The process sequence on an anodizing line
Every EVESS line is engineered around the customer's own requirements, but the concept behind the tank set is common to all of them. Coating quality depends directly on holding the specified regime at every stage.
The five principal stages
The sequence is fixed by the process itself: skip a stage or drift off its regime and it shows up in the finished coating.
- 01
Mechanical preparation
Sets the geometry and surface finish before any film forms: once a part is anodized, the coating cannot be removed without destroying it.
- 02
Chemical pre-treatment
Degreasing and etching. Aluminium oxidises instantly in air and absorbs oils from machining coolant - every contaminant has to come off.
- 03
Anodizing
The core stage: electrochemical oxidation under current in an electrolyte tank.
- 04
Colouring
Where a decorative finish is required - applied immediately after the acid rinse, while the pores are still open.
- 05
Sealing
The mandatory final stage: open pores act as capillaries, and without sealing they will destroy the coating over time.
Degreasing tanks
After machining - turning, milling, polishing - the metal still carries traces of cutting fluid, polishing compound, mineral oils and greases. Those residues undermine adhesion and degrade the coating, which is why surface cleanliness at the preparation stage is critical.
Degreasing tanks are stationary vessels in chemically resistant polymer with compressed-air agitation. Processing runs in alkaline solutions with surfactants added to lift contamination more effectively.
Types of degreasing
Chemical
Alkaline solutions with surfactants at elevated temperature.
Electrolytic
Cleaning under current: more effective, and it needs no high temperature - so there is no aggressive evaporation of the solution.
Combined
In sequence: chemical degreasing first, then electrolytic.
Etching tanks
Etching strips oxide films, scale and corrosion products from the surface - all of which would otherwise compromise the coating.
One thing sets an aluminium anodizing line apart from zinc, chrome or nickel plating lines: etching is carried out in both acid and alkali.
Etching tanks are stationary vessels in chemically resistant polymer with solution agitation, lip extraction, water supply and drain, level and - where required - temperature sensors, and covers.
Types of etching tank
Acid etch
Removes the oxide film and corrosion products.
Alkaline E0 tank
Caustic soda (NaOH) based - the first stage of chemical etching for aluminium and its alloys.
Alkaline E6 tank
Blends out minor defects such as scratches and develops an even matt surface ahead of anodizing.
Desmutting tanks
Desmutting is an activation (pickling) step: it removes the oxides and etch smut left behind before the part goes to anodizing. That gives a more even oxide film and a better coating.
Desmutting tanks are stationary vessels in chemically resistant polymer, most often PVC. Equipment: solution agitation, lip extraction, water supply and drain, level sensor and covers.
Anodizing tanks
Anodizing - electrochemical oxidation - is the principal operation, carried out in aqueous electrolyte in stationary chemically resistant polymer vessels under applied current. The reaction is strongly exothermic, so a cooling system is not optional: a coil and a chiller are part of the scope.
Electrolyte types
Chromic acid (Type I)
Corrosion-resistant coatings up to 10 µm that need neither sealing nor colouring. Suited to riveted and welded assemblies; hardness and wear resistance are low.
Sulphuric acid (Type II)
The most widely used electrolyte, forgiving across most aluminium alloys. Microhardness of 300–500 HV depending on the alloy, with pores that are good dielectrics. Not suited to riveted or welded assemblies. Takes colour well, and the layer is sealed after anodizing.
Combined (Type III)
A sulphuric-chromic electrolyte processed at sub-zero temperature: 400–600 HV hardness and markedly higher wear resistance.
Oxalic acid
Dense, hard, corrosion-resistant films with a natural yellowish cast, and higher dielectric performance on cast alloys.
Phosphoric acid
Highly ordered nanoporous films: gentler oxide dissolution and a larger pore diameter than the sulphuric route, which is why it is specified as a bonding pre-treatment.
Colouring tanks
Colouring is a decisive post-treatment step: it fills the micropores that inevitably form in the oxide layer during anodizing. Done properly it materially raises corrosion resistance and coating life.
Colouring methods
Clear filling
Hydrothermal treatment - a hold in hot water or in dedicated solutions. Hydration of the oxide plugs the pores while the coating keeps its natural appearance.
Metal salt colouring
The pores are filled with solutions carrying metal ions - nickel, for instance. The ions deposit inside the pores, giving a tone and reinforcing the protective properties.
Organic dyeing
Organic dyes penetrate the porous structure, opening up a wide colour range. Sealing afterwards is mandatory - otherwise the dye simply washes out.
Sealing tank
Sealing is the key operation of the finishing stage: it closes the micropores in the oxide layer for good. Porosity is what makes adhesion and colouring possible, but unsealed pores are the coating's weak point - moisture and aggressive media travel straight through them to the metal.
Where the part has been coloured, the sealing tank locks the dye inside the pores so it cannot wash out or fade in service.
What sealing delivers
Corrosion resistance
Closing the pores cuts off the paths electrolytes use to reach the aluminium.
Wear resistance
A sealed layer is denser and harder, and stands up better to abrasion.
Colour durability
The dye is locked into the structure and fades far less in sunlight.
Electrical insulation
Closed pores lower the conductivity of the coating.
Rinse tanks
A rinse tank is a chemically resistant polymer vessel holding demineralised or process water - which of the two depends on the surface cleanliness required and on the next step in the sequence.
Its job is to wash every trace of electrolyte off the parts after an operation. Good rinsing is what stops chemistry being dragged into the following tanks, where it contaminates the electrolytes and produces scrap.
How rinse tanks are classified
By water temperature
Cold (15–39 °C) for standard duty; warm (40–60 °C) after degreasing, after etching light alloys and after chromating; hot (above 60 °C) after alkaline operations and before drying.
By water exchange
Running rinse - a continuous feed of fresh water gives a reliably low contaminant concentration at the cost of higher consumption. Static drag-out tanks capture the electrolyte carried over, and the concentrate goes back to bath make-up or to recovery.
By number of stages
Single-stage is simpler but less economical. Multi-stage counterflow rinsing feeds clean water into the last tank and moves it against the parts: high rinse quality at a fraction of the water consumption.
We will match electrolytes and regimes to your specification
We can work to your existing process, or develop one for you.