MULTI-RESPONSE OPTIMISATION OF ELECTRODEPOSITION OF NANO-CRYSTALLINE COMPOSITE COATINGS Cr-Cr3P IN STATU NASCENDI FROM TRIVALENT CHROMIUM-BATHS

In this paper the effect of parameters such as current density, temperature, electrolytic bath pH and concentration of chromium sulfate and sodium hypophosphite on the electrodeposition process of the Cr-P alloys was investigated. Chemical composition of the coatings was evaluated by scanning electron microscopy and X-ray diffraction. Experimental design 25-1 central composite design was used to evaluate the chemical composition of alloys, current efficiency and the appearance of coatings as well as to optimize the electrodeposition process of the alloy using Response Surface Methodology associated with experimental design. The Harrington desirability function was applied for optimization. The optimal composition of bath (in g/l) and electrolysis conditions are proposed: Cr-2(SO4)(3)center dot 6H(2)O - 285, Al-2(SO4)(3)center dot 12H(2)O - 120, Na2SO4 - 50, NaH2PO2 center dot H2O - 20, CO(NH2)2 - 70, pH - 1.3, temperature - 35 degrees C, current density - 46 A/dm(2). An acceptable current efficiency 13-14 % was observed for an alloy obtained under optimal conditions of 46 A/dm(2), 35 degrees C and pH 1.3 from an electrolyte of optimal composition. Under these conditions, the coating contained 16 wt.% phosphorus. The alloys were X-ray amorphous at a phosphorus content of about 6 wt.%. When the phosphorus content was 16 wt.% the alloys became nanocrystalline and the chromium phosphide - Cr3P phase was released, which can be seen from the peaks on X-ray diffractograms. This indicates the formation of Cr-Cr3P nanocrystalline composite coatings under nucleation conditions (in Statu Nascendi). Therefore, the results of this work show the importance of using optimization techniques to obtain metallic coatings with controlled properties for different types of applications.