Correlating Physical Chemistry with Interfacial Properties: Effect of Salinity on the Partitioning, Distribution and Performance of a Quaternary Amine Corrosion Inhibitor

Corrosion inhibitors provide a cost effective method for internal corrosion control of carbon and low alloy steel infrastructure within the oil and gas industry. The correct selection and validation of inhibitors is essential to ensure successful, safe and reliable operation of infrastructure. Despite a consensus from operators and chemical suppliers that the fundamental properties dictating inhibitor performance are largely understood, the reality is that an extremely complex relationship exists between the physical properties of surfactants, the environmental solution chemistry and the surfactant adsorption characteristics/performance. To demonstrate the complexity of such interactions, the effect of salinity on the partitioning/distribution behavior of a model quaternary amine corrosion inhibitor molecule (alkyldimethylbenzylhexadecylammonium chloride, or BAC-C16) is explored, with the implications of such behavior determined in the context of interfacial adsorption and inhibition performance. Partitioning experiments were conducted at 30°C and pH 4 between toluene and sodium chloride (NaCl) brine (1:1 ratio) over varying salinities (0.1, 1 and 10 wt.%). The partitioning/distribution characteristics of BAC-C16 were firstly reviewed in the context of the surfactant micellization at each salinity through determination of the critical micelle concentration (CMC) using a lipophilic dye (Nile Red) method. The effect of BAC-C16 partitioning/distribution behavior on corrosion inhibitor performance was subsequently quantified using a series of rotating cylinder electrode (RCE) experiments, before discussing the implications of such observations in the context of field deployment and verification of corrosion inhibitors. The work demonstrates that marginal changes in the physical chemistry (salinity in this instance) can have significant consequences in terms of corrosion inhibitor performance, and consequently, asset integrity.