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topic 56917

Problems with Alodine, Iridite, Chem-film not sticking / rubbing off

A discussion started in 2000 but continuing through 2019


Q. Greetings to all! I'd like some advice regarding chem-film finishes for die-cast aluminum parts. Our company's products often use small aluminum housings that are machined from 6061-T6 alloy and chem-filmed per Mil-C-5541 [link is to free spec at Defense Logistics Agency,]. We're now investigating the feasibility of producing some of these parts via die casting.

Recently we received some sample parts that were die cast from alloy A380 aluminum and were chem filmed per MIL-C-5541, Class 1A. Unlike the machined parts (which have colors that are uniform and range from pale to golden yellow), some of the cast parts possess a mottled green appearance that can be easily rubbed off. This is unacceptable.


"Chem film" is a synonym for chromate conversion coating on aluminum, but is mostly used only in the USA.
Alodine, Iridite, & Alochrom are some common trade names from different suppliers.
MIL-DTL-5541 is the most common specification for it.

Also a few other samples I received have a brownish color, and I noticed that heating them to about 125-150 °F causes their color to darken to a deep bronze. The texture of the finish also becomes flatter, almost chalky.

Some individuals have indicated that this result is typical of chem-filmed die cast aluminum parts, especially when using the A360 or A380 alloys. However, I'd like to hear additional, detailed opinions on this topic.

Also, if the chem-film finish is inadequate for this application, what is an economical alternative? Keep in mind that the electrical conductivity that the chem-film finish offers is a very desirable feature.

Keith B. Olasin
- Whippany, New Jersey, USA


A. 360 and 380 both have about 9% silicon in the alloy which makes life quite difficult vs 6061. It will require an acidic deox/desmut that contains a goodly amount of fluoride. Tri acid-sulfuric/nitric/HF being a common one that is extremely strong. If that is the only alloy being run in that chem-film tank, then it could be tweaked with the help of the vendor for optimum performance on the cast alloy. Color changes, minor ones,are moderately common. It should not have major ones and certainly should not be chalky or rub off.

140 °F is the maximum that a chromate coating should be heated to , in most cases.Alternatives would be return to the wrought alloy, change the casting alloy to one that was more amenable and find better vendors for the casting and chem-film. You might have to bring the chem-film "in house" . This may cause more problems than it solves. There may be vapor deposition processes that would give satisfactory tradeoffs, but The price would very probably be higher.

It is worth looking into from some of the folks that advertise here.

James Watts
- Navarre, Florida


A. Hi Keith,

You're pretty much right on the button with your thoughts and what others have been telling you. Die casted chromated parts are nowhere near as uniform as a machined finish because of the porous structure of the casting, you can limit this by polishing down the casting to a smooth surface, but you eliminate the cost savings of going with a casted instead of a machined surface by doing this.

The finish probably appeared rather patchy in areas, but as far as rubbing off, it shouldn't do that no matter what class of chromate you use Class 1/3 whatever. It won't last as long corrosion wise on the casting material as it will on a machined surface. I want to say off the top of my head that 336 is of course the machined surface salt spray requirement in hours and casting material (dependent on the type, I think 380 is the called out material in the ASTM spec) its around 58 hours or something incredibly low like that. Which in my opinion is proof enough to signify the difference! s, the governing bodies already

realized and noted that you will not have good salt spray resistance, and if not mistaken, your electrical requirements also drop down as far as the conductivity goes in regards to the casting material. Are there other options along these lines, maybe, but I don't know of one myself. Good Luck to you, if you have any further questions, feel free to let them fly, I don't have an reference material in front of me to consult and all of this is coming off the top of my head so it might not be 100% accurate, but it does give you a solid A to B starting point to acknowledge.

Matthew Stiltner
- Toledo, Ohio

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.

Alocrom 1200 finish coming off


Q. We have a customer who is experiencing problems with an Alocrom 1200 finish [Chromate conversion coating]. Please could you tell me if the colour should be coming off in:
a) General transportation in cardboard boxes.
b) When rubbed lightly with your finger or a damp cloth.

I have spoken to a couple of our metalwork suppliers who give conflicting answers.
i.e., Once the process has been sealed properly no colour should come off & another who says yes the colour does come off, be very careful what you clean it with.

We have been using this finish for over 12 years & never had a problem before.

If you need to clean the metal after handling it in a production environment, what should be used? Have been told white spirit is ok?

Would very much appreciate your help.

Alistair P
- Peterborough, Cambridgeshire, UK


A. Rubbing lightly with a damp cloth - should not remove the coating.

Shipping in cardboard boxes. In humid conditions the sulfur in the brown cardboard will slowly degrade the chromate but that is unlikely your main problem.

