Methods were devised to use electrical resistance probes in the low-temperature water-cooled Hanford reactors to measure corrosion continuously during reactor operation and during chemical decontamination. The changing resistance of a corroding test element is compared with the resistance of a protected reference element. Typical probe results showed carbon-steel corrosion ranging from a 1 unit/y equilibrium rate to a 1050 units/y maximum rate during decontamination. (An arbitrary rate scale is used to obviate security classification.) Total corrosion from decontamination was equal to about 1.5 months of reactor operation.