Abstract

The combination of dissimilar metals can result in corrosion due to galvanic effects. These are caused by differences in their electrode potentials. The spread difference between these potentials is the driving force for galvanic corrosion. Literature data usually refer to either standard conditions or aerated sea water at ambient temperature. However, the actual electrode potentials are influenced by a variety of factors such as temperature, pH, dissolved gases and salts, flow rates, scale formation, etc. For conditions encountered in the oil and gas industry, there are hardly any data about electrode potentials available. The aim of this work is to create a practical galvanic series under typical upstream conditions for standard oil country tubular goods (OCTG) steels, corrosion resistant alloys (CRA) as well as the gasket material graphite.

The following materials were selected for the tests, which are considered as representative for downhole and topsides equipment: J55, L80 Type 1, X52, Cr13, 316Ti, 2205, 904L and Graphite.

The electrode potentials and polarization resistances of the selected materials were measured in synthetic formation water with dissolved carbon dioxide (CO2) at temperatures between 22 and 130 °C. To account for the influence of bicarbonate and acetic acid these species were added in some of the tests.

In order to also identify overpotential effects the electric galvanic currents were determined between coupled dissimilar materials in the described media at different temperatures by electrochemical noise measurements (CoulCount-Method). For comparison, actual corrosion rates of dissimilar material couples were measured gravimetrically.

The results exhibit the influences of temperature, bicarbonate and acetic acid on the electrode potential of each material. Galvanic corrosion effects are observed between CRAs and OCTG. The highest galvanic corrosion occurs between graphite and OCTG and is also distinct between graphite and CRAs. Temperature has a strong influence on the galvanic corrosion. At high temperatures, galvanic corrosion effects occur also in couples of different CRAs.

The results show a good agreement between the measured electrode potentials, the results of electrochemical noise measurements and the gravimetric corrosion tests.

This work provides a practical galvanic series for materials and conditions that are usually encountered in the upstream oil and gas industry. Based on the results of the work presented it is possible to rate the galvanic material behavior under similar conditions. Being one of the first publications in this field of application, it represents a reference for material selection, engineering and failure analysis.

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