What is it? Anodising is the electrochemical conversion of the aluminium surface to an aluminium oxide. This coating is extremely hard (aluminium oxide is used for grinding wheels). And can be coloured by either organic dyes or inorganic metal compounds.

What material can be Anodised? The only material that can be commercial Anodised is aluminium, alloys of aluminium give varying results, generally the higher aluminium content the better the Anodising result. Most casting alloys do not give a satisfactory decorative finish as the alloying ingredients give rise to grey dull coatings.

How is it done? The commercial Anodising process has five basic steps:
1. Cleaning the surface.
2. Chemical etching the surface to obtain a uniform surface finish. This normally done in a strongly alkaline solution such as caustic soda with special chemical additives to give that silver matt finish.
3. Anodising in a 10-15% sulphuric acid solution where the work is made the anode with a DC current. This solution must be closely controlled in chemical content and operating temperature.
4. Colouring, after Anodising the anodic film can be treated with a colouring process which can be dyes or electrodeposited metallic pigments.
5. Sealing, newly formed anodic coatings will absorb materials into their structure. To prevent this coatings are normally treated in boiling water or with special chemicals to "seal" the surface. Sealing locks in colours and increases the corrosion resistance of the surface.

Recommended uses of Anodic coatings. Anodising is used where you require a finish that retains the appearance of aluminium, a finish that is not going to affect the function of threads or machined surfaces, a heat resistant finish (Anodising will stay intact to near the melting point of aluminium) or a finish that will not peel or chip.

How colourfast are the coatings? Anodic coatings coloured by the electrodeposition of inorganic coatings compounds are very lightfast and are commonly used for full exterior exposure on buildings. Unfortunately these compounds have a limited colour range, from light bronze through to dark bronze and blacks. Colours produced by organic dyes have a wide colour range but do not have the colour fastness of the electrodeposited colours and are generally only used for a interior use.

Where should Anodising not be used? Anodising is sensitive to alkaline chemicals so compounds such as wet cement or machine dishwashing chemicals will attack the anodic coating.

What thickness is Anodising? Anodic films range from about 5 microns to about 30 microns; the thicker coatings being used for external exposure.

What is bright dipping? Bright dipping is a chemical etching process that creates a bright finish on the aluminium surface before Anodising. These are strong concentrated phosphoric and nitric acid mixtures that are operated at temperatures over 100c. Special alloys are manufactured to give the best result for bright dipped finishes. Normal commercial alloys give a semi gloss finish.

Does Anodising change product dimensions? In theory anodic coatings increase the overall dimensions of the component, but because the products are normally etched to obtain a uniform surface appearance most products will finish undersize . This reduction will depend on the degree of etching the product has been exposed to. Always inform us if there are any special tolerance requirements. If your components are heavily etched before Anodising to achieve a "surface finish" your product could finish undersize after Anodising, even with a 20-micron coating.

How flexible are Anodic coatings? Anodic coatings are glass like and will crack if formed after Anodising. Even so cracked coatings will not flake from the surface. Badly cracked coatings will affect corrosion resistance.

Can I machine Anodic coatings? Anodic coatings, particular the thicker coatings will quickly dull the edge of cutting tools. If you must do post Anodising machining it is recommended to use tipped cutting tools.

Does Anodising conduct electricity? Anodising is a very good insulator , particularly when coatings are over the 20 micron thick. If your products need earthing points the Anodising will need to be removed post Anodising (ground off) or masked during the process.

What are the design considerations for trouble free Anodising?. Don’t design products that will trap chemicals during Anodising, spot-welded, riveted or assembled parts generally cause problems in Anodising. Don’t use mixed metals in an assembly. Any other metal. (even stainless steel) will either be attacked by the Anodising process or prevent the part from anodising. Try to avoid making your product from different aluminium alloys as each alloy will give a different finish.

Don’t use marking pens or other inks/paints on your products that will not come off in the cleaning process. The Anodising process could permanently etch these markings into your product, or we could charge you additional for removal. Don’t allow your parts to become wet as even that slight white corrosion can be greatly magnified by the chemical etching process and result in rejects. It is also best to specify coating thickness or tell us the end use of the product.

Can I powdercoat over anodic coatings? Anodising is not the best surface for painting or powdercoating over as adhesion will be greatly reduced as compared to a pretreated aluminium surface. If optimum performance is required it is recommended that the anodic coating be stripped and the aluminium conversion coated before painting or powdercoating.If the product is to be selectively powdercoated or painted the heat in the oven can cause crazing of the anodic coating which may affect the appearance of the product but will not alter its performance in other ways.

What does it Cost? Anodising is one of the more expensive metal finishes as the Anodiser has to meet many problems unique to this industry. Waste disposal of the spent chemicals, expensive plant, high electricity costs and racks that need constant replacement. The thicker the coating the higher the costs. Low volume, heavy items, small items (that have increased labour costs in racking)and special finishers may incur higher charges.

Can Anodic coatings be used in contact with food? Anodising has been used in the cookware industry for many years and is not considered to be any more toxic than the aluminium that the product is made from.

What are chromate conversion coatings? Chromate conversion coatings are formed on the surface of the aluminium that will inhibit corrosion of the aluminium. These coatings are softer than Anodising and are not suitable for wear applications, but are ideal for use on interior components to prevent white rust or as a base for organic coatings. These coatings are normally brownish in colour but can be leached to form a clear film.

What is the maximum size of finished products tanks? Our tanks are 3 meters long by 0.6 meters wide. Generally we consider that 2.8 meters long by 0.4 meters wide is the maximum size we can process.

For more information on Anodising or to find out how Finished Products can help you achieve outstanding results for your surface finishing requirements, please feel free to contact us by telephone on +612 9602 9833 or send us an email by clicking on the "Contact Us" link below.