Corrosion Arc

An interesting component in the Nyquist plots appeared near the completion of this experiment. Samples J through M were cast and tested approximately three months after samples A through I. The IS results from samples A through I were fairly consistent. However, samples J through M exhibited an additional feature in the Nyquist plot such as that shown in Figure 10 from specimen K-2. The data shown in the figure were measured at the beginning of the RCT, just prior to the application of 60 volts DC. The bulk arc can be seen at Z ' < 700 Ω , and the electrode arc can be seen at Z ' > 750 Ω. The interval 700 Ω < Z ' < 750 Ω has an additional arc, possibly due to corrosion accumulating over the duration of the entire experiment. However, the cell was damaged during an attempt to clean one of the brass electrodes in order to confirm the corrosion theory.

Figure 10: Impedance plane plot for Specimen K-2 in the RCT cell. Solid circles represent data at decade frequencies.

The "corrosion" arc presents a possible difficulty in using the RCT cell for determining the sample conductivity. The Nyquist plot of Specimen K-2 in Fig. 10 has two arrows delineating the values R_B and R_o . The value of R_B is an accurate estimate of the sample resistance since one would expect the true value to be at the intercept of the bulk arc and the Z ' axis; one can show this by adding another RC pair in the equivalent circuit shown in Fig. 1 to approximate corrosion and observing the result. The measured value of R_o is affected by the presence of the intermediate arc. Despite this, the values for σ_IS and σ_RCT differ by less than 11 % for specimens J through M.