Microbiologically influenced corrosion (MIC) is one of the operational threats that could lead to catastrophes such as liquid/gas leaks and even explosions, especially in the oil and gas system. MIC is caused by microbial biofilms. Sulfate reducing bacteria (SRB) is one of the corrosive microorganisms commonly present in an anaerobic environment, e.g., in pipelines and reservoirs. Since sulfate is ubiquitous in these environments, SRB are almost unavoidable during operations. They are blamed for the majority of MIC cases. Other environmental microbes such as acid producing bacteria (APB) can also cause MIC. Biocides and physical scrubbing such as pipeline pigging operations are needed to mitigate biofilms and MIC.
In the field environment, microorganisms typically live in synergistic microbial biofilm communities. In enhanced oil recovery (EOR) operations, the hydrolyzed polyacrylamide (HPAM), one of the popular polymers in EOR polymer flooding, is added to increase the injected fluid's viscosity prior to pumping to the downhole oil reservoir. HPAM degrades as a result of multiple microbial species in the consortium. Bacillus cereus can utilize the amide group of HPAM as their nitrogen source, while SRB can hydrolyze the amide group to carboxyl for acquiring a carbon source.
Biocides and physical scrubbing such as pipeline pigging operations are needed to mitigate biofilms and MIC. Large-scale industrial biocide applications pose potential environmental problems when they are discharged. More efficacious biocide uses at lower dosages are desired. Biocide enhancers can be added to improve existing biocides. They can be biocidal or nonbiocidal. For example, a non-biocidal chemical that makes the cell wall more permeable to biocides or encourages biofilms to disperse can be used as a biocide enhancer.
D-limonene is known as a safe and biodegradable antimicrobial agent. It is considered safe because it is widely used in food preparations in the food industry. Its application for MIC treatment in oil and gas fields needs to be investigated. D-limonene was added to glutaraldehyde (a popular green biocide) in order to mitigate Consortium II (a mixed-culture biofilm isolated from an oilfield) MIC on C1018 carbon steel. The 200 ppm (w/w) D-limonene + 100 ppm glutaraldehyde cocktail treatment provided the extra sessile cell reductions of 1.1−log, 1.0−log, and 1.3−log reductions in SRB, APB, and general heterotrophic bacteria (GHB), respectively, in comparison with 100 ppm glutaraldehyde alone treatment in biofilm prevention testing in 125 mL anaerobic vials with 100 mL culture medium. As a consequence of the biofilm mitigation, the weight loss, pit depth, and Tafel analysis corrosion current density (icorr) from Tafel analysis reduction of 68%, 78%, and 79%, respectively, were achieved by the 200 ppm D-limonene +100 ppm glutaraldehyde treatment compared to 100 ppm glutaraldehyde treatment. A synergism parameter (S1) of 1.8 (> 1) was achieved by 100 ppm glutaraldehyde + 100 ppm D-limonene; therefore, the combination of D-limonene + glutaraldehyde enhanced MIC inhibition in comparison with the two individual biocides due to synergy. The linear polarization resistance (LPR) and the electrochemical impedance spectroscopy (EIS) electrochemical corrosion data in 450 mL anaerobic glass cells provided transient biocide treatment efficacy data trends and confirmed the cumulative weight loss data.
Desulfovibrio ferrophilus, a relatively new pure-strain SRB, is reported to be one of the lithotrophic organisms isolated from the marine environment. It is capable of fast extracellular electron transfer (EET), and thus causing fast EET-MIC against carbon steel. Biocide treatments using glutaraldehyde and tetrakis hydroxymethyl phosphonium sulfate (THPS) (another popular green biocide) were conducted to investigate their efficacies in biofilm prevention and corrosion prevention against D. ferrophilus on C1018 carbon steel in enriched artificial seawater (EASW). The results indicated that THPS was more effective than glutaraldehyde in treating D. ferrophilus MIC of carbon steel. The 100 ppm glutaraldehyde treatment yielded a sessile cell reduction of 0.3−log, while 1.8−log reduction was achieved by 100 ppm THPS in biofilm prevention testing compared to the untreated coupon in 125 mL anaerobic vials. The weight loss reductions of 66% and 86% compared to the untreated coupon were achieved by 100 ppm glutaraldehyde and 100 ppm THPS, respectively. The pit depth results indicated that 100 ppm glutaraldehyde and 100 ppm THPS led to 39% and 90% pit depth reduction, respectively in comparison with the untreated coupon. The LPR, EIS, and icorr from Tafel analysis electrochemical data in 450 mL glass cells supported the weight loss trend; therefore, the experiment proved that THPS was more effective than glutaraldehyde in treating D. ferrophilus MIC of carbon steel.
