[cdowns]

i'd just clamp the vicegrips on it and wack it good with a hammer to see if i could loosen it up// the threads may be history because the dissimilar metals so in the end you might have to tap for the next size plug// you can also drill out the center then tap a bolt in the hole and use a welder to weld the bolt to the pipeplug ,that would bring the most heat to the thread area maybe dislodging the corrosion