Letter 42017

Ever notice a buffering effect in a sulfuric anodizing tank?  

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Hello all,

I'm curious if anyone else has ever noticed a buffering effect in a sulfuric acid anodizing tank. We run our 700 gallon tank at or around 27.5 oz/gallon (205 g/l) and use a anopur filter unit to keep the aluminum level at around 1 oz/gallon (8 g/l). Both numbers will fluctuate a bit, but the strength tends to stay between 25 and 29, though the CpK isn't what I would wish of it. The dissolved aluminum will vary between .5 and 1.4 (4-12 g/l) but tends to be pretty consistent at the 1 - 1.2 range. All additions or changes to the tank are made manually.

I check the concentration once a week with a pH meter/titrator from Thermo-Orion, pulling a minimum of two samples and going back for more if there is more than a few percent discrepancy between the two. I just dip the samples out in a lab cup and then pull a 1 mL sample for the titration, but I air agitate the tank vigorously before and while I pull the sample from the tank so the solution should be pretty standard throughout. At times, while trying to improve the CpK of the concentration, I've checked it as frequently as daily - however we run so much product (4000 square feet of surface are is a fairly normal average, sometimes quite a bit more) that I can't seem to get rid of the last bit of variation.

The thing that throws me is this - on an infrequent and very random basis, I'll see sudden and drastic swings in the concentration, almost like what you see during a titration when a buffer runs out. For example, my tank just had a 10 oz/gallon swing (30 oz/gallon to 20 oz/gallon) in a week with nothing abnormal being done, the same operators as always, and a normal amount of work being processed. For the couple of weeks leading up to this, the tank has been abnormally high even though we were adding less acid than normal. During periods when I checked more often (and I'm starting, as of today, to check it daily) I've seen similar swings happen in as little as 2-3 days.

I would half suspect operator error, but all of our addition records (and chemical usage records based off of orders placed from our chem supplier) seem ok. Next I would suspect equipment issues - like an error in the Anopur unit - but every time I check it it tests out as working perfectly. I've come to finally chalk it up to something weird happening in the bath chemically - like the dissolved aluminum or other metals (we run all alloys, but especially 2000, 5000 and 6000 series; not many cast parts) in the tank operating as a buffer that will suddenly tie up or release free acid.

So, has anyone ever heard of something like this happening? If so, any idea on how to prevent it?

Thanks as always,

Jim Gorsich Accurate Anodizing Inc. Compton, CA, USA

Jim, based on your frequent replies, I am assuming that you run a professional shop, so will leave out the normal replies.
I always used a colwissa ( a piece of 1/4" cpvc) and sampled a rather large number of places in the tank into a large beaker, mixed this and took the sample from that.
It takes a large tank a long time to reach a true equilibrium, so are samples taken at least an hout after any chemical or water additions?
Orion, makes a fine instrument, but could it be overshooting or undershooting the endpoint at least differently from day to day. The titrant reaction with the aluminum content variability could affect the outcome.

Drag in could be a factor unless all of the parts are the same.
Good luck in finding your gremlin.

Jim Watts
- FL

Hi Jim -

Have you considered stratification of the bath solution? If the solution has gotten well aged and viscous or if your mixing is not too good you might see this.

Terry Tomt
- Auburn, WA

Jim

We experience some similar situations from time to time.

In a past life in California, our anodize solution titrations were fairly predictable. Type II anodize solution would lose volume through dragout and the Type III would accumulate volume through condensation. Acid concentration would drift lower and aluminum content would steadily climb to 15+ g/l. The trend over many years was consistent.

Now in Colorado, we see the aluminum content peak at 10 - 12 g/l with occasional dips or spikes. The parameters are near the same- similar concentration, same alloy cathodes, same temperature, same maintenance- add acid (tech grade) as required, processing primarily 6000 series. Sometimes I think the aluminum content may be reaching an equilibrium between that dissolved, and dragout, but something is telling me this is not right. I expect our results to be similar to CA results. We are running production parts in one tank, but all three tanks demonstrate the same tendencies.

We titrate a 5 ml sample with 1.00 N NaOH for total, and again with 10 ml KF (10%) for free.

I would be interested as well in hearing about similar situations.

Willie Alexander
- Colorado Springs, CO

I much appreciate all the quick responses - my thanks to all three of you.

I've toyed with the idea of sampling from various areas of the tank before, but always just assumed that vigorous air agitation was sufficient to make it unnecessary - based on the fact that 2 of you both suggested similar things though, possibly I should rethink my assumption.

The time it takes to reach equilibrium I've noticed before - in retrospect I'd say that there is a fairly high probability that that is factoring in... Thank you for pointing it out, it is definitely possible that these extreme swings are just the product of artificially high and low readings caused by the tank not settling down yet after an addition.

It definitely sounds like I should look for the cause in areas like the tank not yet being at equilibrium - it appears my long-shot guess of some kind of buffering action is pretty unlikely, not too surprising seeing how I've never heard of it happening elsewhere!

For Willie - I've heard of an equilibrium being reached like what you suspect, especially when the parts doing the dragout are racked in such a manner as to encourage or discourage dragout (based on the situation). It may be that the Colorado shop is racking the parts differently, or that you are running differently shaped parts now. My understanding is that if you can get such a equilibrium going it turns out to be a great way to run a tank. I've never had the opportunity to try it - the demands bulk anodizing puts on a tank make that way to tricky!

For Jim - as an aside, I appreciate the compliment that you've noticed my replies; I've picked up quite a bit from reading your's over the years! Accurate Anodizing has beeen in business under current ownersip for over 25 years, specializing in bulk anodizing since we started. Our main focus is on automotive fasteners (there is a reasonable chance that the rivets holding your windshield wipers on were done by us!) but we also do aerospace and some straight commercial stuff (my personal favorite is aluminum rings for chain-maille).

I'm going to start watching the time between water or acid additions and sampling a little tighter, it if turns out to be something else, I'll post it for the sake of the discussion.

Thanks again!

Jim Gorsich Accurate Anodizing Inc. Compton, CA, USA

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