Anodizing Aluminum



Callout Requirement
Type IChromic acid bath
Type IBChromic acid - low voltage
Type ICNon-chromic alternative
Type IISulfuric acid bath
Type IIBThin sulfuric acid bath
Type IIIHard anodic coatings
Class 1No dye
Class 2Dyed

MIL-A-8625 is the principal specification covering anodizing of aluminum. AMS-A-8625A has been cancelled and references the latest revision of MIL-A-8625. MIL-A-8625F is the latest revision. It describes anodizing with both sulfuric and chromic acid baths; although oxalic and other acids are mentioned as offering unique properties. See acid-base titrations for analysis instructions for both sulfuric acid and dissolved aluminum in MIL-A-8625 baths. See iodometric titrations for analysis methods to determine concentration of sodium or potassium dichromate in seal baths.

MIL-A-8625 Type II is sometimes called "sulfuric" anodizing. MIL-A-8625 Type III also specifies use of a sulfuric acid bath, but Type III is called "hard coat". The Type III bath normally contains additives and is run at higher current and lower temperature to achieve the harder and thicker coating.

Notes to purchaser ...

A print note reading "Anodize per MIL-A-8625 Type II, Class 1" or "Anodize per MIL-A-8625 Type III, Class 1" is specifying anodizing without a colored dye. A print note reading "Anodize per MIL-A-8625 Type II, Class 2" or "Anodize per MIL-A-8625 Type III, Class 2" is specifying colored anodizing. The sulfuric or Type II anodizing easily accepts dye, and the possibilities span shades of black, blue, gold, brown, red, green and others. It is more difficult to color hard coat (Type III). While anodizing can be colored, it is not quite as easy to match colors and ensure consistent coloration. A competent anodizing shop should be willing to give you a sample to ensure a good color match before you commit to production, and should be willing to include color match in your acceptance criteria.

If your print does not specify a sealing process, Type III will NOT be sealed, while all other types will be sealed. Sealing improves corrosion resistance at the expense of abrasion resistance; to seal or not to seal is a key application factor. Sealing should not be used if the application requires the maximum abrasion resistance. Moreover, the combination of acid type and seal type will affect the corrosion resistance in corrosive atmospheres.

For sealed anodizing, you should specify the type of sealing either on the print or the PO: hot DI water seal works best to improve corrosion resistance of Class 1 (non-dyed) anodizing; nickel acetate or sodium (or potassium) dichromate seal is normally specified for Class 2 anodizing because DI water will leach the dye from the coating..

Acceptance testing and periodic testing are required to prove compliance with the MIL-A-8625 specification. Purchasing due diligence should include inspection of bath records for all process baths, along with acceptance test records and monthly results from coupon testing. If you are serious about the quality of your final product, audit the supplier’s records of monthly corrosion and/or abrasion test results.

Notes to supplier ...

The coloring may be done in one step, with the dye in the anodizing tank, or it may be a post-anodize process. MIL-A-8625 describes various sealing chemistries including hot DI water seal, potassium or sodium dichromate seal and nickel acetate seal. If the type of sealing is not specified as a print note or PO call out, it is wise to clarify with the purchaser.

¶ mentions a "5% aqueous solution of sodium or potassium dichromate". For chemistry nuts, this begs the question as to whether the dihydrate salt (Na2Cr2O7-2H2O) or pure dichromate (Na2Cr2O7) is intended. Given that the dihydrate is the most common form, most anodizers are weighing out 50 g/L of the dihydrate to make up a seal tank. The dihydrate is 13.75% heavier than the pure salt, so the concentration of the pure salt will be 4.4% when the dihydrate concentration is 5%. It would have been better if MIL-A-8625 had followed the lead of the QQ-P-35 passivation specification and made it clear what is meant by "sodium dichromate". Many anodizers also offer passivation per ASTM A967 Nitric 1 or QQ-P-35 Type II, so it is a safe bet that sodium dichromate dihydrate is the salt on hand. See the page on iodometric methods for help with solution maintenance: the titration for sodium or potassium dichromate in a passivation bath is easily adapted to a seal bath.

Although MIL-A-8625 does not specify impurity testing in the anodize bath, it is crucial to process control - aluminum is especially important. A competent anodizing vendor will record aluminum test results and trigger a bath make up when the aluminum concentration tests in the 10-20g/L range. (See acid-base titrations for methods of titration for both sulfuric acid and for aluminum impurities.)

