The effect of sulphate-reducing bacteria (SRB) and carbon dioxide (CO2) on the general and localized corrosion of carbon steel in synthetic produced water (SPW) was investigated. The synthetic produced water was saturated with various concentrations of CO2. Experiments were carried out in a rotating cage apparatus using a mixed culture of sulphate-reducing bacteria obtained from an oil field in Venezuela. During the experiments the pH, planktonic bacterial count, and concentrations of sulphide, sulphate, iron, calcium and magnesium ions in the solution were measured. After the experiment, the sessile bacteria count and mass loss of the carbon steel test coupons were determined, the corrosion products on the metal surface were identified using X-ray diffraction, and the corrosion morphology was determined using optical microscopy, scanning electron microscopy and laser profilometry. Both general and localized pitting corrosion rates of carbon steel were found to nearly double in solutions containing 10% CO2 and 10% SRB compared to solutions containing either CO2 or SRB alone (synergistic effect). Higher CO2 concentrations killed both sessile and planktonic SRB; under these conditions the general and localized corrosion rates were determined by CO2 concentration (sweet corrosion).
Studies have indicated that the annual costs due to corrosion may vary between 1 to 5% of a country’s GDP, and that 15% to 30% of these costs may be associated with microbiologically induced corrosion (MIC).1-5 Internal corrosion, especially in the form of localized pitting corrosion, is one of the predominant failure mechanisms of oil and gas production facilities.
A sample of mixed culture of SRB was obtained from the produced water from an oil field in western Venezuela. Several inocula were prepared in Postgate B medium (PGB) containing 16000 mg/L of chlorides according to procedures described in NACE TM0194.