Peptide A, a 14−mer cyclic peptide (CSVPYDYNWYSNWC) containing a disulfide bond, with its main 12−mer sequence inspired by the Equinatoxin II protein in a sea anemone which has a biofilm-free exterior, is a promising biocide enhancer. It was reported that the THPS efficacy was enhanced by using a sub-ppm concentration of Peptide A in the mitigation of Desulfovibrio vulgaris in ATCC 1249 culture medium and oilfield Consortium II biofilm on C1018 carbon steel in EASW. This thesis investigated the Peptide A enhancement ability for THPS in the mitigation of D. ferrophilus MIC of stainless steel 410 (SS410). The biofilm prevention test results in 125 mL anaerobic vials at 28oC (optimal growth temperature) demonstrated that 100 ppb Peptide A enhanced 20 ppm THPS significantly. The combined treatment provided an extra 0.6−log reduction in sessile cells compared to the 20 THPS alone treatment. The maximum pit depth of 15 µm was found on the untreated coupon, followed by 8 µm for 20 ppm THPS treatment. No clear pitting was detected for 20 ppm THPS + 100 ppb Peptide A or 40 ppm THPS treatment. The 100 ppb Peptide A + 20 ppm THPS cocktail treatment achieved an additional 15% and 19% reductions in weight loss and icorr, respectively, compared with 20 ppm THPS alone treatment. LPR and EIS data confirmed this weight loss trend. The results demonstrated that a tiny concentration of 100 ppb Peptide A enhanced the efficacy of THPS in the mitigation of D. ferrophilus against SS410.
In electrochemical testing, the Tafel analysis of the potentiodynamic polarization (PDP) scans provides quantitative icorr data. In MIC, people automatically adopted the traditional PDP scan scheme used in abiotic corrosion, which is to scan continuously from the most negative voltage, usually −200 mV or −250 mV vs. OCP (open circuit potential), to the most positive (usually 200 mV or 250 mV vs. OCP) using a single working electrode. PDP scans are usually carried out at the end of incubation because of the concern that the wide-voltage scans could alter a coupon’s surface. Ideally, anodic Tafel slope (βa) and cathodic Tafel slope (βc) should be determined from the Tafel lines obtained using two replicate working electrodes, with each working electrode scanned only once starting from OCP. Because the half-scans start from OCP instead of the end voltage imposed externally, the biofilm has time to adapt to the imposition of the external voltage during the scan period. In this study, this “gold” standard (i.e., “orthodox”) scan scheme with minimum distortion was constructed from two independent replicate working electrodes. One was used for the anodic half-scan curve and another for the cathodic half-scan curve. The icorr result, obtained from the two half-scan curves after Tafel analysis, was used as the standard reference to calculate the deviation of icorr obtained from other scan schemes using a single working electrode for one-set of Tafel curves without the need for the second working electrode.
D. ferrophilus MIC on C1018 carbon steel in EASW was used as a model system in this Tafel scan scheme study. The results from 450 mL glass cells show that the icorr obtained by using a single working electrode for two half-scans: anodic and cathodic scans, had a maximum icorr deviation of 7.1% compared to the standard, which was practically negligible. Corrosion potential (Ecorr) was very close to OCP. In contrast, the single continuous Tafel scan from the most negative to the most positive voltage (i.e., traditional scan scheme with a single working electrode) provided a deviation of 88% in icorr compared to the reference. The reversed-direction continuous scan (from most positive to most negative) also caused a similarly large deviation in icorr. The distortion of PDP curve was found in both continuous scan schemes. The continuous scan from cathodic to anodic region resulted in the compression of the cathodic (first half of the scan) voltage range and stretching of the anodic voltage range, which moved Ecorr considerably lower from OCP, while the reversed-direction continuous scan caused compression in the anodic curve, and the cathodic curve was stretched with the Ecorr shifting considerably upward above OCP.
It was also found that the half scan scheme using a single working electrode did not alter the working electrode surface because back-to-back scans produced only a minor difference of up to 7.1% in icorr and a small difference in Ecorr. The results confirmed that continuous scans could alter a working electrode while half-scans starting from OCP did not, even though they all need only one working electrode. This means daily half-scans may be permissible.