Chromic Acid Anodising | Machine Parts Finishing | Penta Precision       +44 (0) 2392 668 334

Chromic Acid Anodising

What is Chromic Acid Anodising?

As aluminium “oxidises” (the natural reaction between air and aluminium) it produces a loose dry white powder, aluminium oxide.

Chromic acid anodising is an electrolytic process using electricity (DC current) and a Chromic acid electrolytic solution causing oxidation of the aluminium surface to form a thin dense flexible grey “harder” film of aluminium oxide under controlled conditions.

The passing of the current liberates high volumes of Oxygen at the surface of the aluminium, thus aiding controlled formation of the oxide layer.

These processes do not produce a coating but are a conversion of the surface aluminium to aluminium oxide and are therefore metallurgically bonded.

This method produces a comparatively thin, flexible but dense oxide film which is usually dark grey in colour. 

It has very good corrosion resistance but isn't really suited to subsequent dyeing.


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Because the film is thin, it doesn't affect close tolerances and is therefore used extensively in the aerospace and defence industries.

If left unsealed it is an excellent base for subsequent painting operations as the first coat of paint soaks in to the pores.

Chromic acid anodising is ideal for structurally important components as is often the case in aerospace and defence applications.

The thin film reduces the possibility of fatigue fracture on the component and unlike Sulphuric acid, Chromic acid protects rather than attacks aluminium; any residual solution that may have been trapped in cracks, pores or folds will not corrode.

In addition, the process also acts as a non destructive flaw detection test due to the brownie/orange colour of the chromic solution and the fact that it is extremely searching and will thus seep in to even the smallest of flaws; thus a stain will become apparent around the flaw. 

This test is used commonly on defence related products.

Why Choose Chromic Acid Anodising?

Please note: chromic acid anodising is not suitable for the following:

Due to the use of hexavalent chromium, chromic acid anodising is gradually being phased out wherever possible.

Laws and regulations are being enacted globally; restricting its use and in some cases, banning it outright.

Therefore if you are designing a new product or updating an existing one, you may want to think about using sulphuric acid anodising.

Similar or obsolete designations and standards


Components, Applications & Industries

Chromic acid anodising is used primarily within the defence and aerospace sectors and industries where subsequent painting or adhesive bonding is required or in applications where crack detection is needed.

It is the preferred process for the treatment of castings and other components where loss of fatigue strength from the anodising process needs to be avoided.

Applications include:

Our Service

Penta Precision is able to offer Chromic Acid Anodising as part of our one stop machining service. As well as our own machining expertise, we are able to access our network of approved quality suppliers for the finishing of your parts, saving you the time and effort.

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"It has been a pleasure to work with Penta Precision with clear, concise and accurate communication and products exactly meeting our specification."Anthony - Quality Manager

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