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topic 34822

Temperature limit for chromate conversion coating on aluminum (Alodine, Iridite, chem film)

A discussion started in 2001 but continuing through 2018


Q. I am a manufacturing Engineer in Wichita KS. We have been experiencing difficulties painting Alodine coated aluminum. We have been using a Zinc-Chromate primer that does not want to stick good to the surface. We use the tape test and it always fails.

Is there any other primer that might work better? Would it adhere better at a higher temperature, like 80 to 90 °F? We are using Dupont 491-17 Etch-Primer.

Dale O [last name deleted for privacy by Editor]
- Wichita, Kansas


A. Hello Dale,

The Alodine coating should provide a decent surface for paint to adhere to without using a primer. Alodining is a chromate conversion coating, supposed to pass 150-hour salt spray. I'd say quit with the zinc-chromate and try to paint right over the Alodined aluminum. I'd also stay away from etching primers, the "etch" is usually an acid, which might negate the usefulness of the Alodine. If you can't get good adhesion, try an epoxy primer, providing you are topcoating with an opaque color (epoxies chalk outdoors if a clear or tinted topcoat is used).

Good luck,

Jake Koch
G. J. Nikolas & Co., Inc.
supporting advertiser
Bellwood, Illinois
nikolas banner ad


A. I do not think that your primer is the problem. The first two things I would investigate would be the Alodine film. Perform the test on the bare Alodine and see if any of the coating is removed. If so, then the coating may be too thick and powdery causing a failure at this interface. The second place I would investigate is the part handling. Are any soils accumulating on the surface? Is the Alodine coating given ample time to dry? When the Alodine is dried is it at an elevated temperature? Normally temps in excess of 150 °F begin to degrade the coating. I hope this gives you some direction in your investigation.

Best of Luck.

Ira Donovan, M.S.F.
Kansas City, Missouri

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


Q. My understanding of chromate conversion on aluminum is to promote paint adhesion, but that the chromate degrades at temperatures over 150 °F. I am also aware that studies have been done that show no degradation at higher temperatures. I need to powder coat a part at 400 °F for 10 min. The print specifies chromate conversion. Will this temperature degrade the chromate and cause loss of adhesion?


High Performance Powder Coating
from Abe Books


A. Hi,

Chromating of alloy is the norm prior to powder coating. Temperature degradation refers to heating in air (i.e. no paint film). Green chromating is more tolerant of overheating, but yellow is better on alloys.

In short - no problems!

Roger Bridger
Walterisation UK Ltd. - Croydon, UK


Q. Most, if not all, technical bulletins and specs state that chromate conversion coatings on aluminum are not to be dried at temperatures over 140 °F. Is there some point after drying when it would be allowable to subject the conversion coated part to temperatures over 140 °F?

We apply a chromate conversion coating to aluminum parts. Then we apply a lacquer maskant to mask selected areas prior to a subsequent anodize coating. After the anodize operation, we remove most of the lacquer maskant by soaking the parts in a solvent. I would like to remove the left-over traces of lacquer in a vapor degreaser, but the degreaser temperature is about 190 °F. Once the chromate conversion coating has "set up", is it allowable to subject it to the 190 °F degreaser?

Kent Kessler
- Seattle, Washington, USA


Chromate conversion coating is a corrosion-fighting and adhesion-promoting process commonly applied to aluminum and to zinc plated steel. It is often used as a final finish, but also often used as a pretreatment for painting or powder coating.

The process usually employs proprietary chemicals rather than homebrew. Alodine is a Henkel trade name for their line of chromate conversion coating processes; Iridite is a Macdermid trade name for their line of chromate conversion coating processes.

"Chem film" is a common colloquialism in the USA for chromate conversion coating of aluminum.


A. Even after the chem-film has "set up", it still exists in a somewhat gel-like form, and temperatures above 140 °F still have the ability to dehydrate it and significantly decrease corrosion resistance (I believe that Wernick, Pinner and Sheasby talk about this, but I don't have my copy at hand).

There are some exceptions to this rule, for example when used as a base for powder coating it is pretty typical to not see negative results during curing; but I think what your proposing would seriously compromise the chem-film coating.

Of course, you could always test it - take two identical pieces and process them the same with the exception of that drying step. Then get them tested to the requirements of Mil-C-5541 [link is to free spec at Defense Logistics Agency,] and see how they compare.

