MICROBE-ENHANCED ENVIRONMENTAL FATIGUE-CRACK PROPAGATION IN HY-130 STEEL

Research was undertaken to characterize the effect of sulfate-reducing bacteria (SRB) on aqueous environment-enhanced fatigue cracking in a high-strength alloy steel Desulfovibrio vulgaris in Postgate C solutio n greatly increased rates of ambient-temperature fatigue crack propaga tion (FCP) in tempered martensitic HY130 steel (MIL-S-24371A) under ca thodic polarization and low-frequency, constant stress intensity range (DELTAK) loading. Crack growth rates (da/dN) in the SRB solution incr eased 50- to 1, 000-fold relative to FCP in sterile sodium chloride (N aCl) solution at -1,000 mV(SCE) and under vacuum, respectively. The pr esence of microbes shifted fatigue cracking from a transgranular path (typical in sterile NaCl) to an intergranular crack path consistent wi th the enhanced growth rates. The SRB reduced fatigue crack initiation resistance, countering the beneficial effect of cathodic polarization for sterile NaCl. Metal embrittlement and increased hydrogen uptake a t the occluded crack tip caused by bacterially produced hydrosulfide ( HS-) and sulfide (S2-) ions were implicated. SRB did not appear to col onize the occluded alkaline crack tip. At constant DELTAK transient en vironmental FCP in the SRB solution was substantial and was most likel y a result of time-dependent bacterial growth and enhanced metabolical ly produced sulfides. Other time-dependent hydrogen sources may have b een important