The Stovebolt Site Forums The Shop Area Paint and Body Shop Phosphoric Acid dilution to remove rust

Rusty46 said: I buy phosphoric acid by the gallon at a local chemical supply. It's a 75% solution, which I'm told is the highest percentage available in WA. I mix 1/2 cup of the 75% solution with 4 cups of water to use to convert rust . I'm using it to aid in removing rust from pitted sheet metal and for storing cleaned metal to prevent rust. One gallon from chemical supply is $15.

The phosphoric acid sold by Tractor Supply Co. (RustBuster)is 25.9% and I forgot to look on the label whether they recommend dilution. I have purchased 75% at a local (KC, MO) supply house and now want to dilute for use.

From Rusty46's statement above I calculate that to be 8%. Is an 8-10% solution normally what is used? I will be doing small parts submerged and sheet metal with spray bottle.

Paul

Hello Paul - 75% solution divided/diluted by 8 parts water = 9.375% .
I have used from 5-20%. 5-10% works the best for spraying. 10% for areas you're scrubbing, 5% for areas you're rinseing. Much over 10% can be sticky when dry. I have not experimented with submerging parts.
Correct procedure is to add acid to water, although there is no chemical reaction in either sequence. Use the same precautions you would use with paint stripper.
And please share your experience.

Paul
I'm in KC too. Could you post the name of the company you got the 75% from??

> 75% solution divided/diluted by 8 parts water = 9.375%.<

Rusty46 I don't follow your math. I would do it this way. Assume 1 gallon of 75% Phosphoric Acid.

1gal. = 3 parts acid and 1 part water [think quarts].

Add 8 parts water [quarts] = 12 parts total [3 acid, 9 water].

3/12 = 0.25 = 25% solution.

In your example of 1/2 cup [4oz.] into 4 cups [32oz.] would be 3/36 = 0.08 = 8%

What say you?

Dave S: Tractor Supply wanted $21/gal of 25.9% and I purchased 75% for $21.50/gal at Terrace Packaging, 2819 Southwest Blvd. KC, MO 816-561-5377. This was the first place I found and easy to get to - there are probably others. This will make more than I will probably use but a bargain never hurts.

I plan to experiment with some items to figure out a reasonable dilution vs time and will post pictures later.

Paul

Paul - I like your math better than mine.
I'm looking forward to seeing your results.

i just read in classic car magazine about using molasses. any one to comment on that. the magazine says it works. the reference was in a letter to the editor. you can buy molasses cheap at a farm store so they say.

WE b - There was some discussion on this topic before. I believe it requires immersion and it's a very slow process.

http://www.stovebolt.com/techtips/rust/rust_molasses.htm

From Barry Weeks in the tech tips....

I've used phosphoric straight as a soak, rinsing well w/ clean water after - works well and the solutuion can be used over and over til it's sludge - another readily available rust remover is washing soda [sodium carbonate] from the grocery store laundry section see the details here - works great and environmentally friendly

Bill

I looked all over for washing soda when I was building my rust cooker and people had never heard of it. I ended up using TSP. It works but I don't know if it's better than the washing soda or not. I'd like to find some.

Thanks Paul,
Funny what you remember.... 20 years ago I was running a charter bus company that ran ski trips to Colorado. We used Windschield Washer solution in the toilets in the Rest Room (to keep from freezing). I contacted Terrace chemical to see what options I had fro something cheaper than WW blue stuff. Didn't buy anything, but remembered the conversation and the name.

I've been trying to come up with a test that might shed some light on what concentration of phosphoric acid to use when soaking rusty parts. After some searching I found a rear fender brace rod. Rust should be nearly the same on each end.

[img]http://www.whcinc.com/images/Acids.JPG[/img]

This is my mostly unscientific setup. Rod goes into each bottle for 1 hour then reversed into the other bottle.

[img]http://www.whcinc.com/images/Acid10%201hr.JPG[/img]
Above is 10% acid for 1 hour.

[img]http://www.whcinc.com/images/Acid25%201hr.JPG[/img]
Above is 25% acid for 1 hour.

[img]http://www.whcinc.com/images/Acid10%202hr.JPG[/img]
10% acid for 2 hours.

[img]http://www.whcinc.com/images/Acid25%202hr.JPG[/img]

And finally, 25% acid for 2 hours. As you can see the 10% seems to do as well if not better than 25% dilution. If I remember my chemistry class correctly, a dilute acid is more aggressive than a concentrated acid (I don't remember why).

So from this simple test I will be using a 10% solution to attempt to clean the inside of a motorcycle gas tank of rust. Will keep you posted.

My original motive was to clean sheetmetal - that is still my main desire - my glass bead blasting cabinet would have done a much quicker and better job of cleaning this fender brace. Acid is just another tool to use on the right application.

Paul

Thanks for sharing your results Paul. The 10% phosphoric acid solution will remove the rust, given more time to work, without the loss of good metal.
When applied to sheet metal, if you cover with plastic wrap, the acid solution will have more time to work on the surface without drying out.
If the acid/rust solution does dry on the surface, wetting again with solution and scrubbing with scotch brite pad will remove the residue. When the metal is derusted and clean, spray on the acid solution and dry the surface. The metal can be stored for an extended period without rusting IF the metal is not exposed to moisture or bare hands.

Good Job!
I would like to see other types of tests like that.
Cool.

I'm curious if anyone in this thread has tried electrolytic rust removal. If not, why not? It's cheap, and really does a great job on serious rusted stuff without eroding away the good iron. The acid etch may be good prior to painting, but why not use your battery charger and plastic tub to save some blasting and acid handling?

I used electrolysis on some old hit n miss engine parts. Slow, but effective.