Alodine, Iridite, Chem-film not sticking / rubbing off

A. 360 and 380 both have about 9% silicon in the alloy which makes life quite difficult vs 6061. It will require an acidic deox/desmut that contains a goodly amount of fluoride. Tri acid-sulfuric/nitric/HF being a common one that is extremely strong. If that is the only alloy being run in that chem-film tank, then it could be tweaked with the help of the vendor for optimum performance on the cast alloy. Color changes, minor ones,are moderately common. It should not have major ones and certainly should not be chalky or rub off.

140 °F is the maximum that a chromate coating should be heated to , in most cases.Alternatives would be return to the wrought alloy, change the casting alloy to one that was more amenable and find better vendors for the casting and chem-film. You might have to bring the chem-film "in house" . This may cause more problems than it solves. There may be vapor deposition processes that would give satisfactory tradeoffs, but The price would very probably be higher.

It is worth looking into from some of the folks that advertise here.

A. Hi Keith,

You're pretty much right on the button with your thoughts and what others have been telling you. Die casted chromated parts are nowhere near as uniform as a machined finish because of the porous structure of the casting, you can limit this by polishing down the casting to a smooth surface, but you eliminate the cost savings of going with a casted instead of a machined surface by doing this.

The finish probably appeared rather patchy in areas, but as far as rubbing off, it shouldn't do that no matter what class of chromate you use Class 1/3 whatever. It won't last as long corrosion wise on the casting material as it will on a machined surface. I want to say off the top of my head that 336 is of course the machined surface salt spray requirement in hours and casting material (dependent on the type, I think 380 is the called out material in the ASTM spec) its around 58 hours or something incredibly low like that. Which in my opinion is proof enough to signify the difference! s, the governing bodies already

realized and noted that you will not have good salt spray resistance, and if not mistaken, your electrical requirements also drop down as far as the conductivity goes in regards to the casting material. Are there other options along these lines, maybe, but I don't know of one myself. Good Luck to you, if you have any further questions, feel free to let them fly, I don't have an reference material in front of me to consult and all of this is coming off the top of my head so it might not be 100% accurate, but it does give you a solid A to B starting point to acknowledge.

A. Rubbing lightly with a damp cloth - should not remove the coating.

Shipping in cardboard boxes. In humid conditions the sulfur in the brown cardboard will slowly degrade the chromate but that is unlikely your main problem.

If the applicator puts the coating on too fast, it will wipe off. Temperature, concentration, and pH affect the speed of the gel formation. 99/100 of the time, the applicator does not measure pH properly in that low range, his pH is too low, the coating forms too fast, the coating wipes off. If he would throw away the pH meter probe which is probably eaten up by the fluoride in the chromate, then dip out a 5 gallon bucket and raise the pH in increments of 0.3 pH with ammonium hydroxide until the coating forms over a period of 1.8 minutes, then it will not wipe off.

A. Years ago we ran into the exact same problem with one of the major vendors in the USA. Possibly the same one. We chased the problem for days and did testing in the lab and could not get a suitable coating in optimal conditions. When we bitched long enough, they replaced the can with another from the same lot. This failed also. They checked with the retained QC sample and agreed that there was a problem and promised to send a can from a different lot #. The same lot # showed a few days later. They only had 4 cans in stock and were all from the same lot#. The problem material just did not look the same, like it was not properly milled or properly mixed. Solution, we switched to Iridite and had no problems. Sometimes the problem is not in your shop.

? Tell us about the cleaning and especially the deoxidizer (concentration, chemicals, age, etc.)

A. Hi Pedro,

You need to give us more details about your process before we can advise you. Can you tell us what your operating parameters are for the precleaning, etching and Alodine processes?

My initial reaction is that the pre-cleaning and etching has not been effective and that is causing your adhesion problems.

A. Important are the conditions of the chromating bath.
Too low pH, too high fluorides, too high CrVI content and a too long treatment time can all cause powdery, badly adhering chromate coatings.

A. Have you tried running it without an etch? Some parts tend to not take chem film if it has been etched prior hand.

A. "tumbles" <= there is the magic word. I suspect the tumble media has been used on iron parts and is embedding fine iron particles into the soft "Kleenex-like" aluminum. Go back and tell them that the tumble media must be designated "aluminum only". While waiting for new media, try a heavier etch to remove and dislodge the embedded iron particles.

A. Micheline

Adding on to Robert's suggestions, see if you can get a part that has not been tumbled and see how it processes. Baring that, take a tumbled part and mechanically clean it with scotchbrite, then process. If you still have problems, alter your rack material. Some heavily alloyed materials react unfavorably on certain (rack) materials.

I would avoid etching, but rather go longer in your (de)oxidizer. Some soaps/cleaner will leave a residue after drying that cause problems.

