What is the context?

Anodized products and components are used in thousands of commercial, industrial and consumer applications, including structures and architectural categories of all types, appliances, building products, furniture, motor vehicle components and others.
Anodizing, or anodising, is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts. Anodizing increases corrosion resistance and wear resistance, and provides better adhesion for paint primers and glues than bare metal. Anodic films are most commonly applied to protect aluminium alloys, although processes also exist for titanium, zinc, magnesium, and niobium.
Electrocolouring is an AC electrochemical process which fills the pores of aluminium oxide layers to give them a decorative coloured appearance. Originally colours ranged from bronze to black, but more recently a wider range of colours can be produced.

What are the issues?

The surface of the aluminium product is converted into a protective oxide layer by connecting the product in an acidic solution to the positive pole of a rectifier. Due to the high electrical resistance of the oxide layer, a large over potential (10 to 30 V) is needed to obtain a reasonable high current density (500 to 3000 A/m2). Therefore a considerable amount of heat is generated (5000 W/m2 of more) on the electrode-electrolyte interface. Traditionally, most of the produced heat is removed by forced convection of the electrolyte. This agitation is not always homogeneous but is a key factor in the overall performance of the anodizing cell.
For the electrocolouring processes, also the hydrogen evolution is an important phenomenon to be accounted for.

What value do we bring?

For continuous anodizing & electrocolouring of aluminium, Elsyca has developed a theoretical model for the entire assembly of electrical circuit, anodizing tank and aluminium product and for the formation of the oxide layer at the anode. The model includes:

• fluid flow of the electrolyte due to forced convection;
• temperature and heat flux distribution in the electrolyte and aluminium product;
• electrical potential and current density distribution in the electrolyte and the aluminium product;
• electrical resistance of wires and connections;
• leakage currents and grounding;
• a non-linear relation between the local surface temperature, potential and current density describing the anodizing reaction (oxide layer growth);
• heat generated in the oxide layer, aluminium and in the electrolyte.

Optimizing both the process parameters and cell configuration of anodizing/electrocolouring processes can be handled in an effective and accurate way within the framework of an engineering project, using an in house software solution that models all of the above phenomena. Previous projects have shown that the benefits for customers can be significant:

• an increase of 10% of the line speed for continuous anodizing of lithographic plates while maintaining the required hardness;
• a decrease of 20% in the energy consumption during the anodiszng of aluminium for domestic and architectural applications.