Sulphuric Acid Anodising



Sulphuric Acid Anodising

Sulphuric Acid Anodising

Sulphuric Acid Anodising

Sulphuric Acid Anodising


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.


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.


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:


*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.

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.


Five seals are available for application after Sulphuric acid anodising:

Surface Conditioning

We offer a range of surface conditioning options including:

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:

Metro Engineering and Plating Works Ltd - Unit 3 - Chartridge Development - Eskdale Road - Uxbridge - Middlesex - UB8 2RT - UK
Tel: 01895 254666 - E-mail:
Sulphuric Acid Anodising - Chromic Acid Anodising - PTFE Coating - Chromate Conversion - Alocrom - Stainless Steel Passivating
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