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How to know "Iridite" is too old and will fail salt spray test?

Q. To whom it concerns: I have a question about Chromate Conversion Coatings on Aluminum.

I work as a C.I. Engineer for a Precision Sheet Metal Fabrication Plant that specializes in Enclosures for a variety of industries from the Medical & Electronic industries to Government & Military use. Many of our products are paint/powder coated and silk screened before we ship to the customer. Because of this, we rely heavily on our two in-house cleaning lines. Our Aluminum cleaning line is equipped with the option to Iridite parts for corrosion protection at the customer's request. We perform monthly Salt Spray testing on 2 separate aluminum alloys (2024 & 6061T3) to keep our MIL Spec. requirements current. We also perform diligent testing on each of our process tanks, and keep detailed records of the information we gather from these tanks, including the frequency in which we "charge" our tanks. This is where my question is derived from.

As I mentioned previously, the recorded data we take from our line is often analyzed to make sure we are gaining the full life and expenditure of our chemical WITHOUT producing bad parts. As it is our experience, the only "trigger" we have to notify us that our Iridite Bath has expired and the chemical is no longer performing to par, is in the Failed results from our monthly MIL Spec Sample plate testing. When we get these poor results, that is our only true testament that it's time to re-charge with a fresh tank. I hate the idea that we have to wait on a failure to determine that our process is bad-- there has to be a way we can see this coming BEFORE we fail a test. So, my question is, would a test such as a "Coating Weight Analysis" tell us if we are getting the type of coverage we need? I can't help but think if we would perform this analysis, say, on a freshly charged tank, you would see a higher coating weight than you would if you performed that same test 10 months later? It would be nice to have that hard evidence to prove that we are indeed losing coverage of Iridite as time progresses and when we see results in the (X?) range it's time to change things up. Am I on the right track here, or is there a simpler way to get the results I am seeking? Is anyone performing these tests on their products?

A. The nice thing about chromate coating (of which Iridite is one trade name) is that visual observation of color and freedom from rub-off tells you much. If the color is normal, orange/gold, and if it doesn't rub off with a piece of tissue when dry, your chromate bath is likely okay. Chromate baths last a long time if properly maintained. A more common salt spray failure is due to faulty etching or desmutting. Build up of metal (especially copper) in either can cause a salt spray problem. Over-etching is a very common problem in salt spray failure. A little etching is good, a lot is a recipe for salt spray failure. Jeffrey Holmes, CEF Spartanburg, South Carolina A. Be sure to check the chromate solution for high sulphate. Most salt spray failures are due to bad panels. Examine your panels with a 10X loupe before processing. Even then you may not see a problem. No matter how good your chromate solution and surface prep, there are some 2000 alloy panels that will never pass. This is due to the age or heat treatment which has caused the copper to come out of alloy and migrate into the grain boundaries where it corrodes away and leaves a pit. One of my clients failed with three panel sources and passed with a fourth panel source. Robert H Probert Robert H Probert Technical Services Garner, North Carolina A. There are many things besides bath chemistry that can influence a salt spray failure, including the quality and age of the aluminum test panels. Operator, pH, temperature tank time, and pretreatment. I do not think that coating weight would be valid as it would be seriously affected by the above items and could still produce a pass or a fail. With everything in normal limits, color of the solution normally gave a warning that the tank was nearing its limits. We normally started running unofficial 2-panel salt spray checks and when they approached half of a failure, we changed chemicals out. We had our own salt spray cabinet and it normally ran all of the time for one test or another, so the extra work was worth not having a failure. James Watts - Navarre, Florida

Hi. I love James' suggestion of extra 'unofficial' salt spray tests and feel that it's the exact right answer! After all, the actual purpose of salt spray testing is to be 'predictive' -- to warn that a process is going south before it becomes evident in other ways.

But once those salt spray results are adopted as a 'legal' requirement, and mean that the processed parts themselves aren't satisfactory (which is the usual routine these days), the test has been completely robbed of all of its predictive value and you've lost the 'predictive' test which is just exactly what you have found yourself now hunting for :-)

Luck & Regards,

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey