Abstract
It is known that starved sulfate-reducing bacterial biofilms corrode carbon steel more aggressively because they use electrons from elemental iron oxidation as an alternative source of energy. This work used carbon source starvation to vary MIC (microbiologically influenced corrosion) severity for studying subsequent MIC impacts on the degradation of X80 carbon steel mechanical properties. X80 square coupons and dogbone coupons were immersed in ATCC 1249 culture medium (200 ml in 450-ml anaerobic bottles) inoculated with Desulfovibrio vulgaris for 3-day pre-growth and then for an additional 14 days in fresh media with adjusted carbon source levels for starvation testing. After the starvation test, the sessile cell counts (cells/cm2) on the dogbone coupons in the bottles with carbon source levels of 0, 10, 50, and 100% (vs that in the full-strength medium) were 8.1 × 106, 3.2 × 107, 8.3 × 107, and 1.3 × 108, respectively. The pit depths from the X80 dogbone coupons were 1.9 μm (0%), 4.9 μm (10%), 9.1 μm (50%), and 6.4 μm (100%). The corresponding weight losses (mg/cm2) from the square coupons were 1.9 (0%), 3.3 (10%), 4.4 (50%), and 3.7 (100%). The 50% carbon source level had the combination of carbon starvation without suffering too much sessile cell loss. Thus, both its pit depth and weight loss were the highest. The electrochemical tests corroborated the pit depth and weight loss trends. The tensile tests of the dogbone coupons after the starvation incubation indicated that sulfate-reducing bacteria (SRB) made X80 more brittle and weaker. Compared with the fresh (no-SRB-exposure) X80 dogbone coupon’s ultimate tensile strain of 13.6% and ultimate tensile stress of 860 MPa, the 50% carbon source level led to the lowest ultimate tensile strain of 10.3% (24% loss when compared with the fresh dogbone) and ultimate tensile stress of 672 MPa (22% loss). The 100% carbon source level had a smaller loss in ultimate tensile strain than the 50% carbon source level, followed by 10% and then 0%. Moreover, the 100% carbon source level had a smaller loss in ultimate tensile strength than the 50%, followed by 10% and 0% in a tie. This outcome shows that even in the 17-day short-term test, significant degradation of the mechanical properties occurred and more severe MIC pitting caused more severe degradation.
Highlights
Sulfate-reducing bacteria (SRB) are the most ubiquitous microorganisms in anaerobic environments, and they are the most important microbes in MIC in the oil and gas systems which are mostly anaerobic and often involve seawater containing sulfate (Little and Lee, 2007; Sheng et al, 2007; Xu et al, 2016; Jia et al, 2017; Jogdeo et al, 2017)
When lactate is used as the organic carbon source, the following oxidation reaction occurs in the cytoplasm of the sulfate-reducing bacteria (SRB) under enzyme catalysis (Xu and Gu, 2011; Xu and Gu, 2014; Xu et al, 2016; Li et al, 2018; Dou et al, 2019; Gu et al, 2019): TABLE 1 | Elemental composition of X80 steel
Morphologies of MIC pits on the dogbone coupons after the 14-day carbon starvation incubation with biofilms and with corrosion products removed were examined under the Infinite Focus Microscope (IFM) before tensile testing (Figure 12)
Summary
Sulfate-reducing bacteria (SRB) are the most ubiquitous microorganisms in anaerobic environments, and they are the most important microbes in MIC (microbiologically influenced corrosion) in the oil and gas systems which are mostly anaerobic and often involve seawater containing sulfate (Little and Lee, 2007; Sheng et al, 2007; Xu et al, 2016; Jia et al, 2017; Jogdeo et al, 2017). Small X80 square coupons in anaerobic bottles were used for weight loss analysis after the starvation testing. The square coupons were cleaned with fresh Clarke’s solution to remove biofilms and corrosion products before weighing to obtain the weight loss data.
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