The authoritative public forum
  for Metal Finishing since 1989
  

  -----

Sulfuric Acid Etching of Aluminum - safety problems

2001

I have recently started etching finished aluminum product with sulfuric acid solution (easier to get then nitric acid).

It had been working great but I just had a very messy accident that I don't want to repeat. Any advise is appreciated.

Here's the story:

I just found that the right combination of sulfuric acid, water and aluminum causes a violent reaction involving lots of heat, sulfur gas, violently bubbling acid, and worst of all expanding sulfur acid foam and lots of sulfur gas.

This was about the 100th time I had acid etched and I was experimenting with different strengths of acid. I had found that about 30% acid 70% water etched faster the 100% acid - faster etching and less acid - sounds like win-win to me.

I figured I'd start at 100% and start diluting using water from the first rinse of rings (i.e. recycling the acid from the first rinse).

Well 100% acid etched fine in about 1 hour - no problems.

10% water 90% acid fizzed a bit at first. Then it started to feel quite warm and fizz more. In my best acid resistant suit and mask I dumped out most of the acid into a neighboring container. Unfortunately there was no stopping the reaction and within a minute the plastic container started to lose its shape. The boiling acid foam started bubbling out of the container and the plastic container melted (still containing over a gallon of sulfuric acid and about 15 pounds of aluminum rings).

Next phase involved huge amounts of sulfur gas combined no doubt with water from the snow I was throwing on the melting lump. The entire garage filled with a thick white smoke that was completely unbreathable and really slowed down my efforts to neutralize the acid with boxes of baking soda [in bulk on eBay or Amazon [affil links] .

One possible contributing factor is my aluminum (5154 alloy) is coated with a very light layer of a light organic oil.

On the plus side it appears that this mix etches the rings extremely quickly.

Jon Daniels
rings - Saskatoon ,SK, Canada

---

A simple one line poem for you.... DO AS YOU OUGHTA..ADD CHEMICAL TO WATTA! Just another tip..obviously the heat generated during your improper mixing techniques has given you the etching results that you like.. so.. add your your acid to the water, and purchase a heater to keep the bath at a temperature that works best for you. Marc Green anodizer - Boise, Idaho 2001 Ohh- lala ! looked like you've been adding water to a concentrated acid. You might have been working with oleum, that's more than a 100% sulfuric acid. I guess (like me) you don't really have a good chemistry background , do you?. I strongly suggest that you leave this experiment to the chemists. You may not be as lucky next time. Dado Macapagal - Toronto, Ontario 2001

---

The reason you should add acid to water and not the other way around is that the addition of water to acid generates a tremendous amount of heat; the liquids boil and present a safety hazard through erupting and melting plastic. Always add acid to water.

James Totter, CEF
- Tallahassee, Florida
2001

---

Just a couple of follow-up points. #1. If you do decide to purchase a heater, make sure its a corrosion resistant one. Secondly, once you are sure of the temperature that works best for you, make sure that your tank is made of a material that won't soften too much if you have to have this solution at a high temp. You didn't say why you were having to etch these rings..but you may want to investigate different solutions that aren't as nasty as the sulfuric solution. Perhaps a sodium hydroxide would work as well...it would clean the oil better, and wouldn't be quite as nasty..and since you are using a 5000 series alloy, I don't think you would have much residual smut left on your parts after a NaOH etch. You were quite lucky with your previous experience, it could have been much worse. Remember..ALWAYS add chemical to water..never the opposite.

Marc Green
anodizer - Boise, Idaho
2001

---

2002

Hi Jon,

Well you've been warned by experts, always add acid to water NOT the other way around. You will still get 'heat' in your mix depending upon the acid/water concentration, a so-called exothermic reaction.

You could STILL use a plastic tank ... if ... you used the right plastic. As it is, your tank, most probably medium density Polyethylene, is not recommended for constant temperatures of over 150 degr. F. But Fibreglass would be OK temperature wise but doesn't like strong sulfuric. Lead would be totally ideal. If your tank is very small (you didn't give any sizes), consider fitting it inside a larger Poly tank filled with cold water. This might work OK. However, drop in your rings beforehand to prevent it from floating! Those open top Poly tanks are available from 10 to 500 imp. gal. and are very reasonable in price.

... I only hope that your garage didn't have any vehicles in it during your experiments. sulfuric does wondrous things to cars!

Cheers!

Freeman Newton [deceased]
(It is our sad duty to advise that Freeman passed away
April 21, 2012. R.I.P. old friend).

I would recommend trying a Phosphoric acid. This is used in aircraft finishing such as Alodine or Aluma-Dyne.

Matt Messley
- Sheridan, Illinois
2007

You add smaller volumes of acid to larger volumes of water so that if the reaction does run away you have more water than acid in the mixture. This solution is more dilute than a water added to acid solution, so will cause less damage to the surrounding environment, and you.

Lyn Lynch
- Mesa, Arizona
2007

---

January 21, 2010

sulfuric acid is a hazardous material. This means safety training is required in the workplace before handling it is permitted.

It would be really smart to read about the hazards:

www.ercoworldwide.com/documents/sulfuricAcid_000.pdf

Adding water to acid can create so much heat that there is boiling, which results in steam, which results in an explosion, which throws acid around with force.

The fumes can be quite harmful - see above website info.

Apparently placing aluminum in sulfuric acid results in hydrogen. It is real explosive if there is fire or a spark.

I am no expert, but plan to do some anodizing with sulfuric acid. I plan to follow my own advice and learn as much as I can about hazards.

Grant Fair
- Toronto, Ontario, Canada

---

Q. Does any know where I can purchase Sulfuric Acid in Toronto, Ontario in small quantities. I just need a few liters for anodizing my aluminum jewelry. Thanks

Paulette Johnson
- Richmond Hill, Ontario
February 24, 2012