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Alkaline Etching Bath gets used up really fast

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Q. On our treatment line, we have an alkaline etch bath which uses a branded blend made up of 80-90% sodium hydroxide and 10-20% sodium orthophosphate (plus whatever additives may be in).
It seems to be losing up to 1.5 g/l of caustic every week (2700L Tank).

I wanted to know from others if this seemed excessive? I feel like it shouldn't be going down so fast and I suspect something else is reacting with the hydroxide but I'm not sure.

G. Bottacin
- Darwen, Lancashire, U.K.
April 24, 2023

A. A lot of factors can contribute to the reduction in your alkaline etch bath. The most common one, and what I suspect this might be, is drag out. Alkaline etching baths tend to feel almost sticky and greasy (Through appropriate gloves of course, always remember your PPE!). Meaning it'll also "stick" to the aluminium and the more aluminium in the etch, makes it more viscous as well. So, if you have an etch, that is quite saturated with aluminium, you'll also have more drag out.

The shape of your parts can also contribute to drag out, by functioning almost like a cup or a bucket, and carrying with it a lot of the etch into your rinse baths.

Maybe you've simply seen an increase in throughput, meaning that more parts goes through your etch, leading to more drag out as well. Do you have several shifts? Day, Evening, Night?

Though, I will say, 4kg /1,5/g of etch over a week, doesn't seem like that much, in the plant where I work, we can see that between 8am and 16pm on the same day; of course, we don't use your precise blend, which might make a difference.

M Stammerjohann
- Horsens, Denmark
April 25, 2023

A. Hi G.
The actual fall in concentration of sodium hydroxide depends on more than one thing, not only the reaction with the parts being etched, but also a reaction between the hydroxide and dissolved carbon dioxide in the solution. This reaction goes on all the time, with carbon dioxide from the air constantly dissolving into the solution to then react with the hydroxide.
This forms sodium carbonate, which gradually builds up in the etch solution. over time this will reduce the etch rate of the solution, and require the solution to be dumped periodically.
You can test for carbonate via barium chloride gravimetric assay, or by titration with phenolphthalein [on eBay & Amazon] to give a value for the carbonate, and then carry on with methyl orange [on eBay or Amazon] for the total caustic value. the following link may be of use to you.
http://www.titrations.info/acid-base-titration-sodium-hydroxide-and-carbonate

Best regards
Mark

Mark Lees
- A forsaken rock in the Irish sea
April 25, 2023

P.S.: I noticed an error in my post, the titration does caustic first then carbonate. I wrongly said it was the other way round.

Q. Hi,

Thanks for your reply! Would drag out lead to a corresponding fall in volume of the tank? Or would the hydroxide stick to the aluminium more than the water?
We do indeed have a day and a night shift. It makes me feel a bit more confident that it is just normal usage.

GB Bottacin [returning]
- Darwen, Lancashire
April 26, 2023

Hi, thanks for your reply!

We do have quite a vigorous agitation on the tank. Could this be introducing CO2 at an increased rate?

I performed the titration you linked and found 205g/L of sodium carbonate. I'm guessing that this is the issue so we will be dumping our bath and starting a new one.

Kind Regards
G. Bottacin

G Bottacin [returning]
- Darwen, Lancashire
April 26, 2023

A. Yes, drag-out would match a reduction in the tank. And you'd also see your first rinse being contaminated by it. A recommendation, to help minimize the lost volume, is to keep your first rinse slightly warm, and using it to top off your etc. At least if you have several rinse baths after etching with overflow from cleanest to dirtiest.

Heat will also help improve the rinsing capabilities, as well as the slight contamination also appears to actually help remove the "sticky" etch from the parts. If you experience that the etch tend to stick to the parts in a visible manner in the following rinse. Introduce a 15-30 sec rest in your first rinsing bath to help alleviate this issue.

As always, be cautious with your chemicals and wear the appropriate PPE, even when handling rinse water, as it is still contaminated with etch and can most certainly ruin someone's day, and eye sight, if you aren't careful.

M Stammerjohann
- Horsens, Denmark
April 27, 2023

A. Consumption rate of caustic soda in etching bath is 250-300 g/l (for 48% caustic) depending on using caustic density of caustic as 48% or 32%. So the first parameter of consumption of caustic soda is surface area of etched aluminium. Second one is, as other contributor said, drag-out issue. Prevention of excessive drag-out from etching bath is proper draining of bars. Slope of bar during drain will minimize drag-out of bar.
Good luck.

Alaattin Tuna
- TURKEY Sakarya
May 3, 2023

A. Hiya cousin Bottacin,

I really like Allatin Tuna's explanation, and it confirms my own experience with a bath of the same approximate volume as yours, running a similar proprietary dry-stock etch as you do.

We do a daily ~5kg add of dry stock, and also automatically pump out of the bath to feed the WWTF pH adjust flocc tank and keep Al low and constant-ish. Pro tip: if you run a pretreatment plant, this actually can self balance your rinse tank effluent!

I don't think what you are seeing is unusual at all.

Remember that as metal hydroxides form and become insoluble, and are removed via filtration, the net effect is an acidification of the bath. You're selectively stripping the -OH off available water molecules to bind with the metal +, ++, +++.... and leaving a mess of lonely H+ to drop the pH and also reduce the efficacy of that aggressive caustic strip you hoped to maintain. If you have a sodium dichromate seal on site, you might have noticed that as the bath ages, it becomes sludgy and acidic. It's the same effect, done differently.

Don't worry, what you see is normal. Just make sure you keep your free aluminum low enough not to further interfere.

Cheers

Rachel Mackintosh
- Greenfield, Vermont
June 9, 2023

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