Hi Nick. This works just fine. It is cheap, easy, and you don't have to breathe the zinc fumes. Dilute the acid, and let it sit a long time. The less it froths, the less collateral rust damage you will have caused by aerosolized HCl.
How much to use? Here is a quick calculation:
A 1/2" schedule 40 galvanized pipe will have about 1.8 oz/sq ft of zinc. For 1 ft of pipe with id 5/8" and od 7/8", this is
7/8/12*3.1416*1.8 + 5/8/12*3.1416*1.8 = .7061 oz/ft
assuming the inner layer is intact. For most cheap blacksmiths, however, the inner layer will be almost missing due to corrosion, else why was the pipe free in the first place.
Pool hydrochloric acid is about 30% by weight HCl. The density is 1.149. HCl has a molar weight of 36.14, so one gram of pool acid has:
.3/36.14 = 0.0083 moles of H+, two of which are required to dissolve one mole of divalent zinc with a molar weight of 63.4.
(.3/36.14)*63.4/2 = .2631 grams of zinc dissolved by one gram of pool acid,
or the same number of ounces of zinc dissolved by one (weight) ounce of acid. This must be debited to convert to fluid ounces, since pool acid is slightly denser than water:
.7061/.2631/1.149 = 2.3356 fluid ounces of acid per foot of 1/2" sch 40 pipe.
Use more to make sure its all gone in a reasonable time, or use less if the pipe is heavily corroded. I have noticed that even if a lot more is used, not all the zinc is dissolved. It can flare up when you hit the hot metal with a hammer dislodging a piece of rust (remember, cheap pipe has lumps of rust inside) which protected some zinc. Do not inhale.
I hope the numbers are correct.
Disposing of the acid? Evaporate to make cheap killed acid soldering flux, or neutralize with baking soda, or pour on concrete, and it will slowly neutralize by itself. Does zinc disposed of in this manner create more water pollution than it would create air pollution from burning it off? I don't know.
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