Like he said above, give it some heat. Galvanic action could indeed be involved, in addition to whatever corrosion might come from exposure of the bolt body to coolant. Another option is if you have a long heavy centerpunch to hold on the bolt head and smack with a 2-1/2 lb. or heavier mallet, possibly setting up a vibration to loosen the bolt. I know the Japaneese used to be notorious for using impact drivers for assembling their cycles.