDAX) and electron-probe microanalysis (EPMA). The results indicated that although all three coatings grew chromia scales during oxidation, the one formed on the CeO₂-modifed coating was thinner, denser and had a fine-grain structure. Moreover, the formation of “Kirkendall” voids and new precipitates, which occurred during oxidation below the scale in the chromium coating on the Ni-plated sample, was prevented in the presence of CeO₂ particles in the coating. The CeO₂ effects on the coating formation and the coating oxidation behavior are discussed in detail.