CORROSION INHIBITION BY AEROBIC BIOFILMS ON SAE-1018 STEEL

Carbon steel (SAE 1018) samples were exposed to complex liquid media c ontaining either the aerobic bacterium Pseudomonas fragi or the facult ative anaerobe Escherichia coli DH5 alpha. Compared to sterile control s, mass loss was consistently 2- to 10-fold lower in the presence of t hese bacteria which produce a protective biofilm. Increasing the tempe rature from 23 degrees C to 30 degrees C resulted in a 2- to 5-fold de crease in corrosion inhibition with P. fragi whereas the same shift in temperature resulted in a 2-fold increase in corrosion inhibition wit h E. coli DH5 alpha. Corrosion observed with non-biofilm-forming Strep tomyces lividans TK24 was similar to that observed in sterile media. A dead biofilm, generated in situ by adding kanamycin to an established biofilm, did not protect the metal (corrosion rates were comparable t o those in the sterile control), and mass loss in cell-free, spent Lur ia-Bertani (LB) medium was similar to that in sterile medium. Confocal laser scanning microscopy analysis confirmed the presence of a biofil m consisting of live and dead cells embedded in a sparse glycocalyx ma trix. Mass-loss measurements were consistent with microscopic observat ions of the metal surface after 2 weeks of exposure, indicating that u niform corrosion occurred. The biofilm was also able to withstand mild agitation (60 rpm), provided that sufficient time was given for its d evelopment.