Chem filmed panels fail salt spray test

A. Hi Alejandra. I think the specifications *require* the test panels to be the same material as the actual parts rather than just "allowing" it.

Luck & Regards,

A. There are two classes of testing called out in this spec, process control testing (at least once a month), and conformance testing (every single lot). Corrosion testing is defined here as part of process control - it is being used to check that the system conforms, not that a given lot of parts conforms.

Now, why define only two alloys as optional substitutes for testing? Well, to me, one benefit of defining these alloys and tempers is that it gives you a nice benchmark between suppliers.

As to why these alloys, they're both fairly standard and probably were considered "representative". Also, 2024-T3 is more likely to fail salt spray than 6061-T6, hence the two-tier system where you can only use 6061-T6 for Class 3, but 2024-T3 for any of the testing.

Great answer, Jenna. Thanks!

Luck & Regards,

A. Dear Lee,

There are several variables that may be potential causes of failure that may have been overlooked. First, I would look at the surface finish/condition of the test parts/coupons. Scratches or hidden occlusions will likely be a source of failure. Another place I would suggest looking is in the final rinse and drying steps. If your rinse is above 140F, you may be degrading your coating. If you are drying with heat, the same rule applies. Not over 140 °F. If compressed air is being used to dry, make sure the air is clean, dry, and oil-free. Lastly, do not place your panels in the salt spray cabinet directly after processing. Allow at least 24-48 hours to allow the conversion coating to "set."

Best of Luck.

A. The single most frequent reason for failing salt spray is the quality of the panel itself. Use a 10X loop and inspect for pits on the raw panel. Even if no pits are there, in the case of 2024, age of the panel can affect salt resistance. Finally, order some more panels from a different source.

A. It is true that the age of 2024 T3-T6 has a detrimental effect for a good conversion coat. However, a few things should be discussed: Using a phosphate alkali bath or baths for initial cleaning? Are they at 135 to 150 °F? Is the pH high or low? Keep it high. We use Oakite 160 and 164 [Chemetall]--these are very strong aluminum cleaners, the pH high for the 160 etchant is 13.6 and the pH high for the 164 cleaner is 12.5. Submerge for 2 to 3 minutes each bath, rinse for 3 minutes between each bath using clean running water.

Next, submerge in a deoxidizer (de-smutter). We use Deoxidizer LNC (also from Chemetall Oakite) for 3 minutes and rinse again. The pH for LNC is .8 to 1.0 (strong acid, keep it low toward .8).

Next, into the Chromicoat for 15 to 45 seconds for class 3 and 45 seconds to 3 minutes for class 1A (times vary depending on the product in use). Try to set the pH for chem film to the high (chromicoat is used at 1.5 to 1.7 pH and I try to put it at 1.650 to 1.700). I can do this using a model 69 pH meter from Engineered Systems and Design (accurate to 0.005 pH). Many physicians use this model to test for blood gas in humans).

Continuing, Dry thoroughly and don't handle while drying. Roughing the panels up mechanically (sanding essentially) helps. Also, you know, I got as good results when I cleaned the panels in nothing more than lacquer thinner [on eBay or Amazon] ! True. MEK / methyl ethyl ketone would do the same. No other pre-cleaning treatment was done. We simply wiped them with lacquer thinner [on eBay or Amazon] and submerged in the chromicoat bath and the results were very good indeed.

Good luck!

A. I found that 1 year was a normal max. It depends how long it set in warehouses before I bought that sheet.

We did our own salt spray and when we started to see the number of pits start to grow, we got new panels.

I do not remember anything in the 6 month range, but the norm to get rid of it was 9 - 15 months. Nothing absolute.

We DA sanded with 240 grit, blew it off, rinsed and dried and then checked for pits of bad spots. We threw away about 1 in ten panels that never made it to the process tank.

A. We ran a lot of machined parts and they required a bit less etch time than sheet aluminum to get clean.
Initially, we were cheap and were trying to get some old panels that were in poor shape to look OK before processing.
For information purposes, we frequently would run a "virgin" panel in with the sanded ones. They frequently would have a couple more spots than the sanded ones, but would pass. We did panels on both 6061 and 2024.

A. What is the surface finish of the test panels? The surface should be mirror polished and panel's surface should be smooth and no pits/ scratches.

Which chemical is used for Chemical conversion coating? Bath parameters (pH, temperature, time and Concentration) plays critical role in salt spray life of test panel. Take trials for increasing bath concentration for 2 gm/lt and 4 gm/lt and maintaining other parameters same for 2024 and/ or 6061 panels. These will give you good results in SST life.

A. Hi Prakash. You can search the site for many other threads about salt spray testing of chromate conversion coated aluminum to get second opinions, but this doesn't sound like a problem to me: your test panels passed the test, and "nothing lasts forever but the earth and sky" --'Dust in the Wind' by Kansas.

In general, the purpose of salt spray testing is to give you early warning that your established process is getting out of control; unfortunately in the specific case of chromate conversion coating on aluminum, the test tells you nothing about your actual substrates and not very much about your process, but a great deal about the age & condition of the test panels. So, please 'take the "yes" for your answer' -- if you pass the test, STOP :-)

Regards,

A. Dave

We had success having a local sheet metal fab shop (they were a customer also) order the material and cut to size. They were able to provide the material certs as well. A 4' x 8' sheet provided a year's worth of coupons although we were running anodize panels as well. We had chem film panel failures early on until we figured out the cleaning steps. Soap and deox only; no etch.

A. Hi Mary. Failure of aluminum test coupons has been a perennial topic of discussion here, and one of the beefs is that the salt spray test demonstrates the quality of the test coupons more surely than the quality of your processes.
Alclad panels have a coating of pure aluminum on them which is more corrosion resistant than the aluminum alloy, and which anodizes/chromates better as well. Using Alclad test panels would definitely be cheating :-)

Regards,

A. Since the intent of the testing is to mimic the performance of your process on parts of the specified alloy- hence the requirement to process identically right down to running panels alongside parts on a 'live' load, which an auditor will check. And since Alclad will have different behavior during processing than bare, you might not be able to process them together at all, depending what the process is.
Ultimately, it tells you nothing about your process success on the given alloy. So it's even more of a waste of money than the testing is already; as Ted and others have noted for years, test coupon material is often inconsistent or poorly stored and handled and those failures are frustrating for all of us.
Luckily this site is here to help you troubleshoot and pass tests legitimately so if you have problems passing on 2xxx bare, come on back for some advice! :)