Along with our other plating services we offer a comprehensive anodising service enabling our clients to treat and colour their aluminium components.

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So what is sulphuric anodising?

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Sulphuric anodising is an electrochemical process that forms a protective, decorative, and corrosion-resistant aluminium oxide coating on the surface of aluminium parts. It is the most common type of anodising and can be dyed in a variety of colors for cosmetic purposes. 

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How the process works

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1. Preparation: The aluminium component is cleaned to remove contaminants. Etching can be used to create a satin finish or remove surface imperfections, though the anodising process itself can degrade the surface finish.

2. Electrolysis: The component is immersed in a bath of dilute sulphuric acid electrolyte. An electric current is passed through the bath, with the aluminium acting as the anode (positive electrode) and a cathode mounted on the side of the tank.

3. Oxidation: Oxygen ions are released from the electrolyte and combine with the aluminium atoms on the surface. This creates a hard, porous layer of aluminium oxide.

4. Growth: The oxide layer grows into and out of the original surface, with roughly 50% of the film penetrating the metal and 50% growing outward. This makes the coating integral to the metal, preventing it from chipping or flaking.

5. Dyeing (Optional): If a color is desired, the porous anodic film is immersed in an organic dye before sealing. The dye is absorbed into the pores, with thicker coatings absorbing more dye and producing richer colors.

6. Sealing: After anodising and dyeing, the porous film must be sealed to close the pores and lock in the color. Sealing is critical for achieving maximum corrosion resistance and light-fastness. Common sealing methods include hot water, nickel acetate, and dichromate. 

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Key properties and applications

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• Corrosion resistance: Sulphuric anodising provides excellent corrosion resistance, especially when sealed.

• Aesthetics: With a wide variety of available dyes, it is the preferred method for producing decorative and colored finishes.

• Wear and scratch resistance: The aluminium oxide layer offers improved hardness and resistance to wear and abrasion compared to bare aluminium.

• Electrical insulation: The resulting oxide film is electrically insulating.

• Cost-effectiveness: It is generally more cost-effective than hard anodising, making it suitable for large-scale production.

• Thickness: The coating typically ranges from 5 to 25 microns. Thicker layers provide greater corrosion resistance and allow for more vivid coloring.

• Applications: It is widely used in industries such as aerospace, automotive, and electronics for components like architectural fittings, domestic appliances, and computer fascias. 

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Factors to consider

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• Alloy composition: The specific aluminium alloy used can affect the final color and quality of the anodised finish.

• Hardness vs. abrasion: While providing good wear resistance, sulphuric anodising is less durable than hard anodising. For parts subject to extreme wear, hard anodising is the better option.

• Jig marks: The electrolytic process requires components to be held by jigs. Small contact points, known as jig marks, will be visible on the finished part.

• Hard vs. standard: Hard anodising (Type III) uses sulphuric acid at a higher concentration and lower temperature to produce a much thicker, harder, and denser coating, but with a more limited color palette.