Corrosion of galvanised steel cord reinforcement in HDPE composite pipes in petroleum production

In this work, corrosion of galvanised steel cord reinforcement in high density polyethylene (HDPE) composite pipes was studied in electrolytes that developed on the cord surface due to permeation of corrosive species through HDPE liner or damage of the liner in petroleum production. Parametric effects, including solution pH, temperature and concentration of chloride ions, were investigated by electrochemical corrosion measurements. A model was developed to illustrate the mechanism of corrosion of the cord. Corrosion of the galvanised steel cord includes anodic dissolution of Zn coating and the steel substrate. A deposit layer of zinc carbonate and iron carbonate forms on the cord surface to provide protection to further corrosion in the environment. Corrosive ions can diffuse into holes or crevices of the cord assembly to cause corrosion of galvanised steel wires in inner layers. The unique geometry of the cord assembly results in an accelerating localised corrosion at the crevices and holes. Generally, corrosion of the cord is accelerated by increased chloride concentration, reduced solution pH, and elevated temperature.