The Effects of Chloride Droplet Properties on the Underoil Corrosion of API X100 Pipeline Steel

The corrosive environment expected to form in diluted bitumen pipelines was explored by simulated exposure with a paraffin oil-covered chloride droplet on API X100 pipeline steel. The effects of droplet volume, chloride ion concentration, temperature, initial pH, and cation species on the underoil droplet corrosion behavior of API X100 pipeline steel were studied by corrosion morphology and product identification combined with corrosion penetration measurements. The corrosion rate in the active region beneath the oil-covered sodium chloride droplets was inversely proportional to droplet volume but increased with increasing temperature and chloride ion concentration. Corrosion attack morphology was found to be dependent on initial droplet pH. At pH 2, uniform corrosion occurred across the entire area exposed under the oil-covered droplet. The oil-covered sodium chloride droplets with initial pH of 4 accelerated the uniform corrosion when compared to the droplet without initial pH control (pH similar to 5.5). However, at a high initial pH of 10, two active regions displaying different general corrosion rates and one inactive region were observed under the oil-covered droplet. At an even higher initial pH of 12, no obvious uniform corrosion was observed beneath the oil-covered droplet. Finally, in the exposures to droplets with varied cation, the uniform corrosion in the active region was reduced by either calcium or magnesium ions in the oil-covered droplet.