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topic 12517

Aluminum Welding issues with Iridite, Alodine, Chem-Film

A discussion started in 2001 but continuing through 2017



Alodine is a Henkel trade name for their line of chromate conversion coating processes.

Q. Hello All,

I have a 5052-H32 Aluminum Chassis that is Fusion welded (TIG) together. After welding I have specified Chemical Conversion Coating (yellow) per Mil-DTL-5541 [link is to free spec at Defense Logistics Agency,]. The areas in and around the welds becomes discolored after Chemical Conversion Coat, and actually some of the weld melted areas are a gray color. I talked to the vendor and he told me he is using Alodine for Chemical Conversion Coating. Does anyone know what is causing this? I have not seen in past.


Mike McLaughlin
- Richardson, Texas

Slang and Acronyms:

"Chem-Film" is widely used slang for chromate conversion coating on aluminum.


A. The filler rod and the amount of heat used have a huge effect on the way the weld area takes the chromate.

Minor differences in the same alloy can affect the weld.

There might also be tiny differences in the platers pre-process and chromate parameters and still be well in the allowable range that could make some difference. The profit on chem film is very small. Virtually no plater can afford to do days of testing in pilot tanks to look for a slightly better look on the weld.

The filler rod is the prime suspect.

James Watts
- Navarre, Florida


A. Mike,

It sounds to me that the parts were not deoxidized enough to clean up the welds properly in the cleaning process before chemical conversion. Also During the TIG welding there was enough heat put to the part that an exact color match is difficult to obtain. Their are proprietary chemicals out there that will help with this.

Lee Moss
- Calgary, Alberta, Canada

What does Iridite mean to welding


Q. We are TIG welding an aluminum frame, and would like an Iridite finish. What does this process entail? Will it effect our welds?

John M Gotsch III
molding co. - Arlington, Vermont


A. Hi John. Iridite is a Macdermid trade name for a brand of chemicals used for Mil-DTL-5541 [link is to free spec at Defense Logistics Agency,] chromate conversion coating of aluminum. Protective finishes like chromate conversion coatings should ideally be applied after fabrication, not before. But you need to protect aluminum from corrosion in some fashion -- so if you don't do a chromate conversion coating, you have to do something else like anodizing or clearcoating.

Chromate conversion coating involves immersing the fabrication in a series of chemical processes, most likely: mild alkaline soak cleaner, caustic etch, desmut, and the conversion coat with intermediate rinses. Many jobshops offer this service. Good luck.

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

Degreasing chromated aluminum for welding

December 4, 2013

Q. Hello All,

Recently I am having problems with welding. I weld Iridited Aluminium 4047 and Aluminium 6061 and am getting blow holes. I would like to reduce the number of them.

I am thinking of degreasing the Iridited Al4047 with a degreaser for a couple of minutes. Do you think it will work? Doesn't it damage Iridited surface? Please note that this part should pass salt spray test.

If it damages the coat, what else can I do?

I appreciate all of your replies.


Tony Kim
- Toronto, Ontario, Canada

December 10, 2013

A. Hi Tony. My understanding is that acetone, isopropyl alcohol, mild detergents, or very mild alkaline cleaning will not harm the conversion coating. However, in these days of highly proprietary RoHS-compliant trivalent chromating, it would be best to check with the supplier to make sure about possible exceptions. Good luck.


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

December 27, 2013

A. When customers ask us the question about welding aluminum here is what we tell them.
When welding aluminum prior to anodizing, use 5356 welding rod only. This rod gives the best appearance after anodize. Never use 4043! This will give you discolored welds after anodize.

Also all welding should be done prior to finishing of any kind. Welding will destroy any chromate conversion coating and the weldment will never pass salt fog testing unless the chromate is re-applied after welding. If that is the case then just wait until after welding to do all the finishing.

Tim Hamlett
- Pompano Beach, Florida USA

Tig welding post chromate coating

June 27, 2016

Q. Hello

I am after some advice on chromate conversion coating.

