Internal cathodic protection to study the erosion-corrosion of AISI 1018 carbon steel

Steel elbows used in pipelines carrying potash slurry may undergo material loss due to mechanical erosion and/or electrochemical corrosion. Cathodic protection was used to study the effects of flow velocity and slurry concentration on the protection current density required to maintain AISI 1018 carbon steel elbows at a set electric potential. A flow loop with a peristaltic pump was used to pump a slurry consisting of silica sand in a saturated potash brine through the steel elbows. The protection current density measurements were performed under varying slurry flow velocities of 2.5 m/s, 3.0 m/s, 3.5 m/s, and 4.0 m/s and varying slurry concentrations ranging from 0-35 wt.%. The results show maximum values of protection current density were required for mid-range slurry concentrations. It is concluded that, within the parameters of study, high flow velocities require higher protection current density than low flow velocities. Furthermore, high flow velocities result in a local maximum protection current density being reached at lower slurry concentrations; conversely, a local maximum protection current density is reached at higher slurry concentrations in lower flow velocities. The contrast in local maximum protection current densities likely occurs due to additional particle-particle collisions at higher velocities causing the maximum mass transfer rate of oxygen to be reached at lower slurry concentrations. SEM micrographs show that wear becomes more evenly distributed with increasing flow velocity due to the homogeneous distribution of particles in the flowing medium. (author)