Comprehensive study on corrosion protection properties of Al2O3, Cr2O3 and Al2O3–Cr2O3 ceramic coatings deposited by plasma spraying on carbon steel

Abstract

In this study, individual Al 2 O 3 and Cr 2 O 3 coatings and Cr 2 O 3 -25, 50, 75 wt% Al 2 O 3 composite coatings were applied on carbon steel by atmospheric plasma spraying method. Corrosion experiments were performed on as-sprayed and epoxy resin sealed coatings including potentiodynamic polarization, electrochemical impedance spectroscopy and long-term immersion in 3.5 wt% NaCl solution. Phase composition and microstructure of the coatings were investigated by x-ray diffraction, optical microscopy and scanning electron microscopy, before and after the corrosion experiment. The results showed that the Cr 2 O 3 coating exhibited the best corrosion resistance, due to the densest microstructure and highest adhesion strength. The Cr 2 O 3 -25 wt% Al 2 O 3 coating had the highest interconnected porosities and thus had the least corrosion resistance compared to other coatings. In general, the as-sprayed coatings induced a maximum increase of 3.93 times the polarization resistance (R p ) in the polarization experiment and a 3.5 times increase in the charge transfer resistance (R ct ) in the EIS experiment, which was not significant. Stresses caused by increased volume of corrosion products in the coating-substrate interface resulted in the spallation of Cr 2 O 3 -25, 50 wt% Al 2 O 3 coatings from the substrate over long-term of immersion. The adhesion strength of the coatings was a determining criterion for the long-term durability of the coatings. The sealing treatment resulted in a significant increase in R p and R ct .