I am about to start fabricating a job using aluminium and needs to be chromated as a primer for powdercoat. My issue is the finished job is to big too chromate post fabrication, so it has been recommended that all the material be chromated before welding and to etch prime the welds after welding. I am worried that the chromate will effect how the material welds and the strength of the weld. Would it be just a matter of wire brushing the weld area before welding? The material grade I will be using is 5005, T5 and T6


Luke Mcfadzean
Sheet metal worker - Melbourne, Victoria, Australia

July 13, 2016

A. I have seen cases where the chromate conversion coating makes it difficult to establish an arc. Seeing as how you will be destroying the integrity of the coating in all the welded areas, it might deserve a second look. I would recommend a good cleaning after welding and then a powder primer under the finish coat.

Ed Peters
- Frederick, Maryland USA

Weld showing after MIL-DTL-5541

October 10, 2017

Q. We purchase A356 cast aluminum parts that get:
Weld areas allowable per spec (minimal areas of short pours or dings, etc.)
Heat treated to T61
Machined (use water soluble cutting fluid which has been the same for years)
Chemical film MIL-DTL-5541
We just ran into a major problem with our castings and have tried multiple solutions with no resolve.
After 25 years of doing the same process with our foundry the weld is showing up in every parts after the etching process when getting the parts chemical filmed. They appear as a dark gray or even black mark after etching.
We have seen this one time in the past and the culprit was the welder used the wrong weld rod material.
We have been working with the foundry to try and figure out what the problem could be as they have not changed any process on their end. Some of the test we have done are:
Trying multiple different manufacturer weld rods (A356 just like parent material)
Different cleaning techniques prior to weld
Multiple different outside welders (one sample weld from an outside source seemed to have the weld show up as "white" in color as apposed to "dark gray".
The ingot has been tested for chemical elements and it falls in with the spec for A356.
Any thoughts or suggestions would be greatly appreciated.
We are preparing to run a test with the actual parent material being used as the weld rod material and also try different plating companies.

Tim Lloyd
- Ontario, California

October 10, 2017

A. If by "etching" you mean alkaline etching, that is part of the problem. you should not alkaline etch 356 castings because it decreases the area of aluminum metal that will take the chromate and increases the area of the silicon which does not chromate and turns gray. Stay out of the etch and use nitric /ammonium bifuoride to decrease the area of silicon.

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

October 17, 2017

thumbs up sign Thank you for the comment Robert. Yes our plating vendor does in fact alkaline etch. We have used the same process for over 25 years with no issues thus far.
We have since welded up more castings with different filler rods and cleanup processes to test.
We sent 2 parts to our current plating company which were alkaline etch cleaned and also sent 2 parts to another vendor who uses the nitric/ammonium cleaning you mentioned. Both processes have failed testing, meaning the weld is still showing up after chemical film, albeit the nitric acid cleaned parts did turn out slightly better, but not acceptable.
We are currently running more sample parts to test.
There seems to be so many variables which could create this issue and we need to figure out what one it is.

Tim Lloyd [returning]
- Ontario, California

October 18, 2017

A356 should be welded using 4043 filler wire, if there are cracking issues with the welds, then 4047 is recommended, 4047 has a very high Si content, and will almost surely be quite a bit darker that surrounding material after caustic etching, it goes away if the desmut tanks can remove Si.

The "light" weld was probably due to using 5356 filler or something like it with almost no Si in it (off topic a bit, this is what I use if I need to anodize welded parts from 5xxx or 6xxx stock, if you use 4043 or 7 filler, you will get very dark looking welds after the part is anodized).

So I suggest you do some in-house testing with different filler wire to see which one matches the A356 best, then buy plenty of that very same filler wire from the very same supplier and use that for your work. If you send out those parts to be welded, send your tig wire with the parts and tell them to use only that and nothing else.

Janis Ziemelis
- Riga, Latvia

October 2017

thumbs up sign  Thanks Janis -- a very informative answer!


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

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