For rust to form in the area you've reffered to, water had to have collected and allowed to remain to attack the metallic surface of the spark plug.
That portion of the plug is plated, either nickel or chrome.
If it is from water, you would deduct that the ellavated temperature of the plug's external surface would evaporate what fluid collected there.
Did the surface contamination readily wipe off?
Were the seat and threads free from the "rust"?
Could this be a combustion leak?
Try again and use an anti-seize on the threads and the seat to see what happens on the new plugs.
There is a torque spec but I can never find it so I tighten to an educated feel.
I've heard screw in till contact in the seat or against the gasket, then 1/4(90 degrees) turn tighter.
Happy Trails Beep Beep from Houston!!!
Donald, El Cheapo