A. Hi Mr Sharifi,
To your question, 'is this normal?'- yes and no. Yes, in that it is not uncommon to see, but no, in that it's not supposed to come off significantly. Alodine needs some time to cure after processing, a couple days is plenty, before you start handling it normally. If by 'the coating comes off the surface' you mean a white cotton glove shows some slight discoloration, I wouldn't worry too much. If the surface is becoming visibly less golden where you rubbed, the possibility and severity of an issue are far greater.

Having seen this issue in the course of doing a LOT of chromate on 7xxx, I can tell you some things that could cause the coating to rub off more easily:

-That alloy isn't ever going to chromate as reliably as 6061. The best possibility for a good coating is going to depend on the pre-cleaning they are doing. They need to be sure their soap doesn't have a bunch of greasy contamination. And regardless as to whether or not they etch (under normal circumstances, they wouldn't for regular chromate, unless you're requiring a mask and anodize step too), it is IMPERATIVE that the deox/desmut tank be working well. That means good sparging, fresh solution, adequate acid and redox potential, some process in place to ensure that silicon is never allowed to build up past 30ppm in the tank. Deox results should be confirmed with a white cotton glove or lab wipe before going to chromate- parts coming out of that tank should never be smudgy, and the glove must be squeaky clean. If not, put the parts back in. Failure to fully desmut WILL result in a poor, powdery finish that doesn't protect as it should.

Obviously running the alodine tank to manufacturer's specs is important- but 90%+ of times I've seen troubles with chromate, it's the deox step to blame.

That being said... We've done a decent amount of destructive testing at our shop and troubleshooting trying to pin down the point of failure vs aesthetics, and parts that APPEARED to have coating rub off and become lighter actually passed 168 hour salt spray in one case!

I guess I would approach this like, 1) is it a functional or aesthetic problem; do these parts pass destructive testing? If aesthetics matter as well, is that clearly stated on your PO? I say this because most shops process to spec, which is a functional spec, and allows for some pretty serious variation in color. You MAY have a functional coating even after rubbing the surface residue off- so you need to ask if that is enough, and if you can test that and prove that it is. 2) is there a way you can confirm tank maintenance and processing best practices with your vendor?

All that being said, you definitely need to address this issue with your vendor and make sure they are giving you the coating you are paying for, whether you care about appearance, performance, or both- Good luck!

A. It should not "rub" off. Non-adherence can be dues to: (1) pH too low, (2) concentration too high, (3) temperature to low OR too high, (4) surface preparation inadequate,(5) floating soil and other contamination, (6) not "cured" for 24 hours before wrapping in a dry low humidity environment, and other reasons.

A. Hi Bobby. We appended your inquiry to a thread which already covers the issue quite well and offers some ideas that I found insightful. The only additional possible cause of poor adhesion that I can add is drying at too high a temperature. This usually causes the chromate coating to get flake-y, so you would probably notice gold colored dustiness in the shipping container.

Luck & Regards,

A. Has the tumble media been used on anything else so that you are imbedding iron particles into the soft Kleenex-like aluminum? Ahead of chromate the tumble media must be designated "for aluminum only'.

A. Hi Bobby!
First, questions!

How long after conversion coating are you or your customer going after it with a Q-tip? Hours, days, or weeks? (The only correct answer is a minimum of 2-3 days)

What color is your scotchbrite? Better be red! The grey stuff is no bueno prior to CC. AlOx only!

Is your plater running a whole mess of cast alloys in the same deox/desmut bath, and what controls do they use to determine a dump schedule? (The correct answer is either no shared bath with cast alloys, or a tightly controlled dump schedule based on ICP analysis of the bath for total Si, dumping at 20ppm buildup, preferably BOTH)

How is the plater determining complete cleaning coming out of deox? Is a smear test being done? Seriously, just a white lab wipe to check for residue.

Is the plater maintaining the acidity of the conversion bath correctly in relation to its concentration? Don't fixate on this question, but be aware of it. Also, an aging hexavalent conversion bath may become depleted of the activators necessary for the reaction to proceed.

Weird one... if the tank is steam heated, how well do the coils pass a pressure test? Even a pinhole can introduce iron and kill a tank. Rare, but it happens. The plater's monthly performance specimen results (assuming they are accredited) will catch that quick- ask for them.

Last, and actually do this first... send out some customer-rejected parts for salt fog testing. I use a lab in Justin Texas who I ADORE for one-off jobs on top of my giant monthly suite. The spec says nothing i can recall about yellow residue on Q-tips. If you don't get pits, you've fulfilled the purchase order and the customer needs to understand that. Aesthetics vs functionality. If they want it to be pretty, too... that's a re-quote ;)

If you want to know how I learned all this mess... you bring the beer (IPA, please); I'll bring the NDA and a pen. I swear I'm still suffering ptsd from the first of many go-rounds on this exact topic with this one particular customer and their ever rotating stable of OV wrangler 'engineers'... it never ends. Passing lab results are your ticket to rejecting an incoming SCAR upon receipt.

When in doubt, send it out.

Best of luck!