Materials Performance

DEC 2014

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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analyses for field use where samples are collected, Eckert says. Various testing methods are used to determine microbiological conditions, such as the species of microorganisms present in a sample, the total numbers of live bacteria/Archaea, and comparisons of the population size of various abundant microorganisms. These methods include microbiological culture techniques; microscopy; measurement of bacteria- produced enzymes, such as hydrogenase, an enzyme produced by bacteria that use hydrogen as an energy source, and adenos- ine phosphosulfate (APS) reductase, an enzyme specifically associated with SRB; and molecular microbiological methods (MMMs), which are genetic-based testing methods. Many times, MIC is caused by the activities of several different organ- isms that form a community, and pipeline analysis has typically included testing for SRB, APB, general aerobic and anaerobic microorganisms, and, in some cases, iron- depositing and iron-reducing bacteria. When using microbiological culture techniques, the objective is to approxi- mate the size of the viable microorganism population in a solid or liquid sample using semi-quantitative estimates, or preferably, the most probable number (MPN) method. 3 Although the culture technique is well established in the oil and gas industry, comments Eckert, one drawback is the types of microorganisms that can be cultured in liquid media may only represent a small percentage of those present in the solid or fluid because some microorganisms, such as Archaea, are very difficult to cultivate using this method. Microscopy is frequently used to determine the overall numbers of micro- organisms present in liquid or sludge samples directly without regard to their species and has been utilized as an analysis method for many years. Epifluorescent microscopy is a technique that helps differentiate microorganisms from debris, and also can be used to examine micro- organisms' specific cellular structures. Samples are treated with a stain that fluoresces when viewed under a specific wavelength of ultraviolet (UV) light. Using energy dispersive spectroscopy (EDS) to examine samples provides information on the elemental composition of materi- als (e.g., scale and corrosion products) and the distribution of the constituents in the sample. Analysis with x-ray diffrac- tion (XRD) identifies the mineralogical compounds present. For example, Eckert says, EDS would reveal that carbon and oxygen are present in the sample, while XRD would show that iron carbonate— typically a corrosion product—is there as well. Methods such as gas chroma- tography-mass spectrometry (GC-MS) or high-pressure liquid chromatography (HPLC) are used in the laboratory to deter- mine if any organic acids are present in the sample that may be a byproduct of bacte- rial activity. Additionally, testing for the presence of hydrogenase, an enzyme produced by bacteria that use hydrogen as an energy source, is a method used to approxi- mate bacteria populations in corrosion deposits and water samples in the field. Measurement of APS reductase, an enzyme specifically associated with SRB, provides an indication of the active SRB concentra- tion present in a bacterial sample. Genetic methods have recently become increasingly available for detecting, quantifying, and identifying microorgan- isms present at corrosion sites. MMMs are culture-independent approaches that analyze samples directly by measuring the genetic material (DNA/RNA) they contain. Because microorganism growth prior to testing is not required, MMMs need only very small samples, with or without live bacteria, for analysis. Once genetic materials are extracted from the sample, assays are performed in the laboratory, which render more precise quantification of various types of bacteria present than culture tests. Eckert notes that two MMMs, quantita- tive polymerase chain reaction (qPCR) and denaturing gradient gel electrophoresis (DGGE), are often used in combination to analyze genetic samples. The qPCR method, a method that is gaining momentum for determining the identity of microorganisms in complex environmental samples, applies a modified PCR to replicate a single or several pieces of DNA across several orders of magnitude so that millions of copies of particular DNA sequences are generated. The qPCR method measures living, inactive, and dead microorganisms and may be used to count the total number of micro- organisms or a specific genus/species of microorganism in nearly any type of sample, including produced fluids, oils/emulsions, and corrosion products. This method uses synthetic DNA tagged with a fluorescent molecule to identify specific targets (micro- organisms of interest [e.g., SRB]) because of their potential influence on corrosion mechanisms. DGGE compares microbial communi- ties across a number of different samples. With DGGE, genetic material in individual samples is amplified by PCR and subse- quently compared by sorting molecules based on their reaction to an electric field (electrophoresis). This MMM is used for identifying dominant groups of micro- organisms in individual samples and for evaluating how the types of microorgan- isms are distributed between samples. Another emerging approach, next- generation sequencing (NGS), enables identification of all the microorganisms in a sample rather than specific targeted species. The approach will not provide specific numbers of particular organisms in the sample, Eckert explains, but instead it delivers a snapshot of the diverse micro- bial population and how it is distributed in the sample. Corrosion Monitoring Because MIC cannot be diagnosed solely with microbiological data, the best approach to determining the presence of internal MIC in a pipeline is to integrate other applicable pipeline and corrosion data with microbiological test results. These data include operating conditions (flow rate, temperature, pressure, pigging frequency, and chemical treatment injection rates); chemical composition of the gases, liquids, and solids transported; data from corrosion coupons and probes that show the rates of FEATURE ARTICLE 30 DECEMBER 2014 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 53, NO. 12

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