Chromate conversion coating failure on aluminum.

A. Hi Erwin. Hopefully you can, and maybe you did, do some sort of material balance to demonstrate to yourself that the chromate didn't get used on the parts. If so, while awaiting input from the readers, the dialog on this thread between Cathy Rogers and Brian Terry may be helpful.

Luck & Regards,

A. Were there any undocumented additions that could have diluted your solution?

Have you tried taking another sample deeper in the tank to see if it's a circulation issue?

A. Hi Pete,

Obviously you want to walk the line and see if anything has changed. Most failures I have seen happen because of improper steps following the chromate tank such as final rinse PH or TDS.

Also make sure the chromate film is never subjected to temperatures above 140 °F [Ed. note: please see letter 34822, "Temperature limit for chromate conversion coatings on aluminum", for a good discussion of this temperature limit].

In some instances it can take up to 24 hrs for the film to fully cure perhaps you are packaging too quickly and causing damage. Also check the paper that they are being wrapped in and make sure it is a good grade of wrapping paper with no sulfur ⇦this on eBay or Amazon [affil links] compounds. And be careful of the rust preventative papers as sometimes they contain compounds that will damage the film.

A. Getting 2024 panels to pass a 168 hr. salt spray can be tricky sometimes. We have seen problems when the Al content rises too high in the chromate solution. Letting the panels set for at least 24 hours is a good idea. A final D.I. rinse helps remove any residues after chromating. I would also consider using a more aggressive etch like a 5% caustic soda ⇦liquid caustic soda in bulk on Amazon [affil link] to help remove any grit that may be left behind. 2024 seems to respond better to a 50% HNO3 (Nitric Acid) deox/desmut than other types of deoxidizers on the market.

A. I got this from an old time anodizer, lots of work and not much technology. It or something worked.

Get a new supply of test plates. Even though they worked before, something happens to them sitting on the shelf. He used a shelf time of one year and trashed the rest.

A. Peter: You'll probably feel like Sherlock Holmes when you solve this problem. The good news is by the time you solve it you'll know a great deal more about the process. Our company has been involved with conversion coating on aluminum for more than fifty years, so I'd like to pass on some of the advice we give to our customers.

Jim Watts is certainly on the right track with checking the process with fresh metal, and he is correct in stating that something happens to the metal. Although we tend to think of aluminum as a solid, and solids as permanent, over time the copper in the 2024 alloy tends to agglomerate, through molecular motion. When the copper agglomerates within the aluminum matrix, there are ideal sites for galvanic corrosion. If and when this happens, it is almost impossible to pass salt spray, as these agglomerates are very difficult to remove.

Our company recommends not using panels that are more than six months old. ( If this sounds creepy, that's exactly what happens!) Some people mechanically clean panels to remove imperfections on the surface which might not be removed by cleaning or deoxidizing, but with good quality metal and good quality cleaner, deoxidizer, conversion coating and rinsing, you should be able to pass salt spray without sanding.

If sanding is proven to be necessary, Scotch-Brite is a better choice than abrasives which could embed themselves into the metal. If sanding is necessary, certainly very light sanding is preferred over heavy abrasion. Our company recommends staying away from alkaline etching to clean 2024 panels which are to be salt spray tested. Since copper is insoluble in an alkaline etch, removing aluminum from the surface can expose more copper on the surface, improving the chances of pitting. Etching (or sanding) also roughen the surface, which can help salt solution cling to the surface during the salt spray test, helping to improve chances of corrosion.

See Section 6.4 of MIL-C-5541E, where you'll see that the U.S. Navy agrees with us. Nor do we recommend using 50% nitric for deoxidizing, because it is a very nasty solution to handle. Actually dilute nitric tends to passivate aluminum surfaces. To get the best chromate conversion coating, it is preferred to activate the surface with a deoxidizer containing a small amount of fluoride. Chromated deoxidizers work best; iron-based deoxidizers are inconsistent. We hope to introduce a non-chrome, non-iron process within the next several months.

Whether or not your problem is with the metal or sanding process, we have always been proponents of "bucket" tests. Since your process worked before, you should be able to pass salt spray by making up new solutions in 5-gallon buckets, place them next to your processing line, then one at a time substitute the new solutions for the old in processing panels. For that matter, using all new solutions should tell you whether or not your metal is bad. As I said earlier, solving problems like yours can involve a great deal of detective work, and incidentally, also check on whether or not the salt spray testing itself has changed in any way. Is the cabinet run according to ASTM B117 ? Has the reject rate from any other process gone up? Good luck!

