Main process parameters and covers characteristics as criteria for evaluating the effectiveness of the of anodic micro-arc oxidation

The article is devoted to the evaluation of the feasibility and efficiency of modernization of the microarc oxidation process (MAO) by changing the conditions and parameters of sparking, as well as the selection of criteria for assessing the quality and predicting the service properties of formed ceramic-like coatings. The predominant role in changing the nature of structure formation and in ensuring the predicted quality and properties of the oxidized alloy is played by the intensity of sparking, which occurs in the process of electric discharges migrating along the treated surface of the object immersed in the electrolyte. Intensity of sparking determines the conditions for obtaining equidistant coatings: the less stochastic the nature of sparking around the oxidized part is, the more evenly and rapidly the thickness of the formed coating increases and its density is higher. These considerations allowed us offering the option of upgrading method of the anode MAO (AMAO), wherein only the anodic component of the current, causing thereby obtaining thin coatings with extensive porosity by stabilizing the process of arcing through the use of the system of quasi cathodes in the electrolysis bath. In this study, we consider the possibility of controlling the parameters of coatings obtained by the AMDO method in its standard and upgraded versions, by fixing in real time the nature of the distribution of spark discharges, indirect evidence of which is the change in the current density over time and the uniformity of the thickness of the coatings created. Taking into account the influence of micro-arc oxidation on the structural and qualitative features of formed layers, it was assumed that the surface of the coating obtained by the modernized method should have a higher quality, one of the indicators of which is a more smooth and uniform relief. Thus, another criterion for the efficiency of the modernized process can serve as amplitude and step parameters of roughness, which, without allowing an objective assessment of the microprofile of the formed coating, can give more extensive information about the nature of the increase in its thickness and quality changes, determining the level of operational capabilities of metal objects with such a coating.

Development and industrial tests of composite material based on TiB2 for repair of local destructions in electrolyzer bottom blocks

The paper presents the developed composition and technology for obtaining a repair mixture consisting of lumped corundum with a TiB2–C composite coating wettable with aluminum for restoration of local bottom block fractures without electrolyzer stops. The proposed technical solution made it possible to reduce bottom wear and increase aluminum electrolyzer service life by 6 months.A mixture of titanium diboride powder and a refractory powder-like binder in a ratio of 50 : 50 (wt.%) was used to obtain the repair mixture with an optimal composition. Then the lumped corundum was coated with the obtained mixture, dried at 150 °C and after that heat-treated under a carbon-bed at t= 700÷900 °C. As a result of reducing firing the TiB2–C composite material with a carbon content of 15–20 wt.% was formed on the surface of lumped corundum. A qualitative evaluation of the properties of the developed composite coating shows that the coating has a sufficiently high hardness, wear resistance and adhesion to the substrate after the heat treatment. For pilot testing, the repair mixture was covered with molten aluminum to obtain an Al–TiB2–C repair mass in the form of plates. The pilot testing of the repair mass on the 400 kA operating electrolyzer in the RUSAL-Sayanogorsk pilot shop showed that the bottom wear have slowed down 3 months after the local fractures were restored without electrolysis bath stops. This fact is evidenced by a 13 % decrease in the average depth of fractures with a stable current value of 4,7–4,8 kA/bloom after repair. Thus, the locallyused repair mass slowed that the overall wear of the cathode surface and allowed to extend the electrolyzer life.

METHOD FOR ASSESSING SURFACE QUALITY OF COPPER CATHODES

In order to stabilize spiral elongation number at the required level (in accordance with the requirements of the European standard EN 12893 not less than 400 mm) the assessment of its distribution on the cathode sheet was carried out. The experimentally obtained results, processed using methods of mathematical statistics, showed significant discrepancies between the values of spiral elongation in the central and other parts of the cathode (upper, lower, side), which indicates an uneven distribution of the spiral elongation number along the cathode sheet. It was found that in order to form a representative analytical sample under cathode copper production of the Polar Division of MMC «Norilsk Nickel», the testing of cathodes should be performed using methods that include all areas of the cathode sheet, for example, cutting of a vertical strip from each cathode which has got in a representative sample, including edges and hanging ears. To determine the dependence of the spiral elongation number on the surface quality of copper cathodes, developed a special technique, based on the detection of the most frequently encountered defects in the surface of the cathode sheet, which assigned a certain numerical value in accordance with the ranking results. The proposed scoring system for the quality of cathodes in appearance allowed us to establish for the first time that stabilization of the spiral elongation number at a level of at least 400 mm is possible if the defectiveness of the cathode surface does not exceed 17 points per one electrolysis bath. The method has found practical application for preliminary quantitative estimation of the most widespread superficial defects of copper cathodes in the conditions of current production.

The Influence of Surfactant CTAB on the Microstructure and Material Properties of Nickel Microelectroforming

The influence of surfactant CTAB (cetyl trimethyl ammonium bromide) on electrodeposited nickel layer has been investigated in this research. Our result shows that CTAB added in the electrolysis bath can effectively remove the hydrogen to obtain a smoother nickel layer. The effect of CTAB on the surface roughness is also significant. It may be ascribed to the adsorption of CTAB on the electrode surface which results in a hindering effect in the electrodeposition process. Increasing the concentration of CTAB, the grain size of nickel deposit was reduced and the microhardness of the nickel deposit was therefore improved. The hardness of the metal layer can be promoted to 450 Hv when 300 ppm CTAB is added.