Sparks/Arcing coming from Air conditioner duct work.

Why do you think it's a chemical reaction?

I turned off the main breaker and it still was sparking... there are no electrical wires near the location of the sparks unless there are wires inside the duct work

To me, this doesn't point to a chemical reaction, but hey...I could be wrong. Let's see what others suggest.

Only two things that I can think of...

A wire has fallen onto the roof and is somehow grounding out on your ductwork. Not very likely though.

The power supply on your A/C unit may have some large capacitors in it that could take a few minutes to drain after the power was cut.

What did you do to finally get it to stop sparking?

It stopped on its own.

If it really happened and the mains breaker was off then the only thing that could cause it was some kind of static electricity discharge. Otherwise maybe you just were dreaming.

Many years ago when I was an adolescent because of my age, I lived with my family on top of a mountain in NE Alabamastan. When I was in the basement, building one of the many Frankenstein machines of my youth, I noticed that large sparks would jump off the metal ductwork that ran the length of the house whenever lightning struck within 5 miles. The house was long and in line with the ridge with the main supply duct about 65 feet long. I had to assume that the EMP from the lightning was being picked up by the ductwork which acted like a big antennae or a longitudinal inductor. It was scary to be sitting there and suddenly hear popping and arcing from the ductwork, then a few seconds later I would hear the thunder. Just a thought, if a home is close to high voltage power lines, a similar thing might happen.

St Elmo's Fire.

fixed! main for the AC is outside next to the package unit. I followed the wires going into the unit and one of the lines was rubbing against the frame of the unit and subsequently energizing the duct work. After insulating with a piece of pvc and plenty of electrical tape, all is well. Thanks for everyones help!

You better havea electrician check it out before something catchers on fire.

Does the main breaker not de-energize the A/C supply? You said you shut the main down and the sparks continued.

Yup..... My first reaction was that your "Main" did not shut off EVERYTHING. I never understood why the NEC allows more than one main in the same building. That should not be allowed. I have several buildings on my property and each building has it's own main, but they are separate buildings. On my power pole there is a main that kills everything on my property. You're lucky you didn't have a fire. I'd consider installing a TRUE MAIN that does shut off the whole place. Probably just a little rewiring so everything including the AC are AFTER the main.

Also, get all wires so they dont touch the duct work. Ducts vibrate and will wear the wire insulation. If you must, run a piece of 2x4 or something next to the duct to staple the wires securely.

Alvin