Rotating cylinder electrode (RCE) methodology with electrochemical impedance spectroscopy (EIS) measurement was applied to study the inhibition behavior of quaternary ammonium compounds on CO2 corrosion of X65 carbon steel. The test program, consisting of 50 tests, investigated the inhibition performance of a homologous series of quaternary ammonium surfactants with different alkyl chain length (C12, C14, C16) at different salinities (0.1%, 1%, 10% wt. NaCl), and temperatures (30°C, 50°C, 70°C). Corrosion rate as a function of inhibitor dosage was determined for each test condition defined by alkyl chain length, salinity, and temperature. The inhibitor dosages were proposed based on a series of proportional CMC (critical micelle concentration) values that were determined from a parallel study using a variety of other methodologies.
Based on measured corrosion rate data, the maximum surface coverage, and adsorption constants (Kads) fitted to the Langmuir adsorption isotherm were obtained. This paper discusses the variation of inhibition efficiency (maximum surface coverage) of quaternary ammonium surfactants with CMC, salinity, temperature and number of carbon of alkyl chain. It also discusses the relationship of adsorption constants as a function of CMC values.
Key words: CO2 corrosion, RCE, inhibitor, surfactant, quaternary amines, critical micelle concentration