An Investigation of the Utility of Instantaneous Corrosion Rate Measurements for Inhibitor Studies^*

Instantaneous corrosion rate measurements were evaluated under laboratory conditions to determine their utility, both in brine solutions and in two-phase brine/hydrocarbon systems containing nitrogen, air, H₂S, CO₂ and mixtures of these gases. Measurements are based on the “polarization resistance” equation first reported by Stern in 1958. Both 3-electrode and 2-electrode techniques were used. In the 3-electrode technique, one electrode serves as a reference and the use of a steel electrode identical to the test electrode was evaluated. Techniques were evaluated from two standpoints: (1) To determine that the data obtained corresponded to polarization characteristics of steel under conditions where a corrosion product layer accumulated; and (2) to establish that with sufficiently constant corrosion conditions measurements will correlate to weight loss. Analysis of the data produced the following conclusions: 1. Use of a never-polarized steel reference electrode is well founded; 2. Three-electrode measurements reliably measure the instantaneous corrosion rate of the test electrode; 3. Even with non-ideal curves, as long as the complex β’s are reasonably constant, the Stern equation can be used to relate ΔI/ΔE to I_corr; 4. There is reason to prefer anodic pulses because of the better shape of the anodic branches of the technique; 5. The 3-electrode technique is preferred because, although 2-electrode data are qualitatively related to the corrosion rate, the measurements exhibit variations which make them less reliably correlated to the corrosion rate of an identifiable electrode; 6. The measurements are uniquely suited for changing corrosion conditions, especially in nearly neutral saline solutions containing air, H₂S or CO₂; 7. The measurements should be useful in all practical solutions where the conductance is high enough to support electrochemical corrosion.