Boric-Sulfuric acid anodizing BAC5632 Issues and Answers

Hello, I.S. There are several other letters on-line here about boric-sulfuric anodize, and several of them mention that the process is Boeing BAC5632. Are you following BAC5632?

A. I.S.

It is a long shot but I would check for chloride contamination, particularly if the pitting coincides with a bath make-up or new solution (container) addition.

A. We really need more information on your process parameters in order to come up with an insightful response but I have found that when I have had pitting problems they usually come from either too weak of a deox/desmut solution or too much time in the deox/desmut solution.

A. I think your problem is not related to anodizing. I had had same problem 4-5 month ago. I noticed source of problem was extrusion and storage conditions. Long time storage profiles's surface getting corroded. After caustic bath, these pits reveal and after anodizing you can see it. Solution: profiles not to wait too long time; avoid humidity storage; and also I wonder do you see these pits for bright profiles after anodizing? I don't see so far... because these profiles not treated with caustic.

Thank you for all your help. Yes, I am using a Boeing spec for the titration procedure. The Procedure No. I used is B-281A of which uses 1.0 N NaOH as the titrant. I have started over and redone the procedure and finally got my end results in reason from what they were. I appreciate all the advice on this and will utilize it to my advantage. Thank you!

A. Such big electrical contact, maybe acid residual when drying leads to these pits.

A. Hi. The pictures might be deceiving me but those pits look so deep & severe that in my limited experience it doesn't seem likely they would happen in a short time solely from chemical reaction. I get the feeling something electrical is happening like a momentary short circuit, or dirty contacts not carrying any current during ramp-up then suddenly breaking through at higher voltage.

Regards,

A. The cause could be a poor electrical contact, probably the surface is uneven so only a small area is doing the electrical contact and that produces the burns.

A. Brandon

What does the surface of the contact plate look like? Does it have the same pitting?

Have you tried unracking the parts prior to seal to see if the problem originates in the anodize bath? If you have residual acid trapped between the part and contact point, elevated temperatures might cause the pitting you are experiencing.

A. I don't suppose this appearance is "pitting" ... pitting is completely different. We can say "stain" as described. My advice: you can change rack design, or degreasing and neutralize duration can be much more, like 5 min. Because maybe profile's surface was dirty from normal level so in that joint area had effected as we see picture.

---

August 2018

  Hi Alaattin. Agreed that this is not what we usually call a "pitting defect", but it sure looks to me like quite deep etched craters, not a surface stain. Pictures can be deceiving: one of us sees stains, but one of us sees deep corroded indentations :-)

Regards,

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

A. Hi Ted, yes, I agree with you about "point of view" issue for some problems like this one ... but I think solution of this problem should be "one". In this situation I follow this way:
1- How often does this problem occur? for example, every rack or some racks ... also does the same problem occur on different profiles?
2- one of the key words in this situation is "survey". Is this problem emerging before/after anodizing or etching ... also sealing.
So we can collect and filter all the results and diagnose; after that, of course, treatment.

A. We have had the same problem as Mr. Boyd, however, not just on the contact areas. The deep pits in Mr. Boyd's photos are all too familiar to me while using boric/sulfuric anodize. I have never been able to nail down a cause. My tank will produce perfect parts for three months straight - then, without warning, an entire lot of parts will exhibit those monster-pits. The pits will appear at contact points AND at random areas all over the anodized parts. The pits are quite deep and generally will cause the entire lot of parts to be scrapped. Our parts are racked on clean and deoxidized racks and the contact points are as small as possible to reduce the likelihood of trapped solution. Pre-clean includes a deionized water rinse after deox.

A. Assuming you mean Boric acid-Sulfuric acid anodizing, as that's what the referenced spec covers, there is no special liner: we have ours in a poly tank.
There are specific cathodes that must be used, which is specified in the spec & subsequent PSD's.
You will need a heater and a chiller, as it has a tight temperature requirement.

A. It is standard white polypropylene.

A. The spec states 237 ppm max, so I wouldn't recommend it.

A. If you run the 2000's, there is nothing you can do to prevent copper, it plates out when the current is on, it partially re-dissolves when the current is off, so take the sample shortly after starting the current on; it does not hurt anything.

A. Copper in anodizing baths tends to lower salt spray resistance.

A. It is possible to remove copper from this anodize bath by electrolysis with a high surface area cathode. This is preferable to decanting or dumping a portion of the bath in order to control copper.

  Hi Micheline. While we're waiting and hoping for a response, I see that you have reframed it in more limited fashion since last time. Please share what if anything you have learned about analyzing for copper in this bath. Thanks!

Regards,

 Ted,

I have not found a great solution for copper. I do like to do everything in house if it is cost effective. I found a lab to do it for now but they say that they cannot do NaCl.

Micheline

A. Hi, Micheline

NaCl in aqueous solution would be present as Na+ and Cl-, so it might be better to ask to analyse for those ions (as NaCl itself would technically not be present).

Something like atomic absorption spectroscopy ⇦this on eBay or Amazon [affil links] could probably do it. Salts can be pretty harsh on some forms of mass spectrometers and HPLC, so that may be why the lab can't do it.

  Thank you so much for your response. I will work hard to get a solution. It may involve buying our own equipment

A. Hi. Fauna previously answered you that a standard white polypropylene tank would be okay, and I think he's right. He also said that Boeing Spec BAC5632 requires very tight temperature control with both heating and cooling. If you are not working to BAC5632 I'm not really sure that MIL-A-8625-1C is truly very meaningful because it says you can only use it if the procuring activity agrees, and it doesn't specify boric-sulfuric anyway, just mineral acid or mixed mineral-organic acid; I think Airbus might use neither chromic, nor boric-sulfuric, but phosphoric acid for similar applications.

There may be shops who do MIL-A-8625-1C boric-sulfuric without adhering to BAC5632 but I don't know, and I don't know of a good way of figuring out ideal heating, cooling, and cathode material, concentrations, etc. without either simply following BAC5632 or being lucky enough that such a shop responds :-)

Sorry, & Regards,

A. I believe that purchasing the most current copy of BAC5632 would behoove you. It is the governing spec for Boric Sulfuric Anodizing.