Inhibitor performance under liquid droplet impingement conditions in CO 2-containing environment

Abstract

Oil and gas production and transport facilities are often constructed from relatively cheap carbon steel and, usually, inhibitors are then applied to mitigate corrosion, CO 2 being a particular threat. A proper inhibitor formulation should reduce the corrosion rate by a factor of at least 10, under actual operating conditions. Severe corrosion may occur when film forming inhibitors are removed from the steel by high liquid shear forces. In gas production, impingement of liquid drolets carried in wet, CO 2-containing gas may cause such erosion-corrosion. Current knowledge about the mechanism of flow induced removal of such protective inhibitors films is limited. The performances of commercial inhibitors under impact conditions are generally unknown and appropriate test methods for establishing critical conditions are lacking. The API RP-14E formula is often used for estimating critical erosion-corrosion velocities in uninhibited corrosive systems, but this formula is not applicable to inhibited systems. This paper focuses at correlating droplet impact velocity with performances of inhibitor films. Impact is simulated by collision of rotating steel coupons, mounted on a disc, against a liquid jet. To avoid drying of the samples between two collisions, the samples are also continuously wetted during rotation. A marked difference in performance between two commercial inhibitors was found. One inhibitor gave protection outside, but severe corrosion within the impact zone. The other inhibitor gave exactly the opposite behaviour. The equipment was shown to be capable of demonstrating differences in inhibitor performance as function of type and concentration. Critical impact velocities have been obtained for different inhibitors. The performance of the inhibitors tested were shown to be temperature and concentration dependent. The potential of an inhibitor to regain its protective properties, after failure above a critical velocity, could also be studied. This appeared to be a function of temperature for the two inhibitors tested. In principal, the equipment may be used for establishing critical velocities for the onset of erosion-corrosion under liquid droplet impact conditions for corrosion product films.