He Jining

The corrosion behaviour of a plasma spraying Al2O3 ceramic coating in dilute HC1 solution

Abstract Al 2 O 3 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 Al 2 O 3 coatings in a boiling 5% HCl solution is investigated. The corrosion rate of the Al 2 O 3 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 Al 2 O 3 ceramic coating itself are also subjected to some corrosion because of pores and nonequilibrium γ-Al 2 O 3 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.

Studies on nanocrystalline TiN coatings prepared by reactive plasma spraying

Titanium nitride (TiN) coatings with nanostructure were prepared on the surface of 45 steel (Fe-0.45%C) via reactive plasma spraying (denoted as RPS) Ti powders using spraying gun with self-made reactive chamber. The microstructural characterization, phases constitute, grain size, microhardness, and wear resistance of TiN coatings were systematically investigated. The grain size was obtained through calculation using the Scherrer formula and observed by TEM. The results of X-ray diffraction and electron diffraction indicated that the TiN is main phase of the TiN coating. The forming mechanism of the nano-TiN was characterized by analyzing the SEM morphologies of surface of TiN coating and TiN drops sprayed on the surface of glass, and observing the temperature and velocity of plasma jet using Spray Watch. The tribological properties of the coating under nonlubricated condition were tested and compared with those of the AISI M2 high-speed steel and Al2O3 coating. The results have shown that the RPS TiN coating presents better wear resistance than the M2 high-speed steel and Al2O3 coating under nonlubricated condition. The microhardness of the cross-section and longitudinal section of the TiN coating was tested. The highest hardness of the cross-section of TiN coating is 1735.43 HV100g.