Rust Removal With Phosphoric Acid

[John Hasler]

What happened to me when I put a cast iron part into my phosphoric acid bath for a long soak, it came out with all of the surfaces soft, rendering the casting to the scrap heap. The cast iron looked as if the carbon had been leached from the steel up to 2 mm deep. I don't know what type of cast iron it was.

That's an odd effect. The phosphoric certainly could not have dissolved the carbon: only the strongest oxidizers will attack it. That part must have been an unusual alloy. I routinely leave cast iron soaking overnight with good results.

 

[British Steel]

You can boil the trapped hydrogen out of the steel by heating it to around 400F for an hour or so. Do a little googling for the exact formula. It's what platers do if steel bolts are acid washed before plating. If you can't boil the hydrogen out for some reason, you can prequalify fastener by torquing them up and letting them set for 24 hours. If they don't break after 24 hours, you won't have a HE problem.

Bruce

Thanks Bruce, I'll remember that!

Dave H. (the other one)

 

[hman]

A few years ago I cleaned some corroded brass and copper plumbing fittings in my acid bath, which worked very well, but after that, the steel parts that I put in the acid bath took on a copper colour hue, which actually looked as if you had coper plated the parts. This stayed like this until I renewed the acid solution in the acid bath

Sounds like what you're getting is electroless copper plating. Very useful in some instances.

 

[LEPLINKS]

I use phosphoric acid to clean rusty tools and other parts. The result is fine most of the times. This old 12" adjustable Bahco wrench has been treated for four hours.

View attachment 124777

I have used white vinegar and it works great with a soak overnight

 

[terryw123]

Use milkstone from tractor supply. Works great and cheap.

 

[StepSide 88]

I buy milkstone remover at the farm store. I cut it 4:1 with tap water and soak the part overnight. I then rinse it and let it air dry, being careful not to damage the phosphate coating. Then I apply a thin coat ot linseed oil and let it dry for a few days in the sun (an hour in an oven would work as well).

I moved onto milkstone remover as well.
Cheaper than Ospho and stronger molarity.
It does contain nitric acid as well however fyi.