INTRODUCTION
Film forming corrosion inhibitors are often selected to control CO2 corrosion and their effectiveness versus microbiologically influenced corrosion (MIC) is desirable in systems that suffer from both forms of corrosion. Traditional corrosion inhibitor tests (e.g., bubble tests) have unfavorable conditions for microbial activity and are inadequate for evaluating MIC control. Biocide screening test methods have been used to evaluate microbial kill with toxic chemicals added batch wise, providing very little direct information about controlling corrosion. Once-through flow cells containing corrosion coupons were inoculated with a field consortium enriched in synthetic produced water to simulate MIC field activity. Maximum pitting rate on the coupons was the key performance indicator for screening inhibitors. Results indicated that many of the corrosion inhibitors tested increased the maximum MIC pitting rates when compared to untreated controls. In at least one case, a less toxic inhibitor provided better MIC control than a more toxic inhibitor. Data suggest that the field microbial consortia used in the testing developed a resistance to an incumbent inhibitor that has been used for many years. The results indicate that inhibitor selection based on MIC control is not simply a function of their ability to control bacterial growth and activity.
Microbiologically Influenced Corrosion (MIC) and Under Deposit Corrosion (UDC) are prevailing corrosion mechanisms occurring in produced water lines. Such lines pose a dynamic environment where changes in nutrient sources (e.g., produced water, seawater, oil, treatment chemicals), as well as, physical conditions (e.g., temperature, flow regime) may induce shifts within the microbial community making the system more corrosive. The use of biocides to control biocorrosion and biofouling problems in produced water systems is well known. However, due to the complex nature of production lines, biocide applications can have highly variable effects from one environment to another. Quaternary amines and other organic film-forming inhibitors have been identified to work in synergy with biocides (e.g., glutaraldehyde) to afford increased effectiveness1. In addition, components of corrosion inhibitors often contain chemical moieties that will attenuate microbial growth and subsequent biofilm accumulation2. These chemical moieties are generally of the cationic/anionic type and include imidazolines, primary amines, diamines, amino-amines, oxyalkylated amines, fatty acids, dimer and trimer acids, napthaneic acids, phosphate esters and dodecyl benzene sulfonic acids. Their mechanism of action is to form a persistent monolayer film adsorbed at the metal/solution interface. While some of these chemical moieties are toxic to microbes, others are susceptible to microbial degradation. In the presence of microorganisms the alteration of the active molecules of the inhibitor can occur by specific mechanisms of microbial degradation 3, 4 potentially altering the performance of the corrosion inhibitor 5.
How corrosion inhibitors affect MIC are not well understood, in part, due to insufficient data characterizing the microbial biofilms involved in pitting. Our results indicate that some corrosion inhibitors may enhance MIC pitting by acting as a nutrient source, while other corrosion inhibitors inhibit MIC due to unknown mechanisms.
