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.
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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.
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http://www.whcinc.com/images/Acid10%201hr.JPG[/img]
Above is 10% acid for 1 hour.
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http://www.whcinc.com/images/Acid25%201hr.JPG[/img]
Above is 25% acid for 1 hour.
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http://www.whcinc.com/images/Acid10%202hr.JPG[/img]
10% acid for 2 hours.
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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
