Mechanism of electrochemical corrosion of carbon steel under deoxygenated water drop and sand deposit

In this work, electrochemical corrosion of X65 carbon steel under deoxygenated water drop and sand deposit simulating the corrosive environments potentially developed in diluted bitumen transmission pipelines was investigated by various electrochemical measurements and a localized electrochemical impedance spectroscopy technique. In the absence of oxygen, corrosion of steel under water drop occurs uniformly on the whole electrode surface at a quite low corrosion rate. This is attributed to formation of corrosion product such as ferrous hydroxide, which “passivate” the steel in the water drop. However, corrosion pits can be initiated under sand deposit. Mechanistically, the sands block transport of corrosion product into the bulk solution, causing the corrosion product to deposit. A galvanic effect is generated, where the edge of the corrosion product ring serves as an anode and the product-covered steel matrix as a cathode. The high electrochemical activity at the edge of the ring enhances preferential dissolution locally, resulting in formation of corrosion pits.