If it's damp enough for a galv. steel box to rust when sunk in a wall, then it's too damp for any flush box in the wall! Socket will be affected by damp which will show up on future IR readings.
Either fit surface boxes, or you could try the method I've used 100's of times over the years - dig an oval shaped hole out of the wall, about ten times the volume of the metal box, rough render the inside of the hole, coat the render with a couple of coats of tanking compound, suspend the box in its exact place with a piece of batten fixed across the front of the box and fixed to the wall either side, and then fill in the hole around the box with mortar.