Good luck,

Jim Gorsich
Accurate Anodizing Inc.
supporting advertiser
Compton, California, USA
accurate anodizing banner

p. 264 of Wernick, Pinner and Sheasby

November 2013

A. Hi. The "Chemical Conversion Coatings" chapter of Wernick, Pinner and Sheasby's "The Surface Finishing and Treatment of Aluminum and its Alloys" [link is to info about book at Amazon] runs 69 pages, so it can't be summarized into a paragraph, but Jim is right that it says the protective value is reduced above 60 °C (140 °F) =>


Ted Mooney, Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey
We need "Aloha" now more than ever



Military spec. Mil-C-5541 [link is to free spec at Defense Logistics Agency,] is often used in specifying the requirements for chromate conversion coating.

The spec describes two classes of coating:

Class 1A, "For maximum protection against corrosion, painted or unpainted" and
Class 3, "For protection against corrosion where low electrical resistance is required".

A. In doing significant electrical bonding research, we discovered that class III Alodine is compromised by high temperatures even after curing.

The temperature limit is about 140 °F. if exposed to temperatures about 160 °F for more than three hours, the conductivity of class III Alodine is no better than class I Alodine.

The lapse of time between the plating and the test seemed to have no significant impact. We were told that Alodine "resides" as a gel with a certain amount of moisture in the solution.

As the moisture bakes out the corrosion resistance goes down and the electrical resistance goes up. The specific amount seems to only be able to be determined empirically and seems to have a host of variables.

In our case we were trying to get a colored anodize after applying the class III Alodine. We had to reverse our process and do the anodizing (with mask) first and the Alodining second in order to maintain the class III.

Larry Sickler
- Rockford, Illinois, USA


A. After the chromate conversion, allow the components to stand for 3 days before baking. We sometimes bake parts at 125 °C after a 3-day wait with no problems.

Chris Matthews
- East Sussex, England

350 °F bake to cure epoxy bonding on chromated aluminum

May 25, 2012 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Greetings,
We are a special process house performing conversion coating per Mil-DTL-5541 [link is to free spec at Defense Logistics Agency,], Type I, Class 1A, and painting. We are having some paint adhesion issues with one customer's parts. On these parts we perform the conversion coating. The parts are returned to the customer for some assembly. This assembly includes some epoxy bonding where the parts are baked at 350 °F for an hour to cure the epoxy. The parts are then returned to us for painting.

We clean the parts using Isopropyl Alcohol and Acetone to remove any grease or oils prior to painting. The parts appear to be very clean prior to paint and the paints are applied in accordance with the applicable specifications, but yet we are having some adhesion issues. We know that high temperature will degrade the corrosion protective qualities of the chem- film but will the high temperature also affect the surface to the point that will affect adhesion?

Peter Doyle
special processing - Phoenix, Arizona, USA

Heat Resistant Chromate Conversion Coatings

"Surface Properties of Conversion Coated Aluminum"
by Raj & Ali
from Abe Books
info on Amazon

May 31, 2012

A. The chromate is on too heavy. For paint base, put the chromate on very light, just barely a color. If it is on too thick the baking dehydrates it and causes the paint to flake.

robert probert
Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina
Editor's note: Mr. Probert is the author of Aluminum How-To / Aluminio El Como
and co-author of The Sulfamate Nickel How-To Guide

June 10, 2012

A. By most specifications and standard practice, baking chem film except the new RoHS-compliant one is not recommended above 140 ° F (60 °C). If you have baked it above this temperature and it is not completely covered by a paint you will lose most if not all if the adhesion characteristics of the film -- not to mention violating most of the specifications in the aerospace industry if the parts are used for that.

drew nosti
Drew Nosti, CEF
supporting advertiser
Ladson, South Carolina

Temperature limits for RoHS-compatible conversion coating

June 26, 2012

"... chem film except the new RoHS-compliant one is not recommended above 140 °F. (60 °C)"

Q. What's a reasonable upper service temperature limit (for salt spray test and/or general corrosion resistance, e.g., occasional [acid] rain, seawater splashing, etc.) for the new RoHS chem-film (MIL-DTL-5541F Type II) vs. older hexavalent Type I? Will Type II hold up to say 70 °C vs 60 °C? Higher?

Stephen Crook
- Wilmington, Massachusetts, USA

July 10, 2012

A. Stephen

You could probably get this information from the Technical Product Bulletin for the material being used to produce the coating. Our Type I TPB recommends <160 °F and our Type II recommends <120 °F for maximum drying temperatures.

If your product is subject to elevated temperatures, your testing might include coupons with simulated environmental conditions.

Willie Alexander
- Colorado Springs, Colorado

2003 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Which is the limit temperatures that Alodine 1200s or Iridite coating treatment over AA 7075 T73, Invar 36/32, and Ti6Al4V could withstand without degradation of performances?

