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?
- 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
- Anodising is a cost effective process
Please note: chromic acid anodising is not suitable for the following:
- Aluminium alloys containing a maximum total of 7% copper, nickel and iron or more than 6% copper
- Colour dyeing due to the thin coating layer as most colours will appear blotchy (except black which is usually effective)
- Welder assemblies or components with blind holes
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
- 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 & 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 contraining 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
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.