An employee was testing a pressurized air tank with the pressure at 6 psi. During the testing procedure, he took a metal grinder to grind down a weld. This caused the head of the tank to pressure. The corner of the tank head broke free, struck him in the side of the head/face, and then threw him back 5 to 6 feet onto the ground. The employee sustained cuts and scrapes, as well as injuries to his head, shoulders, and ribs.