Phil Johnson
- Madison Heights, Michigan
1999

Have we considered the possibility that the failed panels just might be representative of the part themselves? It is much more difficult to solve that problem than to manipulate the test panels so they pass a test. I think we should saw up some actual parts (taking care to seal off the metal exposed by the saw), and see what they do in the salt spray chamber.

If the results are similar, the coating and process is bad. If the parts pass with flying colors, see why the panels are not representative of the parts.

I would also take this opportunity to make the test panels receive the same processing as the work. Unless you jitterbug sand the parts, I would not sand the test panels. You may want to investigate a more representative finish on the panels.

We don't want to become experts in the art of making a test panel look good at the expense of resistance of the parts to the elements.

Dear Pete In case that all the advises you get before me won't work begin to make new solutions on the line, begin with the deoxidizer which seems to me the main reason for failure . After deoxidizer if still the phenomenon continues, replace the rest, the Conversion Coating and in the last the Alkaline cleaner. Replace the rinse bathes completely. Yehuda

Peter, Been there - done that. Get new panels. Check with Tom Morse @ Macdermid in Waterbury Conn. Tom has a 30 year old cleaning cycle for the panels which was put out by the military. Ray

In case you missed it, I am taking the liberty of reprinting my response to a request for test coupon suppliers which I sent to this forum back in July of this year. This may be of help to you also:

"We had many problems getting our 2024 aluminum panels to pass salt spray after chemical conversion. We found that a source of the problem was the condition of the panels before treatment. We solved this aspect of the problem by obtaining panels from Benjamin Metals, Inc. (USA). These panels come with a coating of PVC tape which protects them from corrosion during storage."

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Ed. note: Sorry, Bill, but we don't do brand sourcing on this site (why?). A handful of companies make this site possible for these public discussions year after year, and it just isn't right that they should also have to pay the costs of maintaining distributorship info for the companies who get commercial benefit from their mention. Benjamin Metals and anyone else can advertise here if they wish, then any commercial things they wish to say can be said :-)

A. This long list of responses covers many aspects of troubleshooting chromate conversion coatings with 1 glaring omission.

Most chromate conversion coatings are qualified to meet mil spec 5541 and 81706 using a chromated deoxidizer on 2024 panels !

A. Do you pass Salt spray with a fresh chromate solution? If so problem could be contaminates being dragged into the chromate from the deoxidizer rinse. Chlorides could cause a problem with salt spray results. A deionized rinse before the chromate tank and making up chromate baths with deionized water could help.

A. Pete,

Since this still seems to be a "hot" item, I thought that I would give a more detailed response to my previous one regarding the source of panels:

Getting 2024 panels to pass salt spray after chemical conversion or anodizing is indeed problematical. On one occasion during a customer audit I informed our auditor that we had halted production due to salt spray failure. He said that this was ridiculous and gave us permission to return to production! Unfortunately not all auditors are that lenient.

We have solved salt spray failures over the years by doing one of the following (We never found that changing the chemical conversion baths, the etch bath or the etch product had an effect.):

1) Chemical polishing of the panels prior alkaline cleaning and etching baths used for the parts. I will gladly look up the details again for anyone interested, but we eventually encountered an auditor who said that this was not permitted since the represented parts did not undergo the same process. I appealed this issue to the customer's engineering department and lost the appeal as well.
2) Try a different lab. We once failed to pass a test performed by an outside, certified lab as required periodically by one of our customers whereas the panels from the same batch passed the tests in our lab. We sent panels to a third lab at the suggestion of the customer's auditor and passed the test.
3) Several years ago at my request Amiel G. Forshee of Boeing kindly sent me copies of slides and papers he had written on the subject of salt spray testing which supplement his article which appeared in "Metal Finishing," October 1991. Try to obtain similar literature from him and pay particular attention to the points on identifying and labeling flaws on the panel before the test as well as his discussion of corrosion tails. I think that in the case of chemical conversion coatings of Al the absence of a tail excludes the spot from consideration.

A. Let me just add one more thing, for Pete's sake and for others who happen to check out this popular string. The "E" version of MIL-C-5541 allows processors to test salt spray resistance of either (1) 2024 panels, or (2) the same alloy panels as the parts being processed. There are pluses and minuses to selecting option #2. If the alloy of the parts being processed are not high-copper alloys, it is probably easier to pass salt spray with that alloy- except cast alloys. But, if the processor is conversion coating more than one alloy, then each alloy must be tested. If several alloys are being processed, or if castings are involved, choosing 2024 for salt spray testing would have an advantage.

Since you are in the Pacific Northwest, I'll just add that I have never seen a BAC spec which allows this option.

