As part of a study into damage of hydraulic system components using fire-resistant fluids, the erosion of mild steel and brass in corrosive and noncorrosive waters in a cavitating environment has been investigated. The addition of emulsifying oils and corrosion inhibitors was included in the work. The apparatus used was an ultrasonic (20 kHz) vibrator modified to operate at 15 microns amplitude. The tests show that the synergetic effect of cavitation and corrosion is greater than the separate actions. Corrosion inhibitors and emulsifying oils are both effective in reducing damage, emulsifying oils down to untypically low concentrations of 0.1 percent.