Chromate Conversion (Chemfilm)

MIL-DTL-5541

Callout Requirement
Type I Compositions using hex-chromium
Type II Compositions without hex-chromium
Class 1A Maximum corrosion protection
Class 3 Lower electrical resistance

The MIL-DTL-5541 specification (current revision is MIL-DTL-5541F) covers chemfilm (aka alodine or chromate conversion) on various aluminum alloys. MIL-DTL-5541 supersedes the MIL-C-5541 standard. MIL-DTL-5541 Type I specifies hexavalent chromium bath while MIL-DTL-5541 Type II is normally achieved with a tri-valent chromium bath. A print callout might read "Passivate per MIL-DTL-5541 Type I Class 1A" or "Passivation per MIL-DTL-5541 Type II Class 3". The Class 3 callout is for both corrosion resistance and minimum contact resistance: this is a tough compromise requiring proper coating thickness. The Class 1A callout is for maximum corrosion resistance which demands a thicker coating. Corrosion testing should be done on AA2024 if this alloy is processed during the test period. Monthly specimen testing is required.

Note to purchaser: This process is most difficult on high copper alloys like AA2024, and can also be difficult on AA7075. Suppliers who are able to perform conversion coating per MIL-DTL-5541 on AA6061-T6 may not have a process that will provide consistent results on AA2024. The process for AA6061 is aimed at removing surface silicon, while a process optimized for AA2024 will be aimed at removing surface copper. These are different objectives demanding different processing. If you want to be sure, you should ask to see a few months of salt-spray test results on AA2024 specimens.

Bath Testing
¶ 3.8 & 4.2.1.1: weekly analysis of
chemical concentrations, pH,
and temperature in all process
solutions to ensure within
manufacturer’s spec limits

Coupon Testing - monthly
¶ 3.6.1: corrosion resistance
¶ 3.6.2: paint adhesion
¶ 3.7: contact resistance

Coupon/Part Testing - per lot
¶ 3.5: visual appearance
¶ 3.9: workmanship

Chromate Conversion (Chemfilm)

MIL-C-5541

Callout Requirement
Class 1A Maximum corrosion protection
Class 3 Lower electrical resistance

MIL-C-5541 has been superseded by MIL-DTL-5541. MIL-C-5541 Class 1A indicates maximum corrosion resistance and a more stringent corrosion test is specified. MIL-C-5541 Class 3 receives contact resistance testing.

Bath Testing
¶ 4.3.1: weekly analysis of
chemical concentrations
in all process solutions
¶ 4.3.2: maintain test records

Coupon Testing - monthly
¶ 4.3.1: corrosion resistance
¶ 4.3.1: adhesion tape test

Coupon/Part Testing - per lot
¶ 3.5: visual appearance
¶ 3.9: workmanship

Chromate Conversion (Chemfilm)

AMS2473

The active specification is AMS2473G and is available from SAE International. The specification is similar to the more familiar MIL-DTL-5541 specification; however, there are some twists in the periodic testing requirements. For example, corrosion must be tested on AA2024 panels even if the shop only processes AA6061, and paint adhesion testing is required without being specified. It makes no sense but the specification is clear.

Note to purchaser: If you purchase to the AMS2473 specification, you should be aware that is is entirely possible for a supplier’s process to meet the requirements for corrosion resistance on AA2024 and other high-copper alloys, while not meeting the specification for AA6061 and other high silicon alloys. It is generally assumed that passing the salt-spray test on AA2024 is the most difficult, and the AMS2473 specification requires the periodic corrosion testing to be done on AA2024. However, we recommend that you require all periodic testing to be done on your specific alloy. Don't expect testing on AA2024 specimens to provide assurance that AA6061 parts will meet the specification.

Note to supplier: If you certify to to the AMS2473 specification, and you decide to do your periodic testing on AA6061 instead of AA2024, or to skip the paint adhesion testing, you should put these disclaimers in your certification text (or by contract) to avoid any expensive misunderstandings. It is very difficult to achieve consistent corrosion protection on AA2024 with a process optimized for AA6061. If you do not intend to process the AA2024 alloy for production customers, it would be best to push your customers toward the MIL-DTL-5541 specification since it allows you to test on the alloys that you process.

