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In the present research, Al2O3 and Cr2O3 compounds and Al2O3-Cr2O3 composite compounds were deposited by atmospheric plasma spraying on St37 carbon steel substrates. The porosity of the as-sprayed coatings was measured by the Archimedes water immersion technique. Corrosion tests on the samples were included immersion and electrochemical impedance spectroscopy in 3.5wt% NaCl solution. Microstructure of the coatings were characterized using optical and scanning electron microscopy before and after the corrosion test. The results showed the best corrosion resistance for pure Cr2O3 that it is attributed to lowest porosities (about 4%). The Cr2O3-25wt%Al2O3 coating showed the highest open and interconnected porosities (about 8.2%) consequently the weakest corrosion resistance. Generally, as-sprayed coatings reached up to 3.5 times increase in charge transfer resistance (332.8 ohm.cm2 for uncoated substrate and1149 ohm.cm2 for Cr2O3 coating) that it was not significant. The reason comes from porous structure of the ceramic plasma sprayed coatings causing easy access ability of the corrosive solution toward the metallic substrate surface. After 28 days immersion test in the same solution, corrosion products were observed on the coatings surface. Elemental distribution analysis confirmed the presence of iron on the coatings surface. This shows that the corrosion products floating in the solution and come out through the interconnected and open porosities.

Keywords: composite coating, plasma spraying, Al2O3, Cr2O3, microstructure, corrosion, electrochemical impedance spectroscopy

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