The corrosion behaviour of a plasma spraying Al2O3 ceramic coating in dilute HCl solution

Al2O3 ceramic coatings plasma sprayed on the surface of metals change greatly the corrosion law of metals in strong acid solutions and enhance effectively their corrosion resistance property. In this paper, the corrosion behaviour of a Q235 steel with plasma sprayed Al2O3 coatings in a boiling 5% HCl solution is investigated. The corrosion rate of the Al2O3 coating sprayed on the surface of Q235 steel is only 1/30 that of the 18-8 austenitic stainless steel under the same corrosion condition. However, the substrate and the Al2O3 ceramic coating itself are also subjected to some corrosion because of pores and nonequilibrium gamma-Al2O3 existing in the ceramic coating. The experimental results demonstrate that at the early corrosion stage the corrosion mechanism is channel corrosion with a large length to diameter ratio for the channels; and the corrosion rate is controlled by the diffusion of corrosion products in the channels; while at the late corrosion stage electrochemical corrosion occurs between the Ni-base alloy bond and the substrate. It was shown that channel corrosion can be restrained and the substrate can be significantly protected by choosing a suitable alloying base layer, increasing appropriately the thickness of the coating, and adopting a porous sealing treatment.