broken steel screws in aluminum casting - the nitric acid

I got a little bit of 70% nitric acid, potent stuff. I cut the end off a screw of the same exact type as I previously dissolved in alum, put it in the bottom of a small glass, and added about 10 drops of nitric acid. Naturally, I wore face shield and gloves, and used an exhaust fan. Nitric is pretty nasty. Anyway, it was cooking along nicely for a few minutes but then it seems to have slowed down quite a bit. It's been going about 8 hours now, and I'm wondering if there is enough oomph in the little bit of acid I put on it, barely enough to cover the bottom of a small water glass.

Thing is, my actual screw, which is in an aluminum casting, won't have much acid around it either.

I suppose I could try drawing the liquor out carefully after it slows, and adding a few more drops.

Anyway, tomorrow I'll take the piece out and weigh it and I'll get a rate of dissolution and then I'll compare it to how the alum did.

GWE

Reply to
Grant Erwin
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I'm no chemist, but I think the it's the hydrogen in nitric that does the work. Needless to say, just because you still have a liquid doesn't mean you still have what you started with. Just like consuming gasoline in your car, you've used up the part that made it nitric acid (HN03), so it is now something else. It's likely the action will have ceased completely, and you'll have to add more, or pour out the old and start with new. When I refined gold, the amount of nitric used was critical, due in part to having to get rid of it before I could recover the gold from solution. I found that an ounce of nitric in conjunction with 4 parts of HCL would dissolve a troy ounce of gold. Less would always leave some gold undisclosed.

Harold

Reply to
Harold and Susan Vordos

You do consume the acid as it dissolves the metal. You'll bubble off hydrogen while the iron is being oxidized, ultimately you'll have a solution of ferrous (ferric? ick) nitrate (or whatever) and no acid.

I suspect that if you just drip more acid around the screw you'll get it down to where it needs to do its work -- but you'll also have some dripping out of the hole, which may not be what you want.

Reply to
Tim Wescott

I'm not a chemist either, but I wonder if H+ is the worker here. If it was , it seems to me that sulfuric acid would work as well as nitric and that's apparently not the case. Nitrate (NO3-) is known as a good oxidizer, so I'm thinking that it is the active component in nitric acid, perhaps oxidising the iron in steel to Fe(NO3)2 or some other form. Googling the subject has not been productive for me, but a post to sci.chem would probably turn up the correct reaction. Once that is known it should be a simple matter to calculate how much nitric acid would be needed to dissolve the screw or tap, as the case may be.

BTW, somewhere in the results of a Google search it was suggested that concentrated nitric acid be diluted before application.

Assuming reaction to Fe(NO3)2, you'd have:

2 HNO3 + Fe => Fe(NO3)2 + H2

so you'd need 2 moles of ntric (126 grams) for every mole of iron (55.8 grams) in the tap, or roughly 2-1/4 gram of pure nitric per gram of iron. For a 70% nitric acid solution that would be a litlle over 3 grams nitric per gram of iron and a 1:1 dilution of nitric acid would require around 6 grams nitric acid per gram of iron. The approach is correct, I think, but the reaction could be all wrong. Note that hydrogen is released in the above reaction. If the solution bubbles, some sort of gas is being released and hydrogen is probably more likely than oxygen.

Mike

Reply to
Mike Henry

Grant

You've used up the acid. Add more (no need to decant, it's liquid after all so it will diffuse) and it should eat the metal again. It will probably take 5 - 10 ml of 70% HNO3 to eat a small screw. When you get to taking out the screw from the aluminum, build a dam around the hole with modeling clay and then rise the acid off with a baking soda rinse when you're done. Just rinse it down the drain, the beasties at the treatment plant like nitrates.

By the way, general rule in chem labs is rinse and dump 3 times.

Jim

Reply to
Jim McGill

Reply to
David Billington

I've got a degree in chemistry, and Jim is right on target. No need to remove the old acid - the new will mix thoroughly.

Nitric acid and concentrated sulfuric acid are, in addition to being strong acids, strong oxidizing agents. Which means they will, under certain circumstances, passivate metals such as aluminum and stainless steel. The passivation will actually prevent the metal from corroding.

Keep in mind that the acid will take quite a bit longer to work in the hole, since it can't get at all surfaces of the broken screw as it did with the test piece. It works quicker with a tap, where it will work down along the flutes. Or with a drilled piece.

John Martin

Reply to
John Martin

While I can't confirm what you said, Mike, I recall that on rare occasions I'd have a minor hydrogen fire------which indicates to me that you are on the right track. If the hydrogen was being consumed, there'd be nothing to burn. It's obvious it is liberated, with the metal in question taking its place.

Harold

Reply to
Harold and Susan Vordos

I recall seeing a chart for corrosion capacities for various chemicals vs. steel. Nitric acid was much less corrosive when the steel was immersed than when the steel was suspended in the acid vapor. Unfortunately I can't find that chart right now. This could have something to do with the passivation John Martin mentioned.

Cheers,

Kelley

Reply to
Kelley Mascher

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