2018 Volume 67 Issue 3 Pages 127-131
In seawater, the corrosion rate of iron increases as the flow velocity increases. If a flow velocity distribution exists, a macro-cell is formed due to the differential flow velocity, by which corrosion in low flow velocity areas is promoted, while it is suppressed in high flow velocity areas. As a result, the corrosion rate of the former may be higher than that of the latter. This phenomenon is called “differential flow velocity cell corrosion”, and is regarded as a form of “differential aeration cell corrosion”（“differential oxygen concentration cell corrosion”）. The author considered the mechanism of “differential flow velocity cell corrosion” by examining the flow velocity dependency of the internal anodic polarization curve. A corrosion test on cast iron was conducted in seawater by changing the flow velocity and potential, to obtain an internal anodic polarization curve based on the corrosion rate. It was confirmed that the internal anodic polarization curve moved to the noble side （low current density side） as the flow velocity increased. Consequently, we experimentally verified a hypothesis developed to explain the mechanism of “differential aeration cell corrosion” and “differential flow velocity cell corrosion” of cast iron and steel.