If the applicator puts the coating on too fast, it will wipe off. Temperature, concentration, and pH affect the speed of the gel formation. 99/100 of the time, the applicator does not measure pH properly in that low range, his pH is too low, the coating forms too fast, the coating wipes off. If he would throw away the pH meter probe which is probably eaten up by the fluoride in the chromate, then dip out a 5 gallon bucket and raise the pH in increments of 0.3 pH with ammonium hydroxide until the coating forms over a period of 1.8 minutes, then it will not wipe off.

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


A. Years ago we ran into the exact same problem with one of the major vendors in the USA. Possibly the same one. We chased the problem for days and did testing in the lab and could not get a suitable coating in optimal conditions. When we bitched long enough, they replaced the can with another from the same lot. This failed also. They checked with the retained QC sample and agreed that there was a problem and promised to send a can from a different lot #. The same lot # showed a few days later. They only had 4 cans in stock and were all from the same lot#. The problem material just did not look the same, like it was not properly milled or properly mixed. Solution, we switched to Iridite and had no problems. Sometimes the problem is not in your shop.

James Watts
- Navarre, Florida

sidebar2 July 30, 2013

Q. Did you ever manage to find the root cause of this? I am also battling this issue currently.

Scott Merritt
- Fort Walton Beach, Florida, USA

July 2013

A. Hi Scott. Alistair actually posted long ago and checks in rarely if at all. But hexavalent chromate was the usual finish in 2005, and trivalent is more common these days, so please feel free to offer some info about your situation and ask the readers for updated opinions. Thanks!

Good luck.


Ted Mooney, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha

August 2, 2013

thumbs up signThanks Ted, I guess that's what I get for not paying attention.

Scott Merritt
- Fort Walton Beach, Florida, USA

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Is it common for Iridite not to stick to aluminum?

February 8, 2008

Q. Have a situation where we are able to physically rub off the coating created from the Iridite process.

R. Carney
Engineer - Pataskala, Ohio

simultaneous February 11, 2008

A. Time out -- It is time for definition of terms.
First Iridite is a brand name for chromate conversion coating. Alodine is another similar trade name of another company.
Second, the coating can vary a (relative) great deal in thickness. The coating is a gel, so till it partially dehydrates, it is not difficult to rub off, even when it is a good coating. Notice that you are not supposed to paint it right away.
Third, rubbing is an extremely nebulous term--with what, # of cycles, pressure applied and etc.
From the nature of your inquiry, you are either expecting too much of it; or the metal finisher has a problem with his process.

James Watts
- Navarre, Florida

February 11, 2008

A. Assuming you solution is not contaminated, and that you have a workable surface preparation cycle, then "rub-off" happens when the coating forms too fast. The coating forms too fast with (1) too low pH, (2) too high concentration, (3) too high temperature. so dip some out in a bucket, rasie the pH in 0.2 increments with ammonium hydroxide (NOT CAUSTIC) until you slow it down enough that, after curing 24 hours, it will not rub off.

robert probert
Robert H Probert
Robert H Probert Technical Services
supporting advertiser
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.

March 10, 2011

Alodine 1201

Q. I've just finished the manufacturing of some aluminum (7075-T6) pieces that needed to receive the Alodine 1200 but it is not fixing on the pieces. Is there anything that can avoid the Alodine to fix the aluminum (or this kind of). I've already worked with 7075 previously and it has worked, but not this time.

Pedro Almeida
Engineer - São Jos ã DOS CAMPOS, São Paulo, Brazil

March 11, 2011

? Tell us about the cleaning and especially the deoxidizer (concentration, chemicals, age, etc.)

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

March 14, 2011

A. Hi Pedro,

You need to give us more details about your process before we can advise you. Can you tell us what your operating parameters are for the precleaning, etching and Alodine processes?

My initial reaction is that the pre-cleaning and etching has not been effective and that is causing your adhesion problems.

Brian Terry
Aerospace - Yeovil, Somerset, UK

March 23, 2011

A. Important are the conditions of the chromating bath.
Too low pH, too high fluorides, too high CrVI content and a too long treatment time can all cause powdery, badly adhering chromate coatings.

Barry Groeneveld
chemicals - Heijningen, Zuid Holland, The Netherlands

Aluminum chem film problem parts

May 3, 2015 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

We are having a problem getting chem film to stick to some parts. Material 7075-T651.
Small parts that our customer machines and tumbles and then rinses the cleaner off. We have tried all different variations on our tanks. Different times, you name it we have tried. They end up looking blotchy and when you rub on the part the chem film comes off.
Something is blocking the process.
Our line consist of a cleaner, mild oxidizer/etch and chem film. At this point I do not know what to do.
I suggested to the customer that perhaps their cleaner is not being washed off properly, but we are running them through an aluminum cleaner @ 160 °F.

