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Posted: 10/22/2004 2:16:03 AM EDT
what are the differnt types of anodizing available? i have seen anodized lowers that looks just like spray painted fin, some scratches and wears off easily. What type should be used for lower receivers?
who can you recommend that anodizes like the original finish?
Link Posted: 10/22/2004 6:25:52 AM EDT
There are a number of types of anodization available, as well as mil-specs and 'market hype'.
Of concern to you are the following:

Type II - A thickness of less than 25.4 microns (< 1 mil). Density of the anodized layer is not specified and it is easy to dye.

Type III - A thickness of more than 25.4 microns (> 1 mil). As part of the specification there is a minium density of the anodization layer that is defined. Problem is that the density cannot be determined easily, unless they have an electron microscope in the back room, or they do destructive testing of control samples during the anodization process. No one does this, except for scientific, military/government specs or special circumstances. It is also very difficult to dye. Type III is also known in the industry as Hardcoat...not to be confused with what many AR15 companies say are Hardcoat (or Hardcote).

Mil-Spec - Meets density requirements of Type III Anodization with a thickness of more than 50.4 micron (>2 mil)

Common Usage of Hardcoat (Hardcote) Term - Thickness of anodized layer is greater than 25.4 microns, but does not meet the density requirement . It is just a thicker Type II coating which dyes well. This is what Bushmaster, RRA and Colt uses in their commercially available uppers/lowers which have a thickness of 26-35 microns.

Generally speaking, if you've got surfaces that look like paint, then they are most likely painted and not anodized (unless the paint was applied over an existing anodized surface). An anodized surface does not wear off easily and if it is dyed, the color is actually absobed by the anodization and will not have any of the characteristics of an applied layer such as paint.
Link Posted: 10/22/2004 7:57:20 AM EDT
[Last Edit: 10/22/2004 7:57:33 AM EDT by mongo001]
Link Posted: 10/22/2004 8:10:56 AM EDT
Milspec is Type III with nickel acetate seal. Colt, Bushmaster and LMT all calim they use this.
Link Posted: 10/22/2004 9:35:13 AM EDT

Originally Posted By DevL:
Milspec is Type III with nickel acetate seal. Colt, Bushmaster and LMT all calim they use this.

Partially true - Mil-Spec specifies that:

3.8.2 Type III. Type III coatings shall not be sealed where the main
function of application is to obtain the maximum degree of abrasion or wear
resistance. Where Type III coatings are used for exterior non-maintained
applications requiring corrosion resistance but permitttng reduced abrasion
resistance, the contract or purchase order shall specify that sealing is
required. Sealing for such Type III coatings shall be accomplished by
Immersion in a medium, such as boiling deionized water, in a hot aqueous 5
percent sodium dichromate solution, in a hot aqueous solution containing
n!ckel or cobalt acetate or other suitable chemical solutions (see 6.2). When
Type III coatings are provided unsealed, parts shall be thoroughly rinsed in
cold, clean water and dried after artodlzing.

Sealing performs a couple of functions...it holds in the dye in the anodized cells, and it increases the corrosion resistance. But like everything concerned with anodizing, it has drawbacks..sealing drawback is that it reduces wear resistance.

While sealing in boiling water reduces abrasion resistance by about 25-30%, sealing in nickel acetate reduces abrasion resistance by an additional 2-3%. Sealing with nickel acetate or other chemicals doesn't have much benefit, other than it being a faster method of sealing the surface.

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