Sulphuric Acid Anodising

Sulphuric acid anodising is the most common protective treatment for aluminium and leaves a clear (“natural”) silver coloured finish, which can be subsequently dyed. The anodising process involves the oxidation of the aluminium surface to form a thin layer of aluminium oxide under controlled chemical conditions – this process occurs naturally at a much slower rate for aluminium exposed to oxygen in the air. As anodising does not involve the coating of the aluminium with a different metal (or other man made coating), the process will not hide any surface imperfections or blemishes in the aluminium substrate. In fact, the vigorous nature of the process will generally degrade the surface finish of the aluminium – this should be taken into account when considering the quality of the surface material to be treated if a highly decorative finish is required.

Anodising is an electrolytic process which requires an electrical contact point at all times during the treatment – this means that a very small set of jig marks will always appear after treatment. It is important that we are made aware of any specific jigging requirements where certain surfaces must be free from jig marks for decorative purposes or for critical bores and sealing surfaces.

Sulphuric acid anodising is generally used where a decorative and corrosion resistant finish is required. The resulting anodic film is usually between 5 and 20 microns thick and is electrically insulating. We typically treat components in all industry areas with special emphasis in the electronics, motorsport, aerospace, medical and general engineering sectors.

Materials

Sulphuric acid anodising is available across all commercially available alloys and castings. The most common alloys treated by Metro are 2000, 6000 and 7000 series. Due to the differing levels of metallic elements used in the various alloys, the anodised finish can vary significantly and it is advisable to discuss the choice of alloy with us at the design stage for new components. We have particular expertise in the handling of 2000 series (copper rich) alloys such as 2014A and 2024 where particular care is needed to avoid burning or melting of the alloy during the anodising process.

Castings

We can carry out sulphuric acid anodising but recommend that suitability should be discussed beforehand.

Maximum Size Criteria

The table below outlines the maximum size criteria for sulphuric acid anodising:

Dimension	Clear	Black*
Length	2430mm (8')	1829mm (6')
Width	910mm (3')	910mm (3')
Depth	910mm (3')	910mm (3')

*Dimensions for all other colours should be discussed specifically as our dye baths vary in size.

Coating Thickness

Sulphuric acid anodising provides a coating of between 5-25µm with a thicker coating giving a greater corrosion resistance. Where colour dyeing is required a surface coating of 15-25µm is recommended.

Available Standards

We predominantly work to 3 specific standards as detailed below although we are happy to work to any specification provided.

• DEF STAN 03-25 (formerly DEF 151 Type 1)
• BS1615
• US MIL-A-8625E

PTFE Coating

Anodised parts can be treated by immersion into a highly concentrated PTFE dispersion. This process will produce an abrasion resistant coating with a high degree of lubricity.

Sealing

Five seals are available for application after Sulphuric acid anodising:

• Hot Nickel Acetate Seal - for greater corrosion protection
• Hot De-mineralised Water Seal
• Dichromate Seal (yellow-green finish) - for high corrosion protection in salt water environments
• Cold Seal
• 3 Stage Sealing for outdoor use

Surface Conditioning

We offer a range of surface conditioning options including:

• Pre-treatment/Etching - to reduce surface scratches
• Satin Finish - achieved through increased etching
• Masking - available to customer requirements

Jigs and Component Preparation

Jigs are provided for mounting and dipping components. It is recommended that good jigging features are added to components at the design phase as contact marks will not receive a surface coating and can result in visible blemishes. It is also recommended that welding on components is minimised and that all non-aluminium parts, such as steel inserts, are removed before anodising. Masking can be carried out for holes and threaded sections.

Colour Dyes

Colour options are available with sulphuric acid anodising for an additional charge. We use the selected colours below from the Clariant dye range although other colours are available for large orders:

Gold	Yellow	Aluminium Orange
Aluminium Bronze	Pink	Aluminium Fiery Red
Aluminium Bordeaux	Aluminium Violet	Blue
Aluminium Green	Aluminium Grey	Deep Black
	Olive Drab (for military applications)

Please note that these colours are meant as a guide and no guarantee can be made for a match between this online colour swab and the final colour. Also, due to the varying nature of alloys and dyes, we cannot guarantee colour consistency between orders over time or even across large batches.