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topic 30544

Chem film aluminum panels fail salt spray test

A discussion started in 2004 but continuing through 2017


Q. Test panels to qualify chem film process to Mil-C-5541 [link by ed. to spec at TechStreet]E, Class 3 have failed corrosion resistance testing several times (aluminum sheet, standard size per spec). The panels were salt spray tested in accordance with ASTM ASTM B117 [link by ed. to spec at TechStreet] (except that the panels were angled at 6 degrees from the vertical, per customer's requirements) for 168 hours.

These panels were mechanically cleaned to remove any pits and corrosion prior to processing and packaged in plastic bags to prevent any paper residue attack of the coating when submitting the specimens to laboratory, which is a thousand miles away, via UPS/DHL. Need advise as of possible trouble shooting options.

All process parameters seem to be under control (i.e. solution concentration, temperatures, etc.) Water used is de-ionized and purchased in big containers (not processed in-house). Any orientation is appreciated, some of you might have any advise and/or recommendation in our process.

Aldo Rodriguez
Manufacturing of aluminum components - Monterrey, Nuevo Leon, Mexico


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.

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


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 [linked by editor to product info at Amazon] ! True. MEK / methyl ethyl ketone would do the same. No other pre-cleaning treatment was done. We simply wiped them with lacquer thinner and submerged in the chromicoat bath and the results were very good indeed.

Good luck!

Bill Workman
- Phoenix, Arizona, USA


A. The problem could very well be the method of packaging the panels. You mentioned that the panels are traveling about 1000 miles to be tested, and they are shipped via UPS. With that distance, I envision at least two, and perhaps three or four days of transit if not shipped overnight. During their trip, the panels are subjected to various temperatures and humidity levels. They could be in the back of delivery truck at 100 degrees for several hours, then in an air-conditioned freight terminal overnight, then back onto the hot truck again tomorrow. This scenario could be repeated two or three times.

To make a long story short, moisture could be collecting in the plastic bags and attacking the panels before the test ever begins.

Reggie Catlett
small parts mfg. - Lynchburg, Virginia, USA

July 9, 2009

Q. You mention age of 2024 T3 test coupon has a detrimental effect for a good conversion coat.
Do you have a recommendation on how old is acceptable. ( 6 months, 12 months, 18 months, 24 months ? )

Doug Weisent
I am a quality engineer supporting a plating line - Orlando, Florida

July 13, 2009

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.

James Watts
- Navarre, Florida

July 16, 2009

Q. OK. I'll go along with the practice of using coupons that are not old for periodic salt spray testing for chem film on aluminum. But why sand or scotchbrite your coupons if you are not going to do the same to production parts. If you ran witness coupons with your 2024-T3 parts, will the coupons pass without the mechanical prep?

Willie Alexander
- Colorado Springs, Colorado

July 23, 2009

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 was 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.

James Watts
- Navarre, Florida

August 4, 2009

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.

T.Chandra Sekhar Reddy
plating shop employee - Mumbai, Maharashtra, India

Quality Salt Spray Panel Vendors?

September 12, 2017

Q. Hey Everyone, I've been with a plating company for 15+ yrs. I've seen salt spray passes and failures. While I feel this test is bogus, it's a requirement for the MTL-DTL-5541. I seem to think I've recently been getting pitted panels. So I'm starting with problems from the get-go. Problem is that I have to buy 100 panels to meet the company's smallest quantity. Can anyone shed some light on where you get your panels from? Good experiences with. I'd like to try different vendors. Thanks.

Dave Jr. [last name deleted for privacy by Editor]
plating shop employee - Wilmington, Delaware

Ed. note: Gentle suggestions only, please, readers. No slams.

September 13, 2017

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.

Willie Alexander
- Green Mountain Falls, Colorado

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