Bath Testing
Periodic test plan for all
cleaning and processing solutions

Coupon Testing - monthly
¶ 3.4.1: corrosion resistance
¶ 3.4.2: paint adhesion
(required even if purchaser does not request)

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

Conversion Coating
Low Electrical Contact Resistance

AMS2477

AMS2474 (discontinued)

The active specification is AMS2477 (no revision letter) and is available from SAE International. The AMS2474 specification was discontinued at AMS2474D and is noncurrent, although it may appear on prints. The AMS2477 specification is current. This specification is similar to the more familiar MIL-DTL-5541 Class 3 specification; however, there are some twists in the periodic testing requirements. For example, corrosion must be tested on AA2024 panels (¶4.3.3.1) even if the shop only processes AA6061, while contact resistance is to be tested on AA6061 (¶4.3.3.2). The specification is clear on the material to be tested; however, AMS2477 leaves the frequency of periodic tests to agreement between the purchaser and supplier.

Note to purchaser: If you purchase to the AMS2477 specification, you should ask for a reasonable frequency of periodic testing, or ask for panels to be processed with your lot. You should also be aware that is is entirely possible for a supplier’s process to meet the requirements for corrosion resistance on AA2024 and other high-copper alloys, while not meeting the specification for AA6061 and other high silicon alloys, so it would be wise for you to specify the material for testing.

Note to supplier: While MIL-DTL-5541 (and MIL-C-5541) required the purchaser to request the contact resistance test and determine the acceptable value, the contact resistance requirement is established by AMS2477 but the test does not appear in the periodic test listing in ¶ 4.2.2, so, as in the case of MIL-DTL-5541, it seems to be per customer request. This test is subject to a variety of factors that have nothing to do with the coating properties: surface roughness, probe pressure, panel thickness, etc. If your customer wants this test, he should supply the panels, and the test specifics should be set by contract to guarantee repeatability. Note that primer adhesion and corrosion resistance are required tests: they do not have to be specified by the purchaser.

Bath Testing
Periodic test plan required for all cleaning and processing solutions

Coupon Testing - periodic
¶ 3.3.1: corrosion resistance
¶ 3.3.2: contact resistance
¶ 3.3.4: primer adhesion
Note: contact resistance testing is by
request since it is not in ¶ 4.2.2
Coupon/Part Testing - per lot
¶ 3.3.3: color
¶ 3.4: general quality
See specification for
detailed test requirements.

Passivation for Stainless Steel

FED QQ-P-35

Callout Requirement
Type II Medium temp nitric
with sodium dichromate
Type VI Low temp nitric
Type VII Medium temp nitric
Type VIII Medium temp
high concentration nitric

FED QQ-P-35 has much useful information on preparation and passivating stainless steel. There is a table that lists many types of stainless steel alloys and shows the best passivation type to be used. FED QQ-P-35C was superseded by ASTM A967 which added citric passivation types to complement the nitric passivation with a less toxic acid. See Iodometric Titrations for information on bath analysis for QQ-P-35 Type II passivation baths.

Bath Testing
No bath parameters or tests mentioned in specification Coupon Testing - periodic
no periodic coupon tests
mentioned in specification

Coupon/Part Testing - per lot
¶ 4.3: visual
¶ 4.4.1: immersion/humidity test
¶ 4.4.2: salt spray/copper sulfate

Passivation for Stainless Steel

ASTM A967

Callout Requirement
Nitric 120-25 %v/v HNO3 with
2.0-3.0%w/w Na2Cr2O7
20 minutes at 120-130°F
Nitric 220-45 %v/v HNO3
20 minutes at 70-90°F
Nitric 320-25 %v/v HNO3
20 minutes at 120-140°F
Nitric 440-55 %v/v HNO3
30 minutes at 120-130°F
Nitric 5Other concentrations, temp, time
where parts meet specifications
Citric 14-10 weight% citric acid
4 minutes at 140-160°F
Citric 24-10 weight% citric acid
10 minutes at 120-140°F
Citric 34-10 weight% citric acid
20 minutes at 70-120°F
Citric 4Other concentrations, temp, time
where parts meet specifications
Citric 5Other concentrations, temp, time
with bath pH of 1.8 to 2.2
where parts meet specifications

Current revision is ASTM A967-05e1. The ASTM A967 specification has replaced the FED QQ-P-35C and offers effective passivation treatments with citric acid that were not available with the QQ-P-35 version. See Iodometric Titrations for analysis and bath control for ASTM A967 Nitric 1 baths.

Note to processor: Beware! The familiar QQ-P-35 Type II correlates to ASTM A967 Nitric 1. The familiar QQ-P-35 Type VI correlates to ASTM A967 Nitric 2.

