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Pleasant Appearance and Surface Protection

Aluminum-finishing is generally the last process.

It is applied to aluminum items or constructions after all other fabrication processes are completed.

As such it comes also after welding.

Welding is used for joining separate elements obtained by all available production methods.

Among these are casting, forging, forming, machining etc.

Aluminum-finishing provides an impervious protective coverage on the external surfaces of aluminum bodies.

That assures long time standing in normal environments.

The most known and used processes employed for Aluminum-finishing are described as anodizing.

It consists in electrolytic passivation processes that increase the thickness of the natural oxide layer on the surface of aluminum parts.

Anodizing can be done in a number of ways that convert the aluminum surface into a thin layer of aluminum oxide or alumina, while the parts are the anode in an electrolytic cell.

Among the advantages of this Aluminum-finishing product one remarks:

  • Improvement of corrosion resistance in atmospheric and salt water environment.
  • The anodic coating works as a barrier hindering the penetration of corrodents.
  • The amorphous porous aluminum oxide is sealed by one of several methods.
  • Increased adhesion of paint or clear finishing providing an active surface to which most of the primers attach strongly.
  • Paints over anodized surfaces protect even more from attack in severe corrosive conditions.
  • Ease of subsequent plating if needed.
  • Improvement of appearance for decorative purposes, with a lustrous colored coating providing some abrasion resistance.
  • Different appearances may be produced by selecting the most suitable finish before anodizing.
  • Remarkable increase of wear and abrasion resistance is achieved, when needed, by a special process known as hard anodizing.
  • Development of an electrical insulating layer displaying properties suitable to the application.
  • Capability of detection of surface flaws like fine cracks and of previous welds.

Three are the main types of Aluminum-finishing anodizing, known as:

  1. Sulfuric acid process, is probably the most used, that provides a quite thick oxide layer and good protection against corrosion.
  2. Chromic acid process, applies thinner but more elastic coatings, considered preferable for items subject to alternating stressing (fatigue).
  3. Hard anodizing, specified when a hard, wear resistant surface is required.

Aluminum-finishing Growth

When Aluminum-finishing by anodizing of parts machined to final dimensions with tight tolerances, one should remember that the processes add a certain amount of thickness to the dimensions measured before processing.

The measured growth is partially within the volume of the original part and is partially towards the outside.

Although empirical indications are available, only actual measurements will suggest the best dimension to give to the finish machined part.

Practical testing will permit to obtain the drawing dimension and tolerance after anodizing.

In fact most drawings will indicate: "Dimensions after anodizing".

The thickness of the anodized layer is best determined on a cross section ground and polished, by metallographic examination under an optical microscope with calibrated scale eyepiece.

The thickness can also be determined by measuring with a calibrated micrometer the thickness of an anodized specimen. The Aluminum-finishing anodized coating is then stripped using the solution described in

ASTM B137-95(2009)
Standard Test Method for Measurement of Coating Mass Per Unit Area on Anodically Coated Aluminum
ASTM International / 15-Apr-2009 / 2 pages

The thickness of the same stripped specimen is then measured again.
The anodized layer thickness is given by the difference between the two readings.

If the original thickness of the specimen before anodizing is known, one can easily confirm that its thickness after stripping is less than the original one.

The following Resources may provide additional information if required:

Accepted Standards for the processes are:

SAE AMS-A-8625 (previously known as MIL-A-8625)
Anodic Coatings for Aluminum and Aluminum Alloys
SAE International, 01-Jul-2000
( Cancelled: Jul 2003 )

Anodizing of aluminium and its alloys - General specifications for anodic oxidation coatings on aluminium (ISO 7599:2010)
German version EN ISO 7599:2010
DIN-adopted European-adopted ISO Standard / 01-Dec-2010 / 29 pages

SAE AMS-C-5541 (previously known as MIL-C-5541)
SAE International, 01-Jun-1999
Chemical Conversion Coatings on Aluminum and Aluminum Alloys
Withdrawn: ( Cancelled: Jul 2003 )

These, sometimes known as Alodine, are extremely thin chemical coatings that are mainly used as local repair of damaged or removed anodizing.

Conversion coatings provide enhanced corrosion resistance and improved bonding ability while remaining electrically conductive.

These are not electrical processes, so that they are performed by immersion or by swabbing the surface with the prepared liquid solution.

An Article on Filler Metals for welding aluminum alloys was published (4) in Issue 163 of Practical Welding Letter for March 2017.
Click on PWL#163.

Summing up, Aluminum-finishing by suitable processes should be integrated into any fabricated product as a necessary part of design, not as an afterthought.

Commercial sources can probably assist in the selection of useful processes if consulted before finalizing design.

Watch the Video on Hard Anodizing of Aluminum

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