Chromic Acid Anodising
Your Guide to Chromic Acid Anodising
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Penta Precision is able to offer Chromic Acid Anodising as part of our one-stop machining service in the UK.
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 and helping to streamline your supply chain.
Customers turn to us when they have had quality and delivery issues with their existing suppliers and need a reliable machining partner.
Why Choose Chromic Acid Anodising?
- Anodising is a cost effective process
- Less than 5µm coating thickness
- Corrosion resistant to atmospheric and saltwater environments
- Aluminium film is dielectric so suitable for electrical insulation
- Chemically active surface for painting
- Adhesive bonding improves adhesion and bond strength
- Paint adhesion provides a chemically active surface for painting
Chromic Acid Anodising
Chromic Acid Anodising Process
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.
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.
PLEASE NOTE: As a result of the utilisation of hexavalent chromium, there is a gradual discontinuation of chromic acid anodising wherever feasible. Globally, laws and regulations are being implemented to restrict its usage, and in certain instances, it is banned outright. If you are in the process of designing a new product or updating an existing one, you might want to explore the option of using sulfuric acid anodising instead. Our technical team is available to support you in making informed decisions, so feel free to contact us for assistance.
- DEF STAN 03-24 (formerly DEF 151 Type 2)
- US MIL-A-8625 Type 1
- Def Stan 21-5/2-2
- MIL-STD-171 7.1.1
- TS 112 D1
- JP 213 Method 2
- ASTM B580 Type G
- Type I Anodizing (North America)
Components, Applications and 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.
- Defence industry components – especially those involving explosives, propellants or pyrotechnics where complete removal of electrolye is essential for safety reasons
- Components containing crevices or small blind holes
- Pre-treatment for painting, especially in aerospace applications
- Flaw detection
Flight-critical aluminium components that are subjected to high stresses, such as landing gear
Chromic Acid Anodising is not suitable for:
- Use on aluminium alloys containing a specified maximum total of 7% copper, nickel and iron or more than 6% copper
- Colour dyeing - due to the thin (2-5µm) coating layer as most colours will appear blotchy, however black dyeing is usually quite reliable and effective
- Welded assemblies or components with blind holes
The coating is on metal less than 5 microns thick, the process and resultant coating have less adverse effect upon the properties of the underlying metal.
Although the process is controlled anodising needs to be regarded as an “artisan” process and not a high precision process.
Due to the use of hexavalent chromium, chromic acid anodising is gradually being phased out wherever possible. Laws and regulations are being enacted around the world both restricting its use and in some cases banning it outright. If you are designing a new product or updating an existing one, you may wish to consider using Sulphuric Acid Anodising, there has been developments with mixing sulphuric acid and either Tataric acid or Boric acid as a method of replacing the less environmentally acceptable Chromic acid solution.
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