Note to purchaser: your supplier is not required to do any testing to ensure a passivated surface per ASTM A967, although a number of potential tests are listed. It is recommended that you determine which of the tests in ¶13 apply to your alloy and request that test to be done on a lot sample of your parts - this may be done on your purchase order or by contract with the supplier.

Bath Testing
no bath parameters or tests
mentioned in specification

Coupon Testing - periodic
no periodic coupon tests
mentioned in specification

Coupon/Part Testing - per lot
¶ 11.1: visual
One or more specified tests may
be specified on PO or by contract.

Passivation for Stainless Steel

AMS2700

Callout Requirement
Method 1
Type 1
20-25 %v/v HNO3 with
2.0-3.0%w/w Na2Cr2O7
30 minutes minimum at 70-90°F
Method 1
Type 2
20-25 %v/v HNO3 with
2.0-3.0%w/w Na2Cr2O7
20 minutes minimum at 120-130°F
Method 1
Type 3
20-25 %v/v HNO3 with
2.0-3.0%w/w Na2Cr2O7
10 minutes minimum at 145-155°F
Method 1
Type 4
38-42 %v/v HNO3 with
2.0-3.0%w/w Na2Cr2O7
30 minutes minimum at 79-120°F
Method 1
Type 5
20-25 %v/v HNO3 with
2.0-3.0%w/w Na2Cr2O7
2 minutes minimum at 70-90°F
Part Anodic at 3-5 volts
Method 1
Type 6
25-45 %v/v HNO3
30 minutes minimum at 70-90°F
Method 1
Type 7
20-25 %v/v HNO3
20 minutes minimum at 120-140°F
Method 1
Type 8
40-55 %v/v HNO3
30 minutes at 120-130°F
Method 2 4-10% w/w Citric Acid
30 minutes min at less than 100 °F or
20 minutes min at 100-119°F or
10 minutes min at 120-140°F or
4 minutes min at higher than 140°F

Current revision is AMS2700C with AMS2700D in progress. The AMS2700 specification lists a number of passivation treatments not covered by either QQ-P-35 or ASTM A967, including passivation with voltage applied.

Note to processor: Iron concentration, nitric concentration and sodium dichromate concentration in the passivation bath must be controlled. See Iodometric Titrations for analysis and bath control for AMS2700 Method 1 baths with nitric and sodium dichromate. Class 1 parts have samples taken from lot for corrosion testing. Class 2 parts have corrosion testing done on 1 part per lot. Class 3 parts get visual inspection per lot with corrosion testing done on specimens periodically. Class 4 parts have corrosion testing done on samples from each lot; however, the sampling plan is at a lower AQL than Class 1. The corrosion test may be salt spray, humidity, water immersion or copper sulfate. See specification for details.

Note to purchaser: It is recommended that you determine which of the corrosion tests apply to your alloy and request that test to be done on a lot sample of your parts - this may be done on your purchase order or by contract with the supplier. If you purchase to Class 3, you should specify a test period of at least quarterly.

Chemfilm Materials

MIL-DTL-81706

This is NOT a processing specification.

MIL-DTL-81706 covers requirements for chromate conversion materials and testing. MIL-DTL-81706B is the latest revision. This specification is often improperly referenced as a processing specification. Virtually any process that complies with MIL-DTL-5541 will be using chemicals from suppliers (Henkel Alodine, MacDermid Iridite, etc.) who have certified their chemicals to the MIL-DTL-81706 specification. Plating and anodizing shops might properly ask a chemical vendor for a certification to MIL-DTL-81706, but a machine shop or OEM supplier print should be calling for certification to MIL-DTL-5541.

Black Oxide for Ferrous Metals

MIL-DTL-13924

Callout Requirement
Class 1Alkaline oxidizing process
for irons and low alloy steels
Class 2Alkaline chromate oxidizing
corrosion-resistant steels
(tempered <900°F)
Class 3Fused salt oxidizing process for
corrosion-resistant steels
(tempered >900°F)
Class 4Alkaline oxidizing process for non-
tempered corrosion-resistant steels

MIL-DTL-13924 describes a black oxide finish that is prinipally cosmetic, providing little in the way of improved corrosion resistance. MIL-DTL-13924D is the latest revision.

Bath Testing
No bath parameters or tests
mentioned in specification

Coupon Testing - periodic
no periodic coupon tests
mentioned in specification

Coupon/Part Testing - per lot
¶ 4.3.1: magnified visual
¶ 4.4.1: smut test
¶ 4.4.2: spot test
¶ 4.4.3: salt spray test
¶ 4.4.4: embrittlement