Selective Anodizing: Restoring & Fulfilling OEM Requirements


Selective Anodizing: Restoring & Fulfilling OEM Requirements

What is Anodizing?

Anodizing is the formation of an oxide film on aluminum using reverse current (part is anodic) and a suitable electrolyte. Depending on the particular type of anodizing process used, the resulting anodic coating provides improved wear resistance, corrosion protection, and/or improved adhesive properties for subsequent painting or adhesive repair.

(NOTE: Anodizing is not the same as using a chemical film, and the processes are not interchangeable. Chemical films come in the form of chromate conversion coatings, iridite and alodine. A chemical film is a coating which sits on top of the component and is used as a primer to improve adhesion when painting and/or it is used to enhance corrosion protection while still maintaining the component’s conductivity.)

Types of Anodized Coatings

There are five principal types of anodized coatings: chromic, sulfuric, hard coat, boric-sulfuric and phosphoric. These types of anodizing differ markedly in the electrolytes used, the typical thickness of the coating formed, and the purpose of the coating.


o Providing anodizing on previously uncoated parts for corrosion protection.

o Repairing damaged anodized coating to restore corrosion protection.

o Used as a base for paint.


o Providing anodizing on previously uncoated parts for corrosion and/or wear


o Repairing an anodized area for dimensional reasons.

o Restoring corrosion protection of a damaged anodized coating where final appearance is

not a factor.


o Building up worn or mismachined aluminum surfaces to blueprint tolerances.

o Replacing tank hard coat in new part manufacturing.

o Providing wear resistance and/or corrosion protection.


o Environmentally suitable alternative to chromic acid anodizing.

o Providing anodizing on previously uncoated parts for corrosion protection.

o Repairing damaged anodized coatings to restore corrosion protection.


o Preparing aluminum surfaces for adhesive bonding.

In all anodizing processes, three processes occur simultaneously:

  1. Electrolytic etching of aluminum.
  2. Formation of the aluminum oxide (Al2O3) at the aluminum surface.
  3. Dissolution of some aluminum oxide by the anodizing electrolyte.

While the first two processes are developing the anodic coating, the third one hinders its buildup and causes decreased coating hardness. When the anodic coating hardness is a primary requirement, such as in Type III hard coating, the anodizing process is carried out at temperatures ranging from 32°F/0°C to 55°F/13°C, according to the alloy, to minimize the coating dissolution. This requires the use of high-capacity cooling equipment.

Often, the anodized coating is left as formed and subsequently finished by painting or other similar methods. However, depending on the application requirements, some anodized coatings may require dyeing – while others may need to be sealed as a final step.

Selective Anodizing

Selective anodizing is used when limited, selective areas of large, complex aluminum assemblies need anodizing to restore a previously anodized surface or to fulfill an original specification requirement.

Selective (brush) anodizing utilizes the similar techniques of selective (brush) plating but reverses the current flow. When anodizing, the tool becomes the cathode (negative) and the part becomes the anode (positive). The anodized coating (oxide film) is formed on a localized area of the aluminum surface in the presence of the electrolyte (anodizing solution).

Electrolytes for selective anodizing may be in the form of anodizing solutions or gels. The gel is used when working near critical components that may be damaged by splashed or running anodizing solutions. The gel stays over the work area and does not stray into inappropriate places such as aircraft instrumentation, equipment and crevices where corrosion would start. With the gel there is also less likelihood of damage to the airframe. The operating conditions for the gels are the same as for their respective solutions and they apply coatings of the same quality.

For more information on SIFCO ASC’s anodizing capabilities, please contact our Technical Department at 800-765-4131.