Abstract
Magnetite as the major part of black powder encountered in transmission pipelines all over the world can form galvanic elements with the pipeline steel, provided a liquid film with some ionic conductivity is present. This galvanic element can exist also in the absence of water, provided other compounds are present to assure some ionic conductivity, e.g. organic acids like formic acid, acetic acid, acetates, or ammonium compounds from inhibitor packages. In this galvanic element iron forms the anode and magnetite the surface for the cathodic reaction. The element currents are in the µA range and increase with increasing ionic conductivity, with increasing temperature, with increasing oxygen partial pressure, and specifically with the ratio of cathodic and anodic surface A pronounced increase of the element current was observed at oxygen partial pressures as low as 200 mbar in the gas. Element currents measured under pipeline relevant filming conditions can cause iron dissolution rates well over the target penetration rate of 0.1 mm/y.