Any suggestion would be welcomed.

Micheline Forth
- Rincon, Georgia USA

May 4, 2015

A. Have you tried running it without an etch? Some parts tend to not take chem film if it has been etched prior hand.

Daniel Harris
- Fort Walton Beach, Florida

May 4, 2015

A. "tumbles" <= there is the magic word. I suspect the tumble media has been used on iron parts and is embedding fine iron particles into the soft "Kleenex-like" aluminum. Go back and tell them that the tumble media must be designated "aluminum only". While waiting for new media, try a heavier etch to remove and dislodge the embedded iron particles.

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

May 5, 2015

A. Micheline

Adding on to Robert's suggestions, see if you can get a part that has not been tumbled and see how it processes. Baring that, take a tumbled part and mechanically clean it with scotchbrite, then process. If you still have problems, alter your rack material. Some heavily alloyed materials react unfavorably on certain (rack) materials.

I would avoid etching, but rather go longer in your (de)oxidizer. Some soaps/cleaner will leave a residue after drying that cause problems.

Willie Alexander
- Colorado Springs, Colorado

December 25, 2018 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. We have some Al7075 parts Alodined by a company. When I rubbed my finger firmly on the Alodined surface, I noticed that the coating was removed from the surface. Is this normal? If this is abnormal, what is the reason behind it?

H Sharifi
- Tehran, Iran

January 4, 2019

A. Hi Mr Sharifi,
To your question, 'is this normal?'- yes and no. Yes, in that it is not uncommon to see, but no, in that it's not supposed to come off significantly. Alodine needs some time to cure after processing, a couple days is plenty, before you start handling it normally. If by 'the coating comes off the surface' you mean a white cotton glove shows some slight discoloration, I wouldn't worry too much. If the surface is becoming visibly less golden where you rubbed, the possibility and severity of an issue are far greater.

Having seen this issue in the course of doing a LOT of chromate on 7xxx, I can tell you some things that could cause the coating to rub off more easily:

-That alloy isn't ever going to chromate as reliably as 6061. The best possibility for a good coating is going to depend on the pre-cleaning they are doing. They need to be sure their soap doesn't have a bunch of greasy contamination. And regardless as to whether or not they etch (under normal circumstances, they wouldn't for regular chromate, unless you're requiring a mask and anodize step too), it is IMPERATIVE that the deox/desmut tank be working well. That means good sparging, fresh solution, adequate acid and redox potential, some process in place to ensure that silicon is never allowed to build up past 30ppm in the tank. Deox results should be confirmed with a white cotton glove or lab wipe before going to chromate- parts coming out of that tank should never be smudgy, and the glove must be squeaky clean. If not, put the parts back in. Failure to fully desmut WILL result in a poor, powdery finish that doesn't protect as it should.

Obviously running the alodine tank to manufacturer's specs is important- but 90%+ of times I've seen troubles with chromate, it's the deox step to blame.

That being said... We've done a decent amount of destructive testing at our shop and troubleshooting trying to pin down the point of failure vs aesthetics, and parts that APPEARED to have coating rub off and become lighter actually passed 168 hour salt spray in one case!

I guess I would approach this like, 1) is it a functional or aesthetic problem; do these parts pass destructive testing? If aesthetics matter as well, is that clearly stated on your PO? I say this because most shops process to spec, which is a functional spec, and allows for some pretty serious variation in color. You MAY have a functional coating even after rubbing the surface residue off- so you need to ask if that is enough, and if you can test that and prove that it is. 2) is there a way you can confirm tank maintenance and processing best practices with your vendor?

All that being said, you definitely need to address this issue with your vendor and make sure they are giving you the coating you are paying for, whether you care about appearance, performance, or both- Good luck!

Rachel Mackintosh
Plating Solutions Control Specialist / Industrial Metals Waste Treatment - Brattleboro, Vermont

Iridite rubbing off

March 5, 2019 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. I have multiple 6061 Aluminum parts that get Iridited at a plater. When the parts come back they look fine, however on some of the parts the iridite comes off right when you rub them. It is even more evident when they come back from the welder and get tested. (And yes, we know that Iridited parts shouldn't be exposed to >140 °F). If the parts are being handled the same way pre and post iridite, what could be the reason behind the iridite rubbing off on only a few of the parts? Does the Iridite color rubbing off mean that the corrosion resistant coating has been rubbed off?

Aulla Hamdeh
- Springfield Township, New Jersey, USA

March 6, 2019

A. It should not "rub" off. Non-adherence can be dues to: (1) pH too low, (2) concentration too high, (3) temperature to low OR too high, (4) surface preparation inadequate,(5) floating soil and other contamination, (6) not "cured" for 24 hours before wrapping in a dry low humidity environment, and other reasons.

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

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