The gravimetric weight loss method and the electrochemical polarization resistance methods were combined to evaluate instantaneous corrosion rates for metals and metal alloys in industrial cooling-water systems. Metal and metal-alloy samples were exposed to synthetic cooling water in a bench-scale recirculating system constructed to study corrosion under flow conditions similar to those of a cooling-water system. The measurements yielded by both methods were related through a coefficient, B', to convert polarization resistance measurements to instantaneous corrosion rates. The metals and metal alloys tested in this study included mild steel, aluminum, copper, and cupronickel. B' was observed to change with time initially but approach a constant value for long-term exposure (> 3 days) for most metal materials. B' values varied with metal or metal alloy studied. It was concluded that combining both methods to determine the B' value for a metal or a metal alloy under particular exposure conditions provides a means of obtaining an accurate instantaneous corrosion rate.