Abstract
In oil and gas production and transportation dissimilar materials are frequently combined for technical and economic reasons. However, the combination of dissimilar metals can result in corrosion due to galvanic or bimetallic effects. The driving force for this kind of corrosion is the difference in corrosion potentials of the individual materials in the given environment which causes galvanic currents the magnitude of which depends on the sum of all system resistances (Ohmic Law). Any assessment of the corrosion rates at bimetallic couplings needs information on the free corrosion potentials (rest potentials) of the bimetallic partners in the relevant medium.
Due to the lack of consistent literature data, a practical galvanic series was established in this work for standard oil country tubular goods (OCTG) steels, corrosion resistant alloys (CRA) as well as graphite (as gasket material) in a model sweet upstream environment. From polarization resistance measurements, information was obtained on the corrosion rate of the carbon steels in the absence of bimetallic coupling. Furthermore, the corrosion rates encountered at the partners of bimetallic couples were quantified via mass loss measurements. The results of this work allow better assessment of the severity of bimetallic corrosion and hence a more appropriate selection of materials which prevents corrosion failures, increases the integrity of assets, and saves costs.
