SUSTAINED CHEMICAL BURN TO TOP OF FOOT & FRONT & SIDES OF LEFT ANKLE DURING ENGINEERING WALKDOWN OF PIPING SYSTEMS. REPAIR OPERATIONS WERE BEING CONDUCTED ON OPPOSITE SIDE OF VESSEL. WORKMAN H OSED CAUSTIC LIQUOR TO DRIAN THAT SPILLED FROM OPEN PIPE. WATER SPRAY UNDER VESSEL WET ENGINEE'S PANT LEG. ENGINEER WASN'T AWARE OF CHEMICAL WASH DOWN ON OTHER SIDE.