P.S.
There are some 2xxx series alloys which have a higher copper content than 2024, and which are almost impossible to pass salt spray tests (e.g. 2219). Many cast alloys are also impossible to pass salt spray due to therir inherent porosity where chlorides can be easily trapped. What does it show about the real-life value of a corrosion test when we are allowed to test panels of one alloy which will (usually) pass salt spray, when we know that the parts to be used in real life service will not pass?

Phil Johnson
- Madison Heights, Michigan
1999


1999

A. I have been away from this forum for some time and I am glad that I returned today to pursue the discussion of this topic. We have absolutely gone round robin with this problem and in fact I am now working closely with another finisher to try and uncover some semblance of cleaning consistency to deal with this problem.

We have had rounds of success and failures attempting to chromate 2024 panels and in 30 years we have not been able to come up with a formula to categorically pinpoint a cleaning method that is repeatable.

We have tried experimentation with new chromate conversion baths, four different brands of conversion coating, changed baths on monthly cycles, weekly cycles, deoxidize only cycles, deoxidize and etch cycles and we are currently using one side coated with plastic panels and have not been able to assure successful, consistent passage of the salt spray panels to MIL-C-5541 criteria.

What I am going to do is print this list of responses to this problem and show it to my next aerospace quality auditor as a backup to my insistence that this test is not an accurate assessment of the quality of chromate conversion coating being produced.

We - as I am sure many of the other people who have responded to this forum - have not had a field failure of chromate conversion coating that resulted in a product or component being rejected and sent back for non-compliance.

There is a need to organize and discuss alternatives to evaluate quality of chromate conversion coating instead of trying to substantiate why we cannot consistently pass a salt spray test on a 2024 aluminum alloy that has been conversion coated.

I think the governing body of ASTM may be interested in ideas viable to designing a consistent, meaningful assessment of chromate conversion coating on 2024 aluminum alloy.

A. The variety of opinions about the salt spray test for chromate conversion coatings amuses me. It is easy to tell who has only dealt with the specification and quality records and who has actually been involved in processing parts and panels. In my opinion, for what it's worth, the only value these tests have is to have test certifications to show an auditor. The gentleman who questions whether or not the panels represent parts being processed sounds like an engineer who assumes that these tests are done for a reason and that the results have some value for comparison to the real world. I'm an engineer who thought the same thing until I had to work with this silly test.

I agree that this test is so flawed as to be simply ludicrous, Mr. Lester. Whenever anyone complains that they're failing this test on 2xxx aluminum, the advice and the correct resolution 90+% of the time is don't dare touch their process at all, don't look at your actual parts at all -- but just keep buying new test panels, and testing the test panels until you find a vendor whose test panels will pass without changing anything at all :-)

This obviously has nothing whatsoever to do with the real parts and nothing to do with the chemical process! What is the value of a test that has a 90+% margin of error? :-)

But I don't accept general despair that the concept of testing is hopeless. Mr. Hillock is on the right track: this test absolutely does not work, and it reveals nothing of value at all, so get ASTM and other standards-writing bodies convened to fix it. Regards,

A. I admit to a large amount of cynicism, but it is only for the chromate conversion salt spray tests. Testing is invaluable. I'd hate to fly on a plane that had been manufactured on the basis of analysis only. Good tests are representative of actual conditions, whether they be corrosive conditions or loading/stress conditions, and the results give good useful data to evaluate a part design, a process, or a system. My cynical attitude toward the chromate conversion salt spray test is that it fails both of these criteria for a "good" test.

A. From my experience (17 years) The deoxidizer should be blame. Or more specific the fluoride. Try using deoxidizer without HF.

A. Hi Cathy,

There are many things to look at when trying to work out where failures are occurring in your salt spray.

First thing to do is to see if anything has changed on the shop floor (new operators, change in immersion times, change in drying techniques, etc).

Secondly look at your chemical analysis for the whole process (cleaners, etches/deoxidizers, conversion coating tank, final and inter-stage rinses). Consider if there could be any potential contamination (deoxidizers can be sensitive to iron and copper levels, the conversion coating tank to aluminum and trivalent chrome levels).

Thirdly check that your salt spray is still operating within tolerances required by the test specification as variation there, especially in solution pH can cause problems.

Finally if you have bath chemistry instability you need to find out what is causing this instability, whether there is excessive drag-over into the conversion coating or the tank is old and become unstable.

These simple steps should start to point you in the right direction.

A. Hi Cathy,

The advice about solution pH in salt spray relates to the fact that pH outside of the range stated in the specification can have a marked effect on your results. Both high and low pH can lead to premature failure of the salt spray.