Electrochemical Corrosion Rate Measurement Under Iron Sulfide Deposit

Different from other types of pipeline solids such as sand and iron carbonates, iron sulfide is semi-conductive and electrochemically reactive. This adds extra complexity to laboratory testing compared with inert deposits. Previous laboratory testing has shown that the corrosion rate under iron sulfide deposits can be severely overestimated using electrochemical techniques. In the current work, a laboratory iron sulfide synthesis method and an iron sulfide under-deposit corrosion testing methodology were developed to better assess and monitor this type of corrosion in a simulated pipeline environment. Two different types of iron sulfides were used to investigate the corrosion of carbon steel under such deposits. Scanning electron microscopy, x-ray diffraction, and laser light extinction particle size analysis were utilized to characterize the surface morphology, composition, crystal structure, and particle size of the different iron sulfides. A white light interferometric volume loss method was used to verify the under-deposit corrosion rate continuously measured by the linear polarization resistance (LPR) technique. It was found that the corrosion rate under iron sulfide deposit is sensitive to the potential scan rate when utilizing potentiodynamic LPR technique for corrosion rate measurement. Overestimation of corrosion rate was observed at the typical scan rates of 0.1 mV/s to 0.5 mV/s, which is attributed to the large interfacial capacitance of the iron sulfide powder. Potential scan rates as low as 0.01 mV/s and 0.005 mV/s were applied so that the LPR corrosion rate matched the volume loss corrosion rate. When the potentiostatic LPR was used, a potential hold time of 16 min was needed to minimize the effect of interfacial capacitance.