Guide To Military Specification Anodizing (MIL-A-8625)
SAF’s capabilities for Military Specification Anodizing MIL-A-8625. Note: there are three different anodizing finishes contained in this specification:
- Type I: Chromic Acid
- Type II: Sulphuric Acid
- Type III : Hard Anodic Coatings
TYPE I: CHROMIC ACID ANODIZING MIL-A-8625
This is a non-architectural finish and is nearly obsolete due to environmental considerations. It is a thin gray film that is not as corrosion resistant as sulfuric acid anodize. It is a fair paint pre-treatment.
Chromic is usually used in assemblies of parts made from different alloys. Chromic acid anodizing is insensitive to alloy. A mixed-alloy fabrication, having been sulfuric acid anodized, might be coated too much in some areas and not enough in others. Chromic acid anodizing will solve this problem.
When solution entrapment is a problem, a carry-over of chromic acid is not as corrosive as sulfuric acid to an aircraft or ship.
Chromic acid is carcinogenic and its use is harmful to the environment. Often it is cited in older military specifications. Do not use it where sulfuric acid anodize can be used. A Chromic acid anodized coating is inferior in all respects.
Once upon a time, Chromic acid was inexpensive. Today we know that Chromic acid is bad for the environment. Well-written environmental regulations make the presence of chrome in industry very expensive, and Chromic acid anodizing now costs much more than sulfuric.
If anyone asks for MIL-A-8625, Type 1, be sure that’s what they really want. It is possible they may mean Type 2, Class 1. Very few, if any processors do Type 1. SAF does not do Type 1 Chromic Acid anodizing.
TYPE II: SULFURIC ACID ANODIZING MIL-A-8625
This type is broken into Class 1 and 2. These classes are not the same as AA Class 1 and 2.
If certification is required, SAF requires a $1,500.00 anodizing up-charge to cover the cost of destructive testing procedures such as salt spray and coating weight. It will require extra pieces of customer metal for destructive testing. Test results may add 3 or 4 weeks to obtain certifications.
Class 1: For architectural alloys 3003, 6063, 6061 and 5005 we can certify SAF215 will meet this specification. For any other alloys, contact SAF management.
Alloys 2024 and many aircraft alloys require a dichromate seal to meet the performance specifications of MIL 8625 Type 2, Class 1. Dichromate seal has the same problems as Type I: we try to avoid the chrome processing in our plants unless it is unavoidable.
We might consider Dichromate seal if the job is large enough, but remember that it is not necessary with most standard architectural aluminum alloys.
Class 2: means the anodic coating is dyed. We can use our standard dyes or 2-step with the appropriate aluminum alloy: 3003, 6063, 6061 or 5005. The SAF Color Dye Library includes those in the Federal Standard 595B color chart. For custom dyes, Clariant makes a custom color match. For non-architectural alloys, contact SAF management.
For SAF to represent that we do any military specification, we must comply with all aspects of the specification including the expensive salt spray test provisions. Some of our customers do not wish to spend the money and time for certifications. The only alternative is for us to perform our standard anodizing, and issue a letter similar to the one below.
We cannot refer to “MIL-A-8625” on an order unless we perform the appropriate tests. However, some of our customers think standard anodizing with no certs is adequate. If the below letter is adequate then the anodizing will be applied at our standard charges. This letter is for example only.
SAMPLE LETTER
April 4, 2012
ABC, Inc
30 Nospec Ave
Atlanta , GA 30000
Re: MIL-A-8625F, Type II, Class 1 and your PO 12345
Dear Sirs:
This letter certifies that the material on our invoice 55133 was anodized in accordance with our SAF 215 process. This letter also describes how our process SAF 215 compares to MIL-A-8625E, Type II, Class 1.
Assuming your metal was either 5005 or 6063 and in accordance with paragraph 3.1.1, 3.3.1.2, 3.3.2, 3.4.2, & 3.6.1.1 of MIL A 8625E, the SAF 215 coating we applied to your material is in accordance with the process requirements of MIL-A-8625E, Type II, Class 1 except for paragraphs 3.7.1.1 and 3.7.1.2. 3.7.1.1 requires coating weight testing to be run. 3.7.1.2 requires a salt spray test. These take about 4 – 6 weeks and $1,500.00 of additional time and money. Because of this exclusion, Southern Aluminum Finishing Company cannot be responsible for compliance to MIL-A-8625F, Type II, Class 1 without performing all process and quality assurance processes as required by the specification.
In addition, SAF 215 precludes any requirements for painting or chemical brightening. You should also review 6.10.1, 6.10.1.1, 6.10.2 and 6.10.3 for information about fabrication tolerances.
I hope this letter provides the information you need. If I can be of further assistance, please do no hesitate to call.
Very Truly Yours,
SAF Sales
TYPE III : HARD ANODIC COATINGS MIL-A-8625
This is low-temperature hardcoat, and we will quote if the quantity is large. For small quantities we are not able to do this process now.
Type III sulfuric acid anodizing at 36’F with a coating thickness of 2 – 2.5 mils (very thick!). This kind of anodizing is accomplished with a bath similar to the standard sulfuric process, but with the temperature reduced to about 0° C to slow the dissolution rate. A higher voltage is applied to enable the coating to continue to build after the insulation value of the coating starts slowing down the coating formation.
For a current copy of MIL-8625 see http://www.wbdg.org/ccb/ccb.php for more information.
