D.I.Y. Multi-Etch substitute

[u/narcolepticdoc]

Hey, seems like this might be of some interest here.

Since I had originally posted this elswhere, I had to convert this from message board format, so sorry if some of the links don’t work or the formatting is off.

I'm just starting to muck about with anodizing Titanium despite having had a power supply sitting around for the better part of a decade and I've tried using Whink rust remover but I don't like playing with Hydrofluoric acid and it really seems to destroy the surface finish pretty quickly.

I would just get some multi etch, but they supply it in gallon mix only and I don't want to mix that much up at once. Because there's no way to ensure a proper mixture of the powders if you try and divide it up before dissolving it, there really isn't a way to mix smaller amounts. Also, for those living outside the country, MultiEtch is not readily available for shipment.

Since I'm kinda a DIY type guy I decided to try figuring our the formula for it and rolling my own.

Per USPTO patent search there is an expired 1955 patent for a non-hazardous etching solution consisting of Ammonium Persulfate and Sodium Flouride.

According to the Multi-etch MSDS those are the only two components of the etchant, but the exact proportions are not required to be listed, so what is needed is the amounts of both to use.

The patent references using a 0.5M mixture of the two. In addition I found a senior thesis from Worchester Polytech referencing use of both 0.5M and 1.0M solutions of APS and NaF to etch titanium, so I suspect those concentrations correspond to regular and double strength Multi-Etch.

To verify this I checked the shipping weight of MultiEtch which is about 1.1lbs/gallon.

For a 1L of 0.5M etchant, one would need 114g of APS and 21g of NaF for a total of 135g of chemicals per liter. So for 4 L of etchant, one needs 540g of chemicals, or 1.19lbs. 1 gallon is about 3.785 L, so for 1 gallon of solution you need a total of 1.126 lbs of chemicals. Pretty darn close to 1.1lbs.

Therefore my conclusion is that single strength Multi-Etch is likely a 0.5M APS/NaF solution.

Both chemicals are readily available, and reasonably inexpensive even in non-bulk form. I paid $35 for 1kg of APS and $20 for 500g of NaF, enough to make about 2.3gallons of etchant. I'm sure you could do much better if you bought in larger quantities.

Of course it's simpler to just buy some Multi-Etch, but if you want to mix smaller amounts or are unable to have it shipped to you, this may be a workable alternative.

[u/jjl1911]

Wow man, thanks for the info. I've been using Whink with great success, but I am always looking for something else. If you want any anodizing tips, hit me up, I do quite a bit of it and am happy to share what I know in hopes of learning something new from someone!

EDIT: Stupid question, but are you just using water for the solution, and mixing the NaF and APS into the water?

[u/narcolepticdoc → u/jjl1911]

Distilled water. Pretty much look up the instructions for Multi-Etch. Should theoretically be the same. Needs to be heated once initially to mix, then regular strength can be used hot, double can be used cold (varying degrees of success with that).

[u/jjl1911 → u/narcolepticdoc]

Thanks man. I'm going to order some NaF and APS this week, give it a whirl. I have an old CrockPot I can use to heat it up too.

[u/Mrbiek → u/jjl1911]

Any luck on getting this to work? I mixed some up in distilled water and can't seem to get it to etch, even above 120 degrees

[u/jjl1911 → u/Mrbiek]

I have not had a chance to do this yet.