Anodising is an electrolytic passivation process that creates a thicker natural oxide layer on the base metal.
What Metals Benefit from Anodising
Aluminum is the most commonly anodised metal, however other metals such as titanium, magnesium, zinc, niobium, zirconium, hafnium and tantalum can also benefit being anodised.
Benefits of Anodising
Benefits of increasing the thickness of the oxide layer or anodising include:
- Increasing corrosion resistance
- Increasing wear resistance
- Reduces friction and effectively increases lubricity
- Promotes improved adhesion by paints and glues
- Creating coloured coatings
- Preventing galling of threaded components
- Creating dielectric films for electrolytic capacitors
- Colour-fast coatings
- A range of different gloss finishes available
The Anodising Process
The anodising process is so-named because the part to be treated acts as the anode in an electrical circuit.
It involves immersing the metal to be anodised into a bath containing an acid electrolyte and a cathode. The metal to be coated becomes the anode and an electric current is passed through the acid bath.
The electric current induces an oxide to form on the surface of the anode, with oxygen ions emanating from the acid. This makes anodising a controlled oxidation process.
Anodising can be carried out in both batch and continuous modes. In continuous mode, it can be used to treat coiled materials.
In either case, anodising processes follow 5 steps:
- Cleaning – Acid or alkaline cleaners remove dirt and grease
- Pre-Treatment – Matt surfaces can be produced by etching with sodium hydroxide, which also removes some surface imperfections. Alternatively, a high gloss finish can be produced by treating with a concentrated mixture of phosphoric and nitric acids.
- Anodising – By passing an electric current through the cell as described above, an oxide layer with controlled thickness can be grown on the metal surface.
- Colouring – This can be accomplished using processes including electrolytic colouring, integral colouring, organic dyeing or interference colouring.
- Sealing – Pores in the oxide film are sealed resulting in a surface that is resistant to staining, abrasion, crazing and colour degradation.
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Environmental Impact of Anodising
Some metals that are commonly anodised, like aluminium are also easy to recycle. Their recyclability is not affected by the anodised coating.
The process itself is also environmentally friendly as it uses water-based electrolytes and as a result does not produce any VOCs. In accordance with EPA rules, anodising does not produce any hazardous waste.
Properties of Anodised Surfaces
Anodising changes the morphology of the surface of the metal being treated. Thick anodised coatings are typically porous and need to be sealed to provide corrosion resistance. In the case of aluminium anodised in sulphuric acid, it can be sealed by hydrothermal sealing, precipitation sealing or converting the amorphous oxide coating into a more stable microcrystalline coating. Susceptibility to corrosion can also be a result of the formation of fissures in the coating.
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Anodic coatings also tend to be stronger and have better adherence compared to metal plating and paint which makes them less likely to crack and debond from the substrate. They are also more brittle and hence more likely to crack from thermal stresses due to the fact they take on properties of the oxide which has a lower co-efficient of thermal expansion and thermal conductivity compared to the underlying metal. Thermal cracking will occur perpendicular to the surface and will not result in peeling of the oxide layer.
Applications for Anodised Materials
Anodised aluminum has many applications including:
- Architecture – Window extrusions, balustrades, stair treads, storefronts, curtain walls
- Marine – Yacht masts, pontoons and decks
- Food preparation equipment
- Sporting goods
- Automotive components
- Scientific equipment