-- The Home Page of the Finishing Industry
A website for Serious Education, promoting Aloha,
& the most FUN smiley you can have in metal finishing

on this site
current topics
topic 4558

MIL-C-5541 / MIL-DTL-5541 Class 1a vs. Class 3 Chromate Conversion Coatings

A discussion started in 2000 & continuing through 2017


Q. Mil-C-5541 [link by ed. to spec at TechStreet]-E, Class 1a (gold finish) vs. Class 3 (clear finish)

What is the difference between Gold Iridite and Clear Iridite in terms of process. Is there any favorable impact on cost if we require our vendor to do Clear Finish?

John Chu
electronics - Melville, New York

Ed. note: Iridite is a Macdermid trademark.


A. Yellow chromate is thicker than clear and has more chromate in it so protects the metal considerably longer. Since most people want better protection, most shops do not offer clear. Some will legally bleach a yellow which is another step. Some will give it a very short tank time which will normally provide a substandard product.

Cost wise, you will probably be better off with yellow.

James Watts
- Navarre, Florida


A. Class 1 or 1A yellow is usually called out for corrosion protection or as a pretreatment for paint adhesion, Class 3 is usually called out for electrical conductivity purposes. In actual practice many folks use the same processes for classes 1 or 3. However, many folks will bleach yellow to meet a clear call-out which results in substandard corrosion resistance, and won't meet salt spray specification requirements. There are good products out there for clear chromate which will meet salt spray requirements even for 2000 series. There should not be a cost impact between the two types as long as the shop is set up to run both as the number of tanks, processing times and chemical costs are essentially the same. In terms of passing salt spray for clear, there are many letters at this site about this and most people in the business know the parameters required, some of which involve the salt spray coupons themselves.

Ward Barcafer, CEF
aerospace - Wichita, Kansas


A. John,

The cost for either process is the same for my business, however, the end use of the product would probably dictate which to specify. The conductivity of 1A vs. 3, as Ward mentioned, is often the biggest reason to specify Class 3 (gold or clear). Some customers want the Clear Chromate finish only because they prefer the way it looks, without any regard or need for corrosion protection or relative conductivity. As James mentioned, Class 1A will give the product more protection longer and that may be the most favorable impact to cost.

Lamar Jackson
- Fort Walton Beach, Florida

Ed note: The postings above refer to hexavalent chrome coatings, and predate the world's big shift to RoHS-compatible trivalent chromium conversion coatings. With the new proprietary trivalent coatings and topcoats, it may no longer be true that yellow coatings are thicker or offer greater protection; these days the yellow color is sometimes just dye.


Q. Will clear Iridite process cause poor peel strength to a metal part? If Yes, Why and How significant different is it compared to a raw metal (no Iridite coating )?

Eric De Forzea
- Malaysia


Q. What is the difference between MIL-C-5541 & MIL-C-5541E? Is the implication of Class 1A and Class 3 are the same? This process is chromating, but come across some time call it Alodine or Iridite, what is the difference?

William Wong
- Singapore


A. Hi Eric. "Peel strength" of what -- paint or powder coating on the component? As long as the chromate conversion coating is done right, paint will stick better to it than to raw aluminum, and you will have better corrosion resistance.

Hi William. The "E" is just the revision level in alphabetical order. There was a "D" and there will be an "F". I don't quite understand your second question, but Ward is saying that it is sometimes possible for no adjustments to be required for one tank to produce coatings that meet both standards. Alodine (Henkel) and Iridite (Macdermid) are two of the brand names on the Qualified Products List associated with Mil-C-5541.

Ted Mooney, Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey


Q. Which process is better protection on parts? Is there a significant cost difference?


Nick Ng
Plating of metal work - Chapel Hill, North Carolina


A. Neither and no. They are brand names (actually trade names for a whole series of processes) from two different suppliers. It's like asking if Ford trucks are better than Chevy trucks, Nick.

Ted Mooney,
Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey

To minimize searching and offer multiple viewpoints, we've combined multiple threads into the dialog you're viewing. Please forgive any resultant repetition or failures of chronological order.


Chromate conversion coating on aluminum is most commonly specified to be in accord with MIL-C-5541 / MIL-DTL-5541. This process is also sometimes called "Chem Film", especially in the USA.
The two most popular brands of the required chemistry are Iridite (from Macdermid) and Alodine (from Henkel), so the process is also sometimes known as Iriditing or Alodining.

Difference between Class I and Class III Iriditing


Q. I would like to know the difference between Class I and Class III Iriditing. Is Class III gold? Thanks for any help

Rita Flygar
hobbyist - Rockville, Maryland, USA


A. Class 3 is lighter in color BUT more importantly, less electrically resistant (i.e. MORE CONDUCTIVE). Heavier chromates (i.e. darker), Class 1A, are more resistant to both corrosion and electrical passage.

milt stevenson jr.
Milt Stevenson, Jr.
Anoplate Corporation
supporting advertiser 
Syracuse, New York

Anoplate banner

May 13, 2009

Q. My experience is this process is not very good, the Iridite does not adhere to the base very well; it could be removed by rubber. Is there some method to improve this?

Seaman Fang
- Shanghai, China

May 15, 2009

A. Hi, Seaman. Raw aluminum is not simply dipped into a tank full of Iridite; immersion is this tank follows after a cleaning, etching, and desmutting pretreatment sequence. Are you operating the process in full compliance with the technical data sheets from Macdermid? What alloy of aluminum are you treating, and what is the pretreatment sequence?


Ted Mooney,
Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey

February 21, 2014

Q. Is there contact resistivity differences between Iridite type III yellow and Iridite type III clear in K band?

I assume the resistivity is extremely low for either type; is there some reference material with actual measurements of surface resistivity?

Paul Volk
- Palo Alto, California USA

October 23, 2017 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Need Suggestions:
I am new to the chemical processing side of things, and have a few questions if somebody can help answering:

- Talking about immersion time req'd for Class 1A and Class 3 ALODINE (using Alodine 1200s): is there a table/chart we can use that talks about immersion time against concentration of Alodine in the tank?

- Referring to MIL-DTL-5541F, if parts comply with ALODINE class 3, do they technically comply with Class 1A as well ? (considering both Classes need to comply with Corrosion Resistance but Class 3 also need to comply with Electrical Conductivity, so basically Class 3 is better than Class 1A?, and if I fine tune my immersion time for Class 3, no matter what is called on the PO I can use the same numbers for both classes 1A and 3?

Nav D
- Pickering, Ontario, Canada

This public forum has 60,000 threads. If you have a question in mind which seems off topic to this thread, you might prefer to Search the Site


Disclaimer: It's not possible to diagnose a finishing problem or the hazards of an operation via these pages. All information presented is for general reference and does not represent a professional opinion nor the policy of an author's employer. The internet is largely anonymous & unvetted; some names may be fictitious and some recommendations may be deliberately harmful.

  If you need a product/service, please check these Directories:

JobshopsCapital Equip. & Install'nChemicals & Consumables Consult'g, Train'g, SoftwareEnvironmental ComplianceTesting Svcs. & Devices

©1995-2018, Inc., Pine Beach, NJ   -   About   -  Privacy Policy
How Google uses data when you visit this site.