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 Icorr; 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.