ABSTRACT
Biocides and corrosion inhibitors can decrease corrosion in stagnant and flowing systems, like storage tanks and pipelines. We have used 1 ml syringe columns packed with 60 carbon steel beads (55 mg each), which were continuously injected with the effluent of an SRB continuous culture chemostat, to monitor corrosion under flow conditions. A constant flow rate of 0.5 ml/hr was maintained throughout. General corrosion rates (CRs) were determined after 45 days of flow by measuring the weight loss of acid-treated beads. Medium entering the chemostat contained sulfate (10 mM) and formate (20 mM) for the growth of SRB. Effluent of the chemostat with 5 mM sulfate, 5 mM sulfide and high numbers of SRB was then continuously injected into the syringe columns. CRs of beads in these columns were 0.1 mm/yr. Periodic biocide treatment (2 h of 300 ppm every 5 days at the same flow rate) decreased CRs on average by 60% for two of five biocides tested, indicating control of corrosion in the system. In contrast, a single exposure of the carbon steel beads to a water-dissolved corrosion inhibitor at the start of the experiment decreased CR by 50%, whereas single exposure to two diesel- dissolved corrosion inhibitors decreased CR by 90-98%.
INTRODUCTION
The annual cost of internal corrosion in the oil and gas industry is significant and affects both stagnant (tanks) and flowing systems (pipelines)1,2. The involvement of microorganisms in this process is commonly referred to as microbially-influenced corrosion (MIC)3. Sulfate-reducing bacteria (SRB) are well known MIC agents, although other bacteria, e.g. acetogenic bacteria or other acid producing bacteria (APB) or methanogenic archaea can also be involved4,5. SRB act by producing sulfide, which reacts with steel in a chemical MIC (CMIC) mechanism, or by using iron directly as electron donor for sulfate reduction in an electrical MIC (EMIC) mechanism5. Like other forms of corrosion MIC can be prevented by the use of corrosion inhibitors or by the application of electrochemical methods, such as cathodic protection6,7. In addition, biocides are often used to kill SRB and other MIC-causing microorganisms8. A problem in killing corrosive bacteria is that these can be free-floating or part of the biofilm-scale attached to the tank or pipeline wall9. These are commonly referred to as the planktonic and sessile populations. The latter are often more difficult to kill because the surrounding biofilm- scale may not be easily penetrable by biocide10. Treatment of stagnant systems is inherently easier than that of flowing systems. The volumes of fluid in the latter are large and because biocide is expensive one has to choose between a continuous low dose of biocide or a high dose of limited duration, often referred to as a slug8. Injecting slugs of biocide into a flowing system is considered most efficient, because lower concentrations may not penetrate the biofilm-scale or may lead to a biocide-resistant biofilm11.
