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Caustic Etch Analysis -- aluminium anodizing

Q. I've seen phenolphthalein ⇦ this on eBay & Amazon [affil links] mentioned in a couple locations now (here, and on Anoplex Software [a finishing.com supporting advertiser] website "Caustic Etch Titration" page) regarding caustic etch titration. It seems there are numerous different preparations available. Which preparation should be used for this?

Our machine shop has just started up a small anodizing line to bring in house the anodizing of one product we're making. We were running successfully for a few weeks at the start, but are now learning the failure points (we knew that would happen) and starting to learn how to analyze and manage them. One issue seems to be that the caustic etch bath has become weak and less effective (not removing as much material as before, less visible effect on surface of parts).

Kevin Enter
- Cambridge, Ontario, Canada
August 9, 2021

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Q. I've become responsible for doing this - there are many ambiguities. "Free" sodium hydroxide..."total" sodium hydroxide...my heads in a maze. I have finally managed to get the "right" answer (what the production manager expected) by titrating a 5 ml sample with 1.0N NaOH [1N NaOH on Amazon [affil links] to a phenolphthalein endpoint, and multiplying the titrated volume by 5.1. I do not know where this factor comes from; or its chemical significance.

The vendor provided another procedure that dictated titration to a pH of 11.0 (by pH meter) that yielded consistently low results.

Does anyone out there have any helpful words of wisdom?

Help!

Dave Wichern
Consultant - The Bronx, New York
April 9, 2010

Q. I am still confused.

Using different procedures, I get wildly divergent results.

I gather that there are three bath components that contribute to etching activity: sodium hydroxide, sodium aluminate, and the sodium salt of whatever organic acid they put in the "anti-scale" additive. Is there any simple way of determining each? Is there any way of determining each?

I have found that addition of the anti-scale additive reduces the titrated NaOH concentration substantially. Have others found the same? If so, please inform me.

Dave Wichern
Consultant - The Bronx, New York
May 28, 2010

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Q. Dear friends,
What is the significance of dissolved aluminium in Etching bath? We use NaOH as etchant, does Aluminium play any role in etching bath?

Aijazullah Tajir
- Abu dhabi, UAE
November 11, 2014

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Caustic Etch Analysis

Q. I started a thread about this a few years back. Had some time to think about it - how to put this on a rational, scientific basis.

How about this: measure the viscosity of the bath with an Ostwald viscometer. Then, dilute the bath with DI water, then add back enough sodium hydroxide to keep the alkalinity constant. Now measure the viscosity again. Repeat this for 10% fresh water + NaOH, 20% fresh water + NaOH, etc. You'd need at least 5 - 6 dilutions, as the relationship between dilution and viscosity will not be linear.

Now, evaluate the etch rate you get from each dilution + NaOH empirically, by etching a test coupon and weighing before and after. Very strict attention would have to be paid to keeping agitation and temperature constant.

Now, you have an idea between viscosity and the effective etch rate. One could measure the viscosity and alkalinity of your working bath and add just enough NaOH to get just the etch rate you wanted.

Why should this be? Well, as the bath ages in service, and sodium aluminate, sodium [anti-scale additive] sodium carbonate, and other crappo build up, the viscosity will change. This will affect the rate at which active etchant diffuses into the work piece/process bath interface zone, and hence the etch rate.

Does this make sense? Has somebody done this already? Am I totally full of beans?

Sounds like a lot of work, too. If I had a place to do it I'd do it for free. Might make a nice article for a trade journal. Also, this problem got under my skin and I would like to solve it.

Thanks in advance for any feedback.

Dave Wichern
Consultant - The Bronx, New York
August 14, 2015

I was hoping for some feedback. Oh, well.

I've changed my thinking about this, a lot.

The etch rate determinations aren't necessary. That eliminates lots of work. I found this great Paper in the Journal of Chem & Eng Data - "Viscosity, Density, and Refractive Index of Aqueous Sodium and Potassium Aluminate Solutions." Answered two questions: is viscosity related to aluminate concentration? and, is the change sharp enough to precisely determine aluminate from viscosity?

Yes, and yes. A solution that's 1.28 M in Al has a viscosity of 2.187 centistokes; for 1.09 M, it's 1.798 centistokes. That's an almost 10% change. Viscosity can be measured very precisely if you have good technique and careful temperature control.

So, to make a long story even longer...take a nice big sample of the etch when it's doing good. Now, take two portions out; dilute one 8% with water, another 4%. Take another (large) portion and stick a weighed Al coupon in. Let a bunch dissolve. Take the coupon out, weigh it. Split the sample in half, stick the weighed coupon back in one of the portions, and let a bunch dissolve. Pull it out and weigh again.

Now, you have 5 samples. Add sodium hydroxide to the diluted, and the Al enriched samples to match the original NaOH concentrations. This is easy to calculate. Measure the viscosity of each. Plot the data on a graph: aluminate vs viscosity. Call the original aluminate "A(sub)0. Then you have 5 points; you still don't know A(sub)0. But you now have two points above, and two below, the original aluminate value. From looking at the graph, you should be able to figure that out.

I'll stop here. I may be boring you all, or my math might be bad.

Dave Wichern
Consultant - The Bronx, New York
August 24, 2015

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