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M AT E R I A L S S E L E C T I O N & D E S I G N FIguRE 2 temperature, but 37 °C is likely suffciently high for pipelines in a shallow seabed in a hot climate. This means increased SRB growth with increasing temperature is generally expected in practical situations. It should be pointed out that planktonic cell counts may be used to help indicate the sessile cell health in laboratory tests, but the planktonic cell counts should not be used to correlate with sessile cell counts. Microbial Growth No microbial growth was detected after one month and six months in vials containing untreated GoM seawater. After cleaning with Clark's solution, SEM images showed roughness on the entire surface of a coupon with one-month exposure to the seawater in a vial at 37 °C, and also of a coupon with six-month exposure at 25 °C. Due to lack of microbial activities and a hydrogen sulfde (H2S) smell at the end of the test, the roughness was likely not caused by SRB. Similar roughness was also observed in tests using heat-sterilized GoM seawater. SEM images (a) before acid cleaning, (b) after acid cleaning, and (c) the EDS Quarrayah Seawater The Qurrayah seawater from the Persian Gulf is much saltier than the GoM seawater, as seen in Table 2. Inhouse quantitative PCR analysis did not detect SRB in the Qurrayah seawater. Figures 2(a1) and (a2) show that a mineral layer covered the coupon surface after a three-month exposure at 37 °C. Figures 2(b1) and (b2) show scattered pits after the coupon surfaces were cleaned. They were likely due to factors such as a trace amount of oxygen leaking through the capped rubber septum rather than microbial activities. Oxygen leakage was not a problem, however, in the tests for vials that were a few weeks long. Some threemonth vials were discarded in tests because of visible oxygen rust. A wax seal around the aluminum cap was subseNACE International, Vol. 52, No. 5 analysis before acid cleaning for a coupon after three-month exposure to the untreated Qurrayah seawater at 37 °C. quently used but it did not completely eliminate oxygen egress. An anaerobic chamber would be the last solution other than using an oxygen scavenger to prevent oxygen egress in long-term tests. The EDS analysis of the surface in Figure 2(c) indicates the absence of the sulfur element, which means that SRB activity was likely absent. Due to the lack of native viable microbes and the lack of nutrients, no MIC pitting was observed in untreated seawater samples. To simulate a contaminated hydrotest fuid and to speed up laboratory testing, worst-case scenario tests were carried out by enriching seawater samples and spiking them with the laboratory SRB strain. Figure 3(a) shows the SEM image of the bioflm on a one-week old coupon. Kidney bean-shaped SRB cells are clearly visible. Pits characteristic of MIC attack were revealed after acid cleaning of the coupon surface, as seen in Figures 3(b1) and (b2). An EDS analysis shown in Figure 3(c) indicates the presence of iron sulfde (FeS). Kinetics-Based Mechanistic Model Recently, Gu, et al.13 introduced an electrochemical kinetics-based mechanistic model for MIC using a new biocatalytic cathodic sulfate reduction (BCSR) theory. It assumes that a corrosive SRB May 2013 MATERIALS PERFORMANCE 67