Jorger Rueda Nuñez de Villavicencio
RYMSA - Spain


A. Alodine and Iridite are family names for chromate conversion coatings. Products are available for coating aluminum, and the USA MIL-Spec limits the temperature on bare coatings to under 140 °F. I do not know of any way to chromate Invar and Titanium. Iridite does not have products for Invar and Titanium.

robert probert
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina

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

2003 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Hello,

I have some requests concerning chromate conversion (Alodine 1200) on aluminum alloys (like 6061 T6):

- what is the higher temperature limit that can bear Alodine coated parts? what is the lower temperature at which properties of Alodine coat start to decrease or collapse?
- what is the behavior of Alodine coat when coated parts are placed in vacuum?
- what are the effects on Alodine coated parts when those parts are placed in vacuum at high temperature?


John J. Proctor
- Los Angeles, California, USA


A. Alodine or any chem-film is a very thin gel of aluminum chromate. It does not like temps above 140 °F and it does not like vacuum conditions as it dehydrates the gel too much and turns it to a powder. The longer it has been on the part, the longer the time it will take to degrade, but degrade it will. A good reason why they paint exposed surfaces.

James Watts
- Navarre, Florida

August 31, 2009

A. Dear John,

I'm assuming you are asking this question for use in a space application. I am currently facing a similar problem with a space component which has to sustain high temperatures in low earth orbit. The problem with alternative finishes, as far as I've been able to gather, is that we need the finished part to retain high electric conductivity.

If you have found an alternative, or wish to discuss this matter, please leave a message at the board. I'd really appreciate some insight.

Gustavo Silva
- São Bernardo, São Paulo, Brazil

Powder Coat over Alodine?


Q. We build an assembly consisting of a cast Aluminum part, tack welded to an Aluminum sheet metal part, epoxied to a formed Aluminum sheet metal part with stainless steel threaded inserts. We are presently Polyester Powder Coating the final assembly for corrosion and aesthetics reasons. We are having some corrosion problems in the field (this is an outdoor product) mainly with the Powder Coating flaking off cast Aluminum part. I have read several of the postings related to an Alodine undercoating to the Powder Coating but do not understand what happens to the Alodine at curing temperatures. Most Powder Coats require 300-400 °F to cure. What I have read about Alodine is that is not good for applications over 120 °F. Does the Alodine retain its corrosion resistant characteristics after exposure to high temperatures? Wet paint seems to on its way out in California and an Anodize under coating does not lend itself to the assembly process because of the epoxy and stainless steel inserts. Any further clarifications or suggestions for a corrosion resistant under coating would be appreciated.

John Conner
Mech. Design Engineer - Sacramento, California, USA


A. The temperature restrictions apply to unpainted Alodine®. The powder coating protects the chromate from dehydrating and cracking during the high temperature cure.

Adhesion problems due to improper chromating could be due to:
1) Poor aluminum pretreatment (e.g., over-etching, wrong deoxidizer for castings, dirty rinses),
2) Dirty post-chromate rinses; final rinsing in DI water is best (if not using a Deoxylyte® final rinse),
3) Excessively thick chromate; try Type II per ASTM B449-93 [link by ed. to spec at TechStreet] (twixt Classes 1A & 3 of Mil-DTL-5541 [link is to free spec at Defense Logistics Agency,]).
4) Under-cured chromate (soft gel, lacking strength),
5) Over-cured chromate (powdered & cracked. Metal temperature shouldn't exceed 150 °F).

See the appropriate Alodine product bulletin or Mil-HNDBK-509 [link by ed. to spec at TechStreet] CLEANING AND TREATMENT OF ALUMINUM PARTS PRIOR TO PAINTING for more info. Final rinsing in Deoxylyte® (rather than DI water) is given. Most Deoxylyte versions are dilute chromic acid solutions; Henkel likely has a non-hex. chromium version by now.

What adhesion & corrosion testing is currently performed? In letter 45803, Sheldon Taylor describes D-I-Y testing of X-scribed painted samples in boiling water, similar to ASTM D870 [link by ed. to spec at TechStreet]. Another quick test is adhesive tape testing of crosshatched samples, see ASTM D3359 [link by ed. to spec at TechStreet] or search this site. If successful with these, then do the more rigorous tests. E.g., have scribed samples tested by a lab per ASTM D2803 [link by ed. to spec at TechStreet], 'Standard Guide for Testing Filiform Corrosion Resistance of Organic Coatings on Metal,' either Procedure A or C (both are salt-fog followed by humidity). Letter 38976 "Specifications for Powder Coated Aluminum -- AAMA 2603, 2604, 2605" gives additional tests.

