Effect of current density, electrolyte composition and temperature on hot alternating current anodising
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Hot alternating current (AC) anodising has been established as a fast and reliable pre-treatment process for continuous coil coating. Hydro Aluminium Rolled Products AS (HARP) uses this process as a surface treatment before application of paint for enhanced adhesion and corrosion properties. The objective of this project was to investigate some of the process parameters, such as current density, electrolyte composition and temperature, on the pre-treatment performance. The main focus was to find the amount of aluminium that needs to be converted to remove the outer surface layer, also called the active surface layer, which influences the corrosion properties, and also determine how thick the formed oxide layer should be to ensure adhesion between the coating and the aluminium substrate. The corrosion and adhesion properties of two standard aluminium alloys, 3105 and 1050, were evaluated as a function of the pre-treatment parameters applied. Aluminium samples from both alloys were hot AC anodised, painted with the same polyester coating and then tested for filiform corrosion and adhesion. As-received samples and samples without the anodic oxide layer were also painted and tested, for comparison. A part of the anodised samples was collected from the industrial production, and the other from the simulated process. The simulated process consisted of anodising samples on a laboratory scale, changing one of the parameters at a time. Weight loss experiments were performed to evaluate the effect of the different parameters on the thickness of the anodic oxide layer, and the amount of aluminium consumed. Both as-received and anodised samples were characterised with optical- and scanning electron microscopy, and glow discharge optical emission spectroscopy. The results indicate that the process parameters applied during the industrial hot AC anodising remove more than the activated surface layer. The thickness and characteristics of the activated surface layer are dependent on the aluminium sheet thickness and the type of alloy. Hence the corrosion properties will be different depending on these variables. The absence of an anodic oxide layer seems to have an adverse effect on the adhesion properties and corrosion resistance. The amount of aluminium consumed during the pre-treatment increases linearly as a function of current density and anodising time. The amount of consumed Al is less affected by the electrolyte concentration and temperature, but the chemical dissolution of the metal increases as a function of these parameters. The oxide layer formed is linearly dependent on current density, electrolyte concentration and temperature, and anodising times below 20 seconds.