Masking/stripping when chromate conversion and anodizing on same part

A. Dear Ola

I have a better idea, if it is possible on your parts.

Anodize the whole part and then machine the areas where you want electric conductivity. After this you chromate the part.

Best regards

A. Chromate is still a gel at 10 minutes and masking is going to remove parts of that coating. Look at the tape when you take it off. If it is properly masked, the chromate can be removed in a caustic cleaner tank or some acid tanks without much problem. If they are having leakage problems with lead tape, they are not burnishing the tape edges with a wood or plastic stick. Leakage should be minor with lead tape. I would expect more problem with vinyl tape. There will always be a trace of leakage, but that tiny amount should not be a factor on an aerospace part.

I am not in love with chromating over anodizing as even a non etch cleaner will slightly attack the anodize. The acidity of the chromate tank is not going to help it either. If it passes salt spray on an every lot basis, it might be OK. The OEM procedure must be followed. What do they say?

A. I think your supplier is correct. His suggested process is easier, and will likely produce better quality. There are plastic film tapes which will hold up well in anodizing. Chromate conversion coating can be done after anodizing with no effect on a properly sealed anodize coating.

A. Ola

1 If the part is properly chromated, most plating tapes will adhere properly if not excessively aged. Lead tape (3M #420 or #421) adheres best with its' natural rubber adhesive but does NOT produce sharp edges as it deforms when firmly pressed down. Polyester tapes will produce cleaner edges. Once again, the surface must be clean- perform an acetone [on eBay or Amazon] wipe before taping to be sure.

2 & 3 If the chromate is not subjected to excessive heat during the drying process, it usually can go at least over night without interfering with the strip process. Process some coupons and age them, 1, 2, 3, days and then strip to test. Most chem films will 'cure' in 24 hours.

We have used NaOH (a readily available commercial blend) to strip chromate as well as anodize. The bath must be in good shape- not a lot of dissolved Al. We run ~10 oz/gal @ 125 F (removes Al at the rate of .0001" per surface per minute).

4 We would expect the 'reverse' process to produce more failures. The chromate is what allows the 'mask' to adhere. An alternative would be to anodize all over, machine off areas to be chromated, and then chromate. We do this on Type III Class II SEALED(!) anodize with no effect on the anodize (aerospace application). Not sure the effect on unsealed coatings.

Hope this helps.

A. Good question Dave. I'm not really sure on that one. I may have to try it myself. One concern I would have would be dissolving a lot of hex-chrome in my anodize tank.

I am a little curious as to why you wouldn't want to remove the chromate prior to anodizing ... is it a finish, or a tolerance issue? Like your shop, our S.O.P. is to remove the chromate prior to anodizing. We really haven't noticed any finish, or dimensional issues arise from this practice.

A. Hi Michael. We appended your inquiry to a thread which shows the overall sequence to be common -- if it's done right :-)
I'm not clear on why your anodizing is soft and powder like, but it's not an inherent problem in the sequence.

Regards,

A. Hard anodizing will not strengthen threads. It will increase the wear resistance. That said, hard anodizing would not rank high on my list for threads since there is a rotational crushing effect that I think will cause the hard coat to not last well. If this is internal threads, why not heli-coils? If it is external, why not electroless nickel? But, in answer to your question, you are going to have to mask at least once. Typical is to chromate, mask and then anodize. The exact procedure will depend on the shop and the part. There is no 100% answer.

First of two simultaneous responses --

A. NO, the anodic sulfuric acid will deteriorate the chromate. You must mask the chromate, or mask for anodizing and chromate afterwards. Tobler lists a long line of maskants.

Second of two simultaneous responses --

A. No it won't. Typically one would chromate the entire part, then mask the areas you wish to remain chromated, then anodize.

You could also mask the areas to be chromated, anodize, and then go back and chromate the areas that require it.

For costing purposes, I would base my decision of how to process based on the complexity of the masking of the areas you need chromated/anodized.

Q. Follow up on selective anodizing -- will powder coat serve as a resist. Part is complex and difficult to maintain a uniform coating.

