- Anodizing Type II (Sulfuric) [MIL-A-8625F]
- Hard Anodize Type III - “Hard Coat” Class 1 & 2 [MIL-A-8625F]
- Chemical Film [MIL-C-5541E]
Anodizing is a conversion of the aluminum surface to practically pure aluminum oxide; the anodic coating. Type II is of particular interest to the designer wishing to extol both the virtues of form and function.
This anodic coating is significantly more abrasion and corrosion resistant than the untreated metal. The coating thickness is a combination of both penetration and build-up, in approximately a 50-50 ratio. This coating may be subsequently dyed in a variety of colors, allowing for a very decorative finish both in a satin and a polished surface result.
Conventional Type II is intended to improve surface corrosion protection under severe service conditions or as a base for paint systems.
Hard anodizing is a term used to describe anodic coatings with surface hardness and/or abrasion resistance as their primary characteristic.
These anodic coatings are usually thick, greater than .001" (50% buildup and 50% penetration), by normal anodizing standards, and they are produced using special anodizing conditions. Thick coatings (over .0004") will tend to break down sharp edges. Alloys with a very high copper or silicon content are less suitable for this process.
The color of the natural anodic coating depends on the alloy and the coating thickness, e.g. 6061 have a tan or gray color which darkens to almost black at .003", 6063 has an amber shade which darkens to bronze. Both are considered clear.
Type III: Thickness as specified on drawing. If not specified nominal thickness shall be 0.002" (+/ - .0002).
Class 1: Not dyed or pigmented.
Class 2: Dyed. (Specify color on contract). Hard anodic oxide coatings find application in the engineering industry for components which require very wear resistant surfaces. "Flash" hard anodize may be used instead of conventional anodize for corrosion resistance.
Chemical Film (Chem Film) coatings are gelatinous films which provide a barrier medium to retard corrosion on aluminum surfaces and enhance adhesion of subsequent coatings such as paints and primers. Materials qualified produce coatings that: range in color from clear to iridescent yellow or brown; are electrically conductive; and impart some measure of corrosion protection. Inspection difficulties might arise with clear coatings because visual inspection does not always reveal the presence of a break in the coating.
Applications for this process include: anodized components (where specific surfaces require electrical conductivity); aluminum stampings; electronic components.
Class 1A - Chemical conversion coatings are intended to provide corrosive prevention when left unpainted as well as to improve adhesion paint finish systems on aluminum and aluminum alloys. May be use on tanks, tubing and component structures where paint finishes are not required for the exterior surfaces but are required for interior surfaces.
Class 3 - Chemical conversion coatings are intended for use as a corrosion preventive film for electric and electronic applications where lower resistant contacts, relative to Class 1A coatings and anodic coatings in accordance with MIL-A-8625, are required. The primary difference between Class 1A and a Class 3 coating is thickness.
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