Get yourself some PB Blaster or better yet some Kano Kroil, apply it to the threads (both sides) and let it soak in and do its work before you try to take the pedals off.

Then use a real pedal wrench; regular open-end wrenches are typically not long enough to apply the leverage needed to remove stubborn pedals. Don't even consider adjustable wrenches. As suggested above CONTROLLED application of heat may help loosen up the joint.

Make sure that when you replace them you grease the threads so that they do not seize/corrode into place again. I use Tef-Gel for such dissimilar metal joints and also use stainless pedal washers; they help avoid marring the crank arms and also help keep the pedals from jamming into the cranks. Grease the washers as well.