Salt Spray Results vs. Panel Preparation (Alodine, Iridite, Chem-film, Mil-C-5541)

A. At a previous shop that I worked at we polished the panels first which corrected all of the scratches and irregularities of the panel or differences between panel suppliers. It made a huge difference in salt spray performance. You may need to hand clean the panels to remove any buffing compound prior to processing. I am not positive this is currently allowed, but at the time there was no prohibition against prepping the panels that way.

A. Hi Zander,

There are a few things that stand out to me from your question. You're going through the traditional trial-by-chromate conversion coating [CC so I can stop typing it out] that all us lab dweebs have lost sleep over at one time or another (*cough cough last week).

2024 is objectively harder to conversion coat than 6061 due to the alloying constituents. I think it's the copper that twists the knife in this case, but I'm not a metallurgist.

I'm gonna start by saying, don't ever beat yourself up over passing double-hours salt spray anodize on the same substrate you failed CC. It's not apples to oranges. It's apples to carrots.

Anodize gets a WAY more aggressive etch prior to processing than a cc does. CC should only be routed for a flash etch to the point where the tech observes an even fizz across the whole surface, bang, done, get it out of there. Too long brings way more of the non-Al constituents into play.

Anodize tends to be routed for an etch that actually has a dimensional goal, and is therefore significantly longer and takes off more material. This negates the Mill Roll problem, which we will get to.

After we talk about seal.

I'm again not an expert, but Taber Abrasion, Coating Mass, and Coating Density give data on the quality of your time in the ano tanks... salt spray tells you how well your seal likes its substrate.

You can work MAGIC with a good seal over a good crystalline structure, even over an ATROCIOUS substrate.

Conversion coating gives you neither of those things to work with.

Test coupon substrate is literally the bane of my miserable lab rat existence, and I'm not even being facetious here.

Fact: your customers who need +6 non rohs cc are sending you really sweet precisely finished parts, in a hurry, so fresh off the 5-axis that they still smell like overheated coolant from that new 'engineer' who is on a mission to drive down run times. *SKREEEEEEEEEEEE you know that sound.

Unless there's smearing from a bad mill op, or the coolant is literal ectoplasm, those parts are theoretically ideal.

Fact: your test coupon supplier is not milling a nice surface on your panels. They are either mill roll, or sanded to some degree that you need either a microscope or a salt fog test to confirm adequate. In the second case, you're in trouble by the time you know why. Therefore, they are already not meeting the intent of the [absolutely poorly designed] spec.

In fact, your coupons being rolled sheet stock while your customer parts are probably milled from a fat extrusion or plate [assuming 6xxx or 2xxx] just means you're testing ... the quality of your coupons. Not the customer parts, which come to the CC shop clean except for paw prints and coolant stains. No big whoop. Not much etch required, rock n roll.

You can see how this is problematic.

Can we talk about 'pass' and 'fail'. Your lab gives pit count per panel. In a set of 5, do you have results varying from 0 to >5 within one set, or is it all sorta borderline, or is it a full set of catastrophic failures? This is a HUGE question.

You say "clear", I assume you are running B449? Are you passing coating mass? (Unrelated to salt spray, but Nadcap auditors got sent an alert about this just recently, and it's a hot mess in which actual customers also seem to be like, "wait whut, why do I need that? It passed salt spray dude?", heads up).

You hold Nadcap, and I assume you also trace substrate mill certs; look at your source mills and H/L codes more than your panel suppliers. I hope you have actual mill certs, not just supplier hand me down certs.

In the past 4 years, the aluminum supply has gone off the rails. A previously trusted test coupon (certified as test substrate, adhering to specific ASTM specs) supplier sent me a grand worth of *ahem* imported from outside NATO substrates, since my dumb ass assumed it was sort of you know... a given, for a company selling to various Nadcap shops. My bad for a P.O. missing the statement "please don't send me metal from a country we are in an active trade war with, and also no Uyghur labor, plsthx xoxo #ano4lyf!!" .

I kept them and chopped them up for dye samples for Sales. I did report the issue as a heads up. No refund requested, professional courtesy. We used the bits.

The same supplier then sent a subsequent lot of material with inclusions. Nicely sanded on one side, really did a nice job hiding them... anyone who has worked with Oak wood as a fine finish carpenter will get this best. It's the closest thing to describe the defect.

More sales color chips. Last order from that vendor. I bought a mini air RO sander for $30 and resigned myself to a permanently ratty manicure from now on.

I don't keep my Museum of Crappy Aluminum in my phone, otherwise I'd post examples.

Next up. Did you dump and replace the whole bath, or did you leave a heel? If it was a full dump, what did you do to precondition it? CC, like ano, doesn't work right 'out of the box'. It needs to build up an agreeable level of aluminum to activate fully. Why they don't advertise a new tank supplement is beyond me, lol couple tablespoons of salt and some AlOx powder...

Quick before your tank gets too old, grab a sample and run it through your ICP-OES (or AA) and check aluminum chromium barium iron copper nickel zinc. Hold that data for "fresh tank" reference, as well as titration results from the day you took that sample.

I recall chatting with you previously in 2021, and there's a bunch more info there, specifically about deox... go back and check up... you have a LOT of moving parts in your situation. We will get you straightened out.

Cheers

A. Hi Yoganandan. This is a very complex question. NASA seemed to feel ("Improved Accelerated Corrosion Testing of Zinc-Rich Primers", NASA Tech Briefs, March 2000) that EIS plus natural exposure proved to be a good way to test zinc-rich primers on stainless steel. My limited understanding is that NASA discovered that after a small amount of natural exposure there was a degradation of the coating that could be discovered via EIS long before it became apparent via visual or microscopic examination, and that the degree of this degradation seemed to be a good indicator of how long the coating would last. But what is your basis for believing it is an effective way to test non-chromate conversion coatings on aluminum? (I'm not saying it isn't, just that it sounds like a huge extrapolation from very limited data).

As Cynthia has told us, accelerated corrosion tests are only quality control tests. Whether salt spray or EIS, I believe an accelerated test correlates to real exposure life only to the degree that has been proven, if that much.

Regards,

A. In the greater Seattle area, following Boeing & NADCAP guidelines, the Alodine 1200S panels are subjected to salt spray analysis for 168 hours. The Boric/Sulfuric Anodize are 336 hours.

A. Hi Adriano. Buy new test panels from at least two different vendors before wasting any more time on process work. This problem has been addressed several times in this forum, but what the salt spray test mostly tests is just the quality and age of the test panels :-)

Regards,

A. Adriano, I would leave out the nitric acid brightener dip as nitric acid can passivate aluminum to such an extent that the conversion coating will not form properly. Instead use a proprietary de-oxidizer with sulfuric acid. Our experience is that sulphates in the rinse water prior to conversion coating will greatly improve the color and coating weight of your coating and the chance of meeting the salt spray requirement.

Regards,