Bath Testing
¶ 4.3.1: analysis to be done on
process baths at least bi-weekly
results and adds to be recorded

Coupon Testing - monthly
¶ coating weight
¶ corrosion resistance
¶ light fastness (dyed)
¶ abrasion resistance

Coupon/Part Testing - per lot
¶ dimensional

Impurities - bi-weekly
recommended due to acid etching

< 5 g/L Al

< 40ppm Cr

< 40ppm Ni

< 50ppm Cl

< 80ppm Cu

< 100ppm NO3

< 150ppm Fe

Anodizing Aluminum


Callout Requirement
-X X mil ± 0.5 mil
Unsealed unless sealing specified

The active specification is AMS2469H, and it is available from SAE International. AMS2469 covers hard coat anodizing only. AMS has other specifications covering sulfuric (Type II) anodizing.

Notes to purchaser ...

The specifications for coating weight, corrosion resistance and abrasion resistance match the MIL-A-8625 Type III requirements, so certification to AMS2469 is virtually equivalent to the hard coat variety of the MIL-A-8625 specification. AMS has different specifications for each of the MIL-A-8625 types and classes. AMS2471 and AMS2472 specify MIL-A-8625 Type II varieties. While AMS2469 is a close match to the MIL-A-8625 Type III, the other AMS specifications differ slightly from the MIL-A-8625 Type II specification. (See later discussion.)

Purchasing due diligence should include inspection of process bath test and addition records, even though AMS2469 does not specifically list the exact tests that are required. Purchasers should also check supplier records of monthly testing of corrosion and abrasion specimens and encourage the use of independent labs.

Notes to supplier ...

See previous discussion on MIL-A-8625 for more information. See acid-base titrations for analysis instructions for both sulfuric acid and dissolved aluminum in anodizing baths. See iodometric titrations for analysis methods to determine concentration of sodium or potassium dichromate in seal baths.

Bath Testing
No specific bath parameters or tests
mentioned in specification

Coupon Testing - monthly
¶ 4.2.2: coating weight
¶ 4.2.2: abrasion resistance
¶ 4.2.2: corrosion resistance
(for sealed parts only)

Coupon/Part Testing - per lot
¶ 4.2.1: thickness
¶ 4.2.1: color
¶ 4.2.1: quality
See specification for
detailed test requirements.

Anodizing Aluminum

AMS2471 & AMS2472

Specification Type
AMS2471 Sulfuric, non-dyed
Dichromate seal*
AMS2472 Sulfuric, dyed
Nickel-cobalt acetate seal*
*Unless otherwise specified

The active specification is AMS2471G for non-dyed and AMS2472F for dyed: both are available from SAE International. These specifications are similar to the MIL-A-8625 Type II specification; however, there are some twists in the periodic testing requirements and in the coating specifications.

Notes to purchaser ...

The minimum coating weight requirements exceed those of the MIL-A-8625 Type II specification and are specifically listed by aluminum alloy. Most shops are more familiar with the MIL-A-8625 specification, so you should check the specification before you sign a certification or include in your Nadcap scope.

Notes to supplier ...

Coating weight and corrosion must be tested on AA2024 panels even if your shop only processes AA6061. This makes no sense, but the requirement is repeated twice in the specification. Other twists include specification of current and temperature control during anodizing as well as detailed specifications on the sealing bath composition, pH, etc. These changes from the MIL-A-8625 specification make the AMS2471 and AMS2472 problematic and worthy of study before you sign a certification or include them in your Nadcap scope.

Bath Testing
Periodic test plan for all
cleaning and processing solutions

Coupon Testing - monthly
¶ 3.4.1: coating weight
¶ 3.4.2: corrosion resistance
(assumes sealing, so no abrasion tests)

Coupon/Part Testing - per lot
¶ 3.6: quality
See specification for
detailed test requirements.

Anodizing Aluminum


Callout Requirement
A 50 µm or 1.97 mil
Engineering Hard Coat
B 18 µm or 709 µin
Architectural Class I
C 10 µm or 394 µin
Architectural Class II
D 8 µm or 315 µin
Automotive - Exterior
E 5 µm or 197 µin
Interior - Moderate Abrasion
F 3 µm or 118 µin
Interior - Limited Abrasion
G 1 µm or 39.4 µin
Chromic Acid
*Unless otherwise specified

ASTM B580 specifies very little. Type A is to be unsealed unless otherwise specified, but all processing requirements and test parameters are left to separate agreement between purchaser and supplier. It is not clear what is accomplished by referencing this specification other than to establish the application of the finished product and to establish a thickness for the coating. It is unclear what a supplier should do with the knowledge that a particular chunk of aluminum is going to be used on the exterior of an automobile. If you are an engineer looking for a way to insure that your supplier uses a proven process and delivers consistent product, you should call MIL-A-8625.

Anodizing Titanium


AMS2487 and AMS2488 cover anodizing on titanium and are still in review by the SAE at this time. In the meantime, Metalast offers a technical bulletin that describes techniques for anodizing titanium and controlling the color. This technical white paper should help purchasers and suppliers develop specifications and test plans.