Awareness of microbiologically influenced corrosion (MIC), which threatens assets in the marine, oil and gas, water utilities, power generation, and various other industries, is growing. At least 20% of all corrosion losses can be attributed to MIC. MIC can also cause mechanical property degradation, resulting in metal fracturing, rupturing, collapsing, and cracking that reduce equipment service life and threaten safety. Sulfate reducing bacteria (SRB) such as Desulfovibrio vulgaris and Desulfovibrio ferrophilus (strain IS5) are a major type of microbe that cause MIC. The latter is several times more corrosive. SRB are anaerobic bacteria that can use sulfate as the terminal electron acceptor in their respiration. Unlike soluble organic carbon, elemental iron releases electrons extracellularly, and then the electrons are used in sulfate reduction inside the SRB cytoplasm under bio-catalysis, which requires extracellular electron transfer (EET). Thus, this type of MIC is labeled as “EET-MIC”, which is the result of the demand for energy by sessile cells in biofilms that can perform EET. In practical applications, mechanical property degradation and stress corrosion cracking (SCC) caused by MIC can result in disastrous consequences such as pipeline ruptures and support beam collapses. In the past, most studies on MIC only investigated the MIC mechanisms that lead to pinhole leaks. The effects of microbes and MIC on mechanical property degradation and SCC are also important, if not more. In this work X80 carbon steel was used as an example of pipeline steel.The following topics and results are reported in this dissertation:(1) The relationship between tensile stress and D. vulgaris MIC was explored to study biotic SCC. In the presence of an applied tensile stress most pronounced in the outer bottom of an X80 U-bend, D. vulgaris MIC initiated crack formation in the ATCC 1249 culture medium at 37oC in an anaerobic bottle. The biotic corrosion weight loss of the X80 U-bend coupon was found to be lower than that of the X80 square coupon after a 14-d SRB incubation, which was supported by electrochemical measurements. Moreover, after a 6-week incubation in the presence of D. vulgaris, a pre-cracked U-bend coupon caused SCC failure. (2) D. ferrophilus grown in enriched artificial seawater (EASW) at 28oC and D. vulgaris grown in ATCC 1249 culture medium at 37oC were used to investigate the MIC impact on X80 U-bend (with stress) and square (without stress) coupons. The weight loss and pit depth of the X80 U-bend coupon were larger than those of the X80 square coupon, suggesting that the mechanical stress accelerated the D. ferrophilus MIC for X80 steel. After a 14-d incubation, D. vulgaris caused significant cracks, whereas D. ferrophilus did not. This was attributed to the fact that the much faster corrosion rate caused by D. ferrophilus prevented or dissolved crack tips, whereas the much slower D. vulgaris MIC rate did not. For the first time, the abiotic SCC phenomenon that fast corrosion does not necessarily cause more severe SCC has been expanded to biotic SCC. (3) Carbon source starvation was used to vary MIC severity for studying microbial degradation of X80 carbon steel mechanical properties. Pre-grown biofilms (after 3-d of D. vulgaris incubation) on X80 coupons were moved to new ATCC 1249 culture media with varied caron source levels (with the original strength being 100% carbon source) for additional 14-d of incubation. After the 14-d starvation incubation, the sequence of sessile cell count (cells cm-2) on dogbone coupons in the anaerobic bottles was 0% < 10% < 50% < 100% of carbon source levels. The sequence of weight loss and pit depth on X80 dogbone coupons were both 0% < 10% < 100% < 50%. The 50% carbon source level resulted in carbon source starvation but without suffering excessive sessile cell loss. Thus, its weight loss and pit depth were both the highest. In addition, the results showed that even in the 17-d short-term test, mechanical properties were significantly degraded, and more severe MIC pitting caused more severe degradation in terms of ultimate strength and ultimate strain losses, which were 22% and 24%, respectively for 50% carbon source starvation compared to the fresh dogbone coupons. (4) X80 pipeline steel dogbone coupons and square coupons were immersed in 150 mL D. vulgaris broths for 14 d to test the impact of headspace volume change on X80 mechanical properties degradation. The headspace volumes in the anaerobic bottles were increased from 150 mL to 200 mL and 300 mL to increase MIC severity. After the 14-d of SRB incubation, the sessile cell counts were 6.5 × 107 cells cm−2 for 150 mL, 2.3 × 108 cells cm−2 for 200 mL, and 1.4 × 109 cells cm−2 for 300 mL headspace volumes, respectively, owing to reduce the H2S cytotoxicity in the broth with a larger headspace because it allowed more biogenic H2S to escape from the broth. The sessile cell count results were in agreement with the weight loss results of 1.7 mg cm−2, 1.9 mg cm−2 and 2.3 mg cm−2 for 150 mL, 200 mL, and 300 mL headspace volumes, respectively. Moreover, the results show that more severe MIC pitting led to a higher ultimate strain loss of up to 23% (300 mL headspace) compared to the abiotic control dogbone, which means SRB MIC made the X80 dogbone less ductile. The ultimate strength losses for all headspace volumes were relatively small (3% and lower). (5) The D. ferrophilus MIC rate of X80 carbon steel was investigated with different incubation times. The corrosion rate of the coupons was the fastest for the first 7-d incubation. For 14 d and 21 d, the corrosivity of D. ferrophilus gradually decreased due to sessile cell starvation loss, resulting in corrosion rate decreasing. The ultimate strength and strain degraded noticeably with increasing incubation time. After the 21-d incubation, the weight loss was 24.8 mg cm−2 with a pit depth of 30.6 μm, which resulted in the ultimate strength and ultimate strain losses of 9% and 18%, respectively compared to the fresh X80 dogbone coupon without incubation.(6) Tetrakis hydroxymethyl phosphonium sulfate (THPS), a green biocide, was used to mitigate the MIC caused by D. ferrophilus in EASW for the purpose of reducing SRB degradation of X80 mechanical properties. The weight loss of X80 square coupons decreased from 19.5 mg cm−2 for the no treatment control to 1.1 mg cm−2 with 100 ppm (w/w) THPS in EASW after 7 d of incubation. There were clear differences in pit depth and pit (surface) diameter of no treatment (maximum pit depth of 13.5 µm and maximum pit diameter of 36.7 µm) and with 100 ppm THPS treatment (no apparent corrosion pit). The sessile cell count and planktonic cell count with 100 ppm THPS mitigation declined by 3-log and 4-log, respectively compared with the untreated control. After the 7-d SRB incubation, 100 ppm THPS mitigation led to lower ultimate strength loss (0% vs. 6%) and ultimate strain loss (3% vs. 13%) compared to the abiotic control. This work demonstrated that mitigating SRB led to MIC mitigation and also alleviation in microbial degradation of mechanical properties.