A. Although there are probably some powder coatings that would not hold up, I'm confident that there are some that will. I've seen powder coating successfully used as a resist for decorative chrome plating in a process cycle that included electrocleaning, cyanide copper plating, and hexavalent chrome plating, so I certainly think it can withstand the anodizing process. But how will you get it off -- or are you implying that the powder coating would stay on as a permanent finish?

A. Unless the areas to be conversion coated are of an extremely complex geometry, and difficult to mask... have you considered having the part conversion coated first, and then have the appropriate areas masked from the anodizing process?

The "softening" of sealed hardcoat (compared to unsealed) is real, but it is rather minimal. Is the reason for the hardcoat simply for abrasion resistance during post machining operations..or for use in the field?

Too add to that, seeing how it's a dyed coating, I would think you'd want to seal it in the first place, to prevent the color from fading over a period of time.

The conversion process normally requires some type of etch, which very well could effect an unsealed coating, especially a dyed one. If I were you, I'd process as follows:

Machine part to final dimensions
Conversion coat
Mask areas important for conductivity
Anodize (seal if necessary)

The masking step will indeed add cost to the process (especially if the masked areas are complex), but it's really the proper way to do it.

Or, I'd just seal the coating (if the masking step adds too much cost) and conversion coat afterwards. To me, when I think "enclosure", it sounds like just a cover for something that won't see much friction in the field, after it's installed. But I definitely could be wrong.

[rant]This is why all design engineers should really take some type of "coating feasibility/cost analysis" course in school. I've seen way too many products come through my door where engineers haven't thought "Finish First". [/rant]

A. Since you state that your hardcoat is dyed black, it would seem to me that your finisher is already sealing your part in order to keep the dye inside of the pores, rubbing the part with an acetone [on eBay or Amazon] soaked wipe would prove this very easily. A proper seal should allow the hardcoat to survive through a very mild chromate cleaning cycle, and subsequent chromate with no problem. If sealing the part raises concerns over the hardness of the part, have your anodizer run a 4x4 panel through the process and have it Taber tested to ensure it will still meet your requirements.

A. No worries on my "rant", Nathan...wasn't specifically referring to you. As a matter of fact, over the last several years I've had the absolute pleasure of working with a mechanical engineer who's one of the brightest guys I've met..and has helped me solve many a problem created by engineers that...shall we say...didn't think finish first.

Anyhow, about all I can say is this. I hardcoat, 10's of 1000's of parts a year, a lot of which either require masking, or a machining step afterwards, or both. All of them are sealed, as they are used in a plasma/corrosive environment. From your last post, it really sounds to me as if the sealing step would be more of a benefit, as opposed to a detriment.

If indeed your masking requirements are that complex, I would seal the coating, post machine, and then conversion coat. If your coater can't seal his hardcoat to withstand a simple conversion coat process, then find another coater.

The big key here is, to make sure your post coating machining process does not subject your parts to dings and the like. Your machinists have to be careful, especially if there are a lot of sharp edges/corners on the part. If there are, and that's what you're concerned about getting damaged, then it wont matter if the coating is sealed or not. Anodizing, while extremely abrasion resistant.. is a very brittle coating. It takes very little effort to knock the coating off of a sharp 90 degree edge.

I'll also add, that my company makes quite a few parts that are Type 2 anodized, that are also sealed, and require a secondary machining operation. No problems at all, and I say again..as long as the machinists are CAREFUL while loading, and unloading the parts from the mills/lathes.

Out of curiosity..could you attach a picture of the part in question? Might help me a bit.

There are many different sealing processes available to anodizers. And it is possible that your current coaters sealing process for a type 3 Class 2 coating may not be adequate to prevent damage to the coating during a conversion coating process afterwards. However, it definitely can be done.

Properly sealing a type 3 coating to withstand a conversion coating process may require more time in the sealing bath, and/or variations in the anodizers operating parameters. Again, I say...it can be done. And yes, there is a difference between properly sealing a type 2, and a type 3 coating.

To your other question "has the hardcoat been harmed". Probably not, just a simple loss of color, which is to be expected on a type 3, class 2 coating which hasn't been sealed. The actual coating itself should be just fine as long as aesthetics aren't a huge concern.