Nanostructured PEO-coatings on silumin as environmental catalysts

Abstract

The process of the formation of mixed oxide coatings on a high-silicon aluminum alloy in the cobalt-containing pyrophosphate electrolyte by the plasma-electrolytic oxidizing (PEO) method has been studied. It has been shown that AK12M2MgN chemical composition heterogeneity causes the consumption of part of the anode current to homogenize the treated surface, which is reflected in minimizing the content of doping components at the initial processing stage. It has been established that the growth of mixed oxides Al2O3·CoOx relative mass is a function of time with the maximum at 55 min. The chemical, phase composition and surface morphology of the formed oxide layer depend on the oxidizing time. The catalytic component content in the surface oxides varies from 0.2 to 23.3 at.% with an increase in processing time from 10 to 60 min. Maximum cobalt incorporation into the oxide layer occurs at PEO of 35–50 min, while the silicon content in the surface layers does not exceed 2 at.%, which is favorable for the catalytic properties. The cobalt oxide, preliminary Co3O4, incorporation in alumina α-Al2O3 matrix is visualized by the blue-violet color spheroid surface structures at the sites of micro-arc discharges. The mixed alumina and cobalt oxides layer are characterized by a developed micro globular surface consisting of conglomerate spheroids with an average size of 1–2 µm. There are some amorphous phases in the structure of mixed oxides due to non-equilibrium PEO conditions. The set of detected factors is a prerequisite for the high catalytic properties of oxide coatings. A promising field of Al2O3·CoOx systems application is an intra-cylinder catalysis in the internal combustion engines.