METHODS TO CALCULATE CORROSION RATES FOR ALLOY 22 FROM POLARIZATION RESISTANCE EXPERIMENTS

The general corrosion rate may be measured using immersion tests or electrochemical tests. Electrochemical tests are fast and can be used for a rapid screening of environmental effects such as temperature and electrolyte composition. Electrochemical tests are described in ASTM standards G 59 and G 102. The basis of these tests is to calculate the resistance to polarization (R{sub p}) in a voltage vs. current plot and to convert these values to corrosion rates using Faraday's law. Commercial software can calculate the corrosion rate based on inputs from the operator. This paper discusses three ways of calculating the corrosion rate (Methods 1, 2, and 3) based on a fixed set of acquired data of voltage vs. current. It is concluded that the way the corrosion rate is calculated does not greatly impact the absolute value of the corrosion rate. Variations in the acquired data (current, potential) from one experiment to another seem more important than the manner in which data is fitted with the R{sub p} slope.