Inhibition of corrosion of steels by self-assembled-monolayers of caprylate

The interaction of a sodium salt of octanoic acid, sodium caprylate (SC), with a carbon steel (CS) and stainless steel (SS) surface was investigated over a wide pH and temperature range, using a range of experimental techniques. The aim of the research was to investigate the possibility of using SC as a corrosion inhibitor, and to determine the mechanism of its corrosion inhibition.It was shown that SC acts as a good CS and SS general and pitting corrosion inhibitor, respectively, yielding a maximum corrosion inhibition efficiency of 97% at pH 2. This high inhibition efficiency is maintained even at higher temperatures. Kinetic measurements demonstrated that inhibition efficiency of ca. 93% is achieved within fifteen minutes after the immersion of the CS surface in the SC solution, while complete equilibrium, and thus maximum efficiency, is achieved after one hour. It was determined that SC inhibits both partial corrosion reactions, and can thus be considered to be a mixed-type inhibitor. The corrosion inhibition efficiency was shown to be directly proportional to the surface coverage by SC. The adsorption of SC on the CS surface at low pH was described by the Langmuir adsorption isotherm. It was found that this process is spontaneous, irreversible and driven by the entropy gain.It was postulated that SC forms a self-assembled-monolayer (SAM) on the steel surface. In an aqueous solution, a bi-layer structure is formed. The inner SAM layer is irreversibly attached to the CS surface through the carboxylate head group, with the hydrophobic tail oriented towards the aqueous phase. The outer SAM is physiosorbed on top of the inner SAM, through hydrophobic interactions of the two SAM tails. In this configuration, the bi-layer structure represents an effective barrier for the diffusion of charged (corrosive) species and water molecules to the metal surface, thus protecting the metal from corrosion