Anodizing is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts. Anodic films are most commonly applied to protect aluminum and titanium alloys in the aerospace, automotive and consumer electronics industry.
For hard anodizing the distribution of the oxide thickness is mainly governed by the non-uniform current density distribution while for high speed anodizing additional non-uniformity due to local heat production becomes important. Typically, most of the produced heat is removed by forced convection of the electrolyte. This agitation is not always homogeneous and is a key factor in the overall performance of high speed anodizing processes
Optimizing both the process parameters and cell configuration of anodizing 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 relevant phenomena. Previous projects have shown that the benefits for customers can be significant:
- an increase of 10% of the line speed for continuous reel-to-reel anodizing of lithographic plates while maintaining the required uniformity;
- a decrease of 20% in the energy consumption during the anodizing of aluminium for domestic and architectural applications.
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.
Elsyca has performed successful projects for leading international companies using reel-to-reel anodizing and electrograining processes on aluminum strips, for workpiece anodizing and for continuous anodizing and electrocolouring. The main goal is to increase the production speed and/or to decrease energy consumption while maintaining the same product quality and oxide thickness uniformity by changing the cell design and the process conditions.
Anodising is a process in which the surface of an aluminium product is converted into a protecting 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 overpotential (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 will not always lead to homogeneous cooling of the aluminum parts, but is a key factor in the overall performance of the reactor. A considerable fraction of this heat might also be accumulated into the product itself.
To quantify all the effects mentioned above, a theoretical model for the anodising reactor, the aluminium product and for the formation of the oxide layer at the anode is proposed. This 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, and in the aluminium itself,
- leakage currents and grounding when multiple cells are used,
- a non-linear relation between the local temperature, potential and current density describing the anodising reaction (oxide layer growth) depending on the type of aluminium and electrolyte,
- dependence of the material properties (conductivity, viscosity, density) on the temperature
- heat generated by the formation of the oxide layer, and due to the ohmic losses in the electrolyte and the aluminium.
Elsyca developed an advanced in-house numerical simulation tool to accurately take all these effects into account. This tool allows an accurate calculation of the local current density and temperature distribution both in the electrolyte and in the aluminium. Based on these data the oxide layer thickness is determined and the morphology can be extrapolated from experiments.
Together with academic partners Elsyca has significant experience in quantifying the anodising process through dedicated polarization measurements.
- An example of simulations for batch anodising of a workpiece is shown here.
- For an example on continuous anodizing of aluminum foil, click here.