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ANODISING

Index

General

Anodising or anodic coating is an electrochemical process aiming at the production of stabile films of aluminium oxide on the surface of parts in aluminium and aluminium alloys.

The oxide film is made of a barrier layer, compact, in contact with the metal surface, and of an external porous layer.

The anodic film is obtained using direct or pulsing current and a suitable electrolyte.

The specific electrolyte and the way the process is carried out determine the relevant properties of the anodic film, and in particular its porosity. If required, the porosity can then be sealed with a suitable post-treatment.

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Scope

Anodising is carried out for one or more of the following purposes:

In this latter case paramount importance is given to proper selection of alloy and product, and to the definition of the properties expected from the process.

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Types

Depending on the specific electrolyte used, the thickness of the film and the operational details of the process, the following main types can be defined:

Porous films can subsequently be subjected to coloration processes, if required.

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Anodising is a special process

Based on the definition of para. 7.5.2 (Validation of Processes for Production and Service Provision) of ISO 9001: processed for production provision, where the resulting output cannot be verified by subsequent monitoring or measurement and, as a consequence, deficiencies become apparent only after the product is in use.
For these processes the referenced standard requires to define and implement, as far as applicable, the following items: criteria of review and approval of the process, approval of equipment and personnel qualification; use of specific methods and procedures; records.
Specification AMS-A-8625 requires that the anodiser prepares a process control document including at least the following items:

And that periodic analysis of each process bath are performed.

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Aluminium alloys and anodising

Aluminium alloys are more or less suitable to the anodising process bases on their chemical composition; This is particularly true when specific appearance requirements are expected from the process.
In general, the higher the alloying elements content, the lesser the alloy is suitable for anodising, and so the best alloys are those with low mechanical properties..
Without taking into account appearance, AMS-A-8625 sets fort a limit of:

The Aluminum Anodizers Council (AAC) defines the concept of “Anodizing Quality” alloys, that is alloys perfectly suitable for architectural components, and defines the following grading of alloys, as far as attitude to anodising is concerned, in particular for sulphuric acid anodising:

Series (AA)

Alloying constituents

Metal properties

Coating properties

Uses

Anodizing Quality
Types

Non Anodizing Quality types

1XXX

None

Soft
Conductive

Clear
Bright

Architectural
Cans

None

1100
1175

2XXX

Copper

Very strong
Hard
Low elongation

Yellow
Poor protection

Aerospace
Mechanical

None

2011
2017
2219
2224

3XXX

Manganese

Strong
Small grain

Greyish-brown

Architectural
Cans
Lighting

None

3003
3004

4XXX

Silicon

Strong
Fluid

Dark grey

Architectural
Lighting

None

4043
4343

5XXX

Magnesium

Strong
Ductile
Fluid

Clear
Good protection

Architectural
Lighting

5005
5657

5052
5252

6XXX

Magnesium
Silicon

Strong
Ductile

Clear
Good protection

Architectural
Structural

6063
6463

6061
6101

7XXX

Zinc

Very strong

Clear
Good protection

Aerospace
Mechanical

None

7029
7046
7075

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Sulphuric and hard sulphuric acid anodising

Hard sulphuric acid anodising is a technical process aimed at building thick (up to 0.08 mm), hard anodic coatings in order to improve the wear and abrasion resistance of the base metal.
The process is designed in order to maximise such functional properties, regardless to appearance.

Sulphuric acid anodising (for corrosion protection and appearance) is the most used process in general engineering; it is expected in general to give, besides good corrosion resistance, an uniform appearance.
This requires that the process is performed in skilled and controlled manner.

Both in Europe and in Italy have been established Associations, e.g. AITAL (Associazione Italiana Trattamenti superficiali Alluminio) that perform activities in the field of R & D, spreading of knowledge, advising and technical definition of processes, and Agencies, e. g. QUALITAL (Istituto di Certificazione Industriale dell’Alluminio ed altri Materiali) operating in the field of quality certification, with particular attention to surface treatments and aluminium products.

In order to help the user to review the implemented process, in the continuation some data sheets are enclosed, that can be downloaded from the AITAL website.

Download (*)
Schede monografiche sull’OSSIDAZIONE ANODICA DELL’ALLUMINIO Anodising data sheets
   

(*) For pdf documents require installing of Adobe Reader

 

 

 

 

 

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