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

On 14 Dec 2005 09:54:14 -0800, "John Martin"
wrote:


Jim McGill wrote:
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

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