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The porosity within plasma electrolytic oxidation (PEO) coatings strongly affects their properties, such as the wear resistance. Two typical types of coatings, i e. bi-layered coatings with large internal pores, which show low wear resistance, and single-layered coatings with excellent wear performance, have been found following PEO of aluminium alloys under pulsed current regimes with an aluminate electrolyte. In this paper, the mechanisms of formation of the different coatings are investigated and discussed based on a thorough investigation of PEO of an Al-Cu-Li alloy using systematic variations of 1000 Hz pulsed current waveforms and electrolyte concentration. Both parameters have important roles in determining the structure of the resultant coatings. The coatings formed in a dilute electrolyte, containing 5 g l-1 NaAlO 2 , were bi-layered and contained both large pores and pancake structures irrespective of the application of a negative pulse. In contrast, the application of negative current pulse favored the formation of single-layered coatings that contained fewer pancake structures in an electrolyte 32 g l-1 NaAlO 2. A more concentrated electrolyte, containing 56 g l-1 NaAlO 2 , resulted in relatively compact, single-layered

Pure CU2S and copper matte consisting mainly of CU2S have been electro refined by packing coarse particulate material between an inert anodic current distributor (graphite) and a cloth diaphragm.Under these conditions 60-75 % of the copper can be removed anodically without serious polarization. High grade cathodes were produced from N oranda Process white metal even after 20 anode cycles without electrolyte purification.The main cause of anode polarization at low copper extractions has been shown to be CuS04 ·5 H20 crystallization in pores, under conditions where the transport of aqueous Cu 2 + out of the anodic dissolution zone is too slow to prevent super-saturation in CUS04 ·5 H20. These conditions prevail when (a) the anode porosity is too low, as for massive anodes or packed fine powders or (b) where the electrolyte is operated too close to CUS04 ·5 H20 saturation as at low temperatures.At copper extractions above 50 % the cause of polarization is no longer CUS04 ·5 H20 crystallization, but is more probably insulation of residual sulphide particles from the current distributor by elemental sulphur (for pure CU2S) supplemented by lead sulphate (for Noranda high grade matte).Experimental data showing copper extractions and polarization curves are provided for anodes 10-30 mm thick in copper refinery electrolytes containing 40g1-1 Cu and 200g1-1 H 2 S0 4 , operated at 30-50°C at current densities of 300-500 A m -2. Results are also shown for cyclic tests with Noranda high grade matte, including analysis of cathodes, electrolyte, and anode residues.-