Ken Vlach
- Goleta, California
contributor of the year honored Ken for his countless carefully
researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work
which the finishing world continues to benefit from.


Your problem sounds more like a curing defect. Have you tried to re-bake a defective component for a second time? We had a customer with the same problem on cast aluminium - a longer bake cured the problem.

Terry Hickling
Birmingham, United Kingdom

May 8, 2013

Q. We are experiencing similar problems with paint adhesion on Alodined parts and cannot seem to nail the root cause.
Are there any other things other than above that we should be looking for? We have ran numerous test panels and found from that testing that letting the primer sit for more than 3 hours will result in a very high failure rate.
We have also found that flash times are not as important as we first believed.
If there is there something that we are missing we would appreciate your help.


Gary Scholl
aerospace - Ontario, Canada

May 13, 2013

A. Hi Gary,

Ira's previous answer is a good starting point. I do not know which paint system you are using, so specific answers cannot be given.

Firstly I'd start by looking at the handling of parts, are all the operators wearing gloves? Is there any other way parts may get contamination on the surface? If there is any way that there may be a contamination issue you will need to have some form of pre-clean before you prime (something like a solvent clean using a clean solvent such as IPA or acetone).

Secondly look at any mixing. I don't know if you are using a 1 part or 2 part mix, but if it is a two part check that the right mixing ratios are used. Check that the correct viscosity is obtained and ensure the full induction time is used before application.

Thirdly look at your environmental conditions. Primers can be quite sensitive to atmospheric moisture, so if you are regularly going over 70% RH you will need to look at restricting painting during these times or consider introducing some environmental controls (that will mean considerable investment though).

Finally look at the curing cycle. Are you leaving it for a sufficient time? To be honest 3 hours seems to be far too quick for a room temperature drying primer. You will probably need to leave it for at least 24 hours and full cure will probably not be reached for 7 days. Most primers will be able to be heat cured, look at the manufacturer's Technical Data Sheet (TDS), this will tell you if it can be accelerated, alternatively talk to the manufacturer and see if they have a recommended heat cure cycle if there isn't one on the TDS.

That should get you started.

Brian Terry
aerospace - Yeovil, Somerset, United Kingdom

June 19, 2017

A. I suspect the chromate coating is too heavy. For painting, the chromate must be very thin, down in the iridescent area.

Editor: I cannot hardly read the fine print of the blog.

robert probert
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina

Ed. note: Are you sure your computer is set to display pages at 'actual size', rather than 'zoomed out'? We can easily make things bigger if people are having difficulty with it, but we've tried to display stuff at about the same size as we see on popular sites like Google & Amazon, so people don't have to zoom in or zoom out for this one site. We've upped the size of the small red text, and we'd be happy to hear additional comments on the sizing.

Alodine 1200S lowest service temperature

November 28, 2013

Q. Hi,

What are the temperature limits for Alodine 1200 S? I only have found the upper limit, around 140 °F, but I also need to know the lower limit.

Best regards

Juan Peiro
Mechanical Engineer - Coimbra, PORTUGAL

December 6, 2013

A. The manufacturer recommends a lower limit of 70 °F. and a high limit of 100 ° F. The 140 °F you mention is not an application temperature, but the temperature at which the still wet or new coating will begin to deteriorate, meaning your rinses and drying should not exceed that temperature.

I have seen operations that work OK at room temp, down to 55 °F, but cooler temperature do interfere with the reaction, creating a poor coating. Salt Spray testing may help in establishing limits outside of the manufacturers recommendations.

Paul Slater
- Cedar Rapids, Iowa, USA

December 9, 2013

A. Alodine (or Bonderite) 1200S:
If you're inquiring as to the temp of the bath, it's supposed to be between 70 & 100 °F. However, it also states to be oven dried at temperatures up to 140 °F. This high of a temperature can be problematic - causing the coating to become powdery & ineffective. Many reliable sources recommend to NOT use oven temps over 130 ° F, or to air dry - to not put them into an oven at all. This is especially true when using 2024-T3 aluminum, as it's unusually high copper content is uniquely affected by high temps and the copper molecules migrate to the surface causing corrosion - especially in salt spray corrosion chambers.

Fauna Tester
- Seattle, Washington, USA

January 4, 2017

A. Hi Juan,

As I read it you want to understand what temperatures an Alodined surface can survive in service.

Chromate conversion coatings are used extensively in the Aerospace industry. Typical operating temperatures will be down to -55 °C, in extreme cases it may be as low as -65 °C.

I am afraid I do not know what the performance would be like in cryogenic use (liquid nitrogen/liquid oxygen type temperatures).

