The Assistant Driller (AD) identified a leak on a hose that was difficult to reach, he asked a Roustabout (RA) to bring ‘something’ to stand on. The RA fetched an empty Caustic Soda drum (used and punctured from below) from the top of the tank and started walking towards the stairs. While passing over the walkway grating above, the AD who was standing below looked up. Residue of the Caustic Soda powder fell from the bottom drum onto the AD’s face resulting in chemical burns to his right eye and cheek.