AVS 59th Annual International Symposium and Exhibition
    Biofilms and Biofouling: Marine Medical Energy Focus Topic Thursday Sessions
       Session MB+BI-ThA

Invited Paper MB+BI-ThA1
The Role of Oxygen in Microbiologically Influenced Marine Corrosion

Thursday, November 1, 2012, 2:00 pm, Room 23

Session: Marine Biofouling
Presenter: B.J. Little, Naval Research Laboratory
Authors: B.J. Little, Naval Research Laboratory
J.S. Lee, Naval Research Laboratory
R.I. Ray, Naval Research Laboratory
Correspondent: Click to Email
Two microbiologically mediated processes dominate the literature on microbiologically influenced corrosion (MIC) in natural marine environments – ennoblement and sulfate reduction that leads to sulfide derivitization. Both are global phenomena and both depend on the presence of oxygen for aggressive attack. Marine biofilms cause a noble shift, or ennoblement, in corrosion potential (Ecorr) for most passive alloys. Ecorr ennoblement increases the probability for pitting and crevice corrosion initiation and propagation for those passive alloys where Ecorr is within a few hundred millivolts of the pitting potential (Epit) (e.g. 304L and 316L stainless steels). Numerous researchers have shown that increased cathodic oxygen reduction reaction (ORR) rates accompany ennoblement of Ecorr, but do not agree on a universal mechanism for acceleration of ORR by biofilms. The role of oxygen in accelerating marine sulfide influenced corrosion has not been precisely defined. Past experiments have demonstrated that dissolved oxygen (DO) in stagnant natural seawater (8 ppm) exposed to corroding carbon steel will be depleted to the detection limits of an electrochemical probe (100 ppb) within 48 h due to aerobic microbial respiration and corrosion reactions. Furthermore, most solid surfaces in contact with seawater are anaerobic because the rate of microbial respiration within a biofilm is greater than the rate of oxygen diffusion. Even in an oxygenated bulk environment, sulfate-reducing bacteria (SRB) dominate anaerobic niches in marine biofilms and produce aqueous sulfides that can derivitize some metals and alloys (e.g., carbon steel and copper-based alloys). In the absence of oxygen, corrosion will slow or cease as bare metal, surface oxides and metal ions are derivatized forming protective surface metal sulfides. Whereas in the presence of oxygen, the protective surface-bound sulfides are oxidized thus allowing more corrosion reactions can take place. In addition, introduction of oxygen (e.g., flow after stagnation) dramatically increases instantaneous corrosion rates owing to metal sulfides being more efficient oxygen reduction cathodes compared to metal oxides. The end result is deeper metal penetration when compared to strictly anaerobic environments. More recent experiments have demonstrated that even transient DO in the bulk medium can influence the microflora and corrosion rates of carbon steel. Using optical DO probes (detection limits 4 ppb) bulk concentrations of DO in the bulk medium are being correlated with corrosion rates and pit depths in carbon steel. Persistence of aerobic bacteria at ppb DO is being followed.