Brian Terry
Aerospace - Yeovil, Somerset, UK

Ed. note: Thanks Brian.
(The strange dates on this "current" topic is because a recent posting was deleted at the poster's request).

October 25, 2018

A. Hi, I am now retired but spent many years in aerospace manufacturing as head of engineering and Manufacturing V.P.
If you have adhesion problems try increasing your Metal prep (phosphoric acid) immersion time. The surface must have a frosted look before Alodine. We followed the Boeing spec.

Harry Menian
Aeromarine Consulting - Fonthill, ON, Canada

December 2018

thumbs up sign  Hi Harry. I've heard of chromic-phosphate coatings for aluminum, but it sounds like either you are not correctly recollecting what what "Alodine" is or you are introducing us to a technology I'm not familiar with :-)


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
We need "Aloha" now more than ever

November 30, 2018

Q. We have been having adhesion issues recently with our primer (Amerlock 2) and top coat (PSX 700) system failing at the substrate. We conversion coat (Iridite 14-2) in a bath immersion application. After several days we bake in an oven around 200 °F for 5 hours in a bag under a vacuum for another process. After this, we prime and paint. I believe our adhesion failure is due to baking the Chem-film and "sucking" the moisture from it. If this is the case, where can I find supporting info to back me up that it not only degrades corrosion protection, but also its adhesion properties. I can't find many studies on the effects on adhesion and the TDS just states it may degrade corrosion protection. Thank you.

Alan Doan
- Indianapolis Indiana USA

December 4, 2018

A. Please see this article:
Also, BAC5719 specifically states not to heat above 130 °F.

Fauna Tester
- Seattle, Washington, USA

December 6, 2018

Q. Fauna, thanks for your response, but the link you provided did not answer my question. It states a loss in corrosion protection values but does not state the effects on adhesion. Also I am looking for an academic article rather than a blog. Thanks again.

One would assume that if you "dry out" the Chem film by exposing a bare, coated part to Temps over 140 °F, it would affect adhesion of a future paint system.

Alan Doan
- Indianapolis Indiana USA

December 2018

"Surface Properties of Conversion Coated Aluminum"
by Raj & Ali
from Abe Books
info on Amazon

A. Hi Alan. This book looks like the most applicable one I can find =>
but I haven't read it nor found any reviews. There is also an old book by Fred Perlstein, "Heat Resistant Chromate Conversion Coatings". A conversion coating book that I do have, Biestek & Weber "Electrolytic and Chemical Conversion Coatings" doesn't seem to cover it. "The Surface Finishing and Treatment of Aluminum and its Alloys" [link is to info about book at Amazon] simply says that MIL specs forbid unpainted conversion coated aluminum at above 65°C.

Metal Finishing Information Services / Finishing Publications in Birmingham U.K. offers their free search of metal finishing literature. The site has appeared to be down for a couple of weeks but they also offer the same database with abstracts on a CD ROM. A quick scan of my rather dated CD ROM of that database seemed to indicate that you're going to have trouble finding a technical article with twenty seven eight-by-ten colour glossy pictures of the quantitative effects of temperature on adhesion for the various types of zinc plating and zinc brighteners, and the various proprietary trivalent and hexavalent chem films, and the various organic coatings.

Google scholar list some articles, but I don't know if any are applicable.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
We need "Aloha" now more than ever

December 7, 2018

Your Q, A, or Comment:


Per chapter 16 of "Advanced Surface Technology" by Moller and Nielsen:
"The reason for this destruction [corrosion inhibition] is associated with a transformation of the hexavalent chromium compounds into insoluble compounds by heating. The formation of an insoluble chrome compound prevents the soluble hexavalent chromium compounds from acting as an anodic inhibitor."
I realize this is referring to the corrosion resistance but I would think any disruption in anodic activity on the surface could reasonably lead to primer/paint adhesion issues. I couldn't find any studies exploring this further but have you tried to perform an adhesion test before and after your bake? I've never used it myself but ASTM B201-80 [link by ed. to spec at TechStreet] has a soft gum adhesion test for chromates.

Ted: I see what you did there ;) Don't forget the circles and arrows and the paragraph on the back of each one.

Jameson Grout
- Agawam, Massachusetts USA

December 2018

? Hi again Alan. This is not really a response to your question, but thinking out loud and wondering if other people are thinking similarly. When a chromate coating is covered with a powder coating, not only is it less prone to drying out, but it's not directly exposed to the oxygen in the air, the decomposition into "... insoluble chrome compound ..." is probably significantly slowed.

You could be correct that your vacuum process is sucking out the moisture and creating a problem despite the paint layers. Can you bake a test part separately without the vacuum?


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
We need "Aloha" now more than ever

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