A MAINTENANCE MECHANIC WAS WELDING A NUT ON THE END OF A TUBE RETRIEVAL TOOL. HE WAS WEARING "" LOW QUARTER"" STEEL BOOTS AT THE TIME. A PIECE OF WELDING SLAG FELL INSIDE ON HIS LT. BOOT & CAUS ED A 3RD DEGREE BURN TO THE MEDICAL REGION OF THE THE FOOT. NO LOST TIME; RESTRICTION PENDING.