Abstract
Chemical corrosion inhibition of steel in CO2 and/or H2S environments under high flow rates, hence high shear stresses, has long been a puzzle from an mechanistic point of view. Certain misconceptions had to be removed first before progress could be made. It had been believed that if inhibitors adsorb on the metal or corrosion product surface the resulting corrosion rate would only be dependent on concentration and not on flow rate. It had furthermore been a general belief that above a certain critical fluid velocity corrosion inhibition would no longer be possible. Systematic progress, however, had shown that there existed a relationship between fluid velocity and inhibitor concentration for equal corrosion rate, thereby opening the possibilities of corrosion inhibition at ever higher flow rates, albeit with higher inhibitor concentrations. High flow rates, however, lead to flow induced localized corrosion (FILC) which was shown to be due to the destruction of protective surface layers (corrosion products or combinations of it with inhibitor). It had been calculated that the destruction of such layers would require shear forces of the order of MPa (106Pa), while measurable wall shear stresses even in highly turbulent flow range only in the order of Pa and, hence, are by far too small to be the right parameter to quantify the hydrodynamic forces responsible for initiation of FILC. It has now been shown on the basis of theoretical calculations and measurements using micro technologies that wavelets of such magnitude could indeed occur. Near-wall „freak waves" are postulated with energies large enough to break corrosion product layers. It has then been shown that the energy contained in such freak waves can be greatly attenuated by chemicals, such as surfactants and corrosion inhibitors, hence providing a solid foundation for the understanding of inhibition of FILC under most severe conditions.
