Investigation on Structure and Corrosion Behavior of Plasma Sprayed Al۲O۳-Cr۲O۳ Coatings in ۳.۵%NaCl Solution

In the present research, Al۲O۳ and Cr۲O۳ compounds and Al۲O۳-Cr۲O۳ composite compounds were deposited by atmospheric plasma spraying on St۳۷ 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 ۳.۵wt% 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 Cr۲O۳ that it is attributed to lowest porosities (about ۴%). The Cr۲O۳-۲۵wt%Al۲O۳ coating showed the highest open and interconnected porosities (about ۸.۲%) consequently the weakest corrosion resistance. Generally, as-sprayed coatings reached up to ۳.۵ times increase in charge transfer resistance (۳۳۲.۸ ohm.cm۲ for uncoated substrate and۱۱۴۹ ohm.cm۲ for Cr۲O۳ 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 ۲۸ 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 , Al۲O۳ , Cr۲O۳ , microstructure , corrosion , electrochemical impedance spectroscopy , پوشش کامپوزیتی , پاشش پلاسمایی , Cr۲O۳ , Al۲O۳ , ریزساختار , خوردگی , طیف سنجی امپدانس الکتروشیمیایی.