Fate of Cr+6 from a Coating in an Electrolyte with Microorganisms

Under the laboratory conditions described, a microbial consortium of the fungus Aspergillus niger and bacteria influenced the corrosion-inhibiting effectiveness of hexavalent Cr (Cr+6) leached from chromate (CrO4-2)-containing coatings into artificial human urine (AHU). In the absence of microorganisms, migration of Cr+6 from coatings to corroding AA2024-T3 was demonstrated using an occluded cell and Raman spectroscopy. In the presence of the microorganisms, Cr+6 was removed from solution and was associated with the cell mass as Cr, but not Cr+6. Pitting in AA2024-T3 was suppressed by Cr+6 in exposures without microorganisms compared to severe pitting measured in the presence of microorganisms with the same initial Cr+6 concentration. Pit depth, polarization resistance, breakdown potential and passive current density were used to compare the consequences of biotic and abiotic AHU exposures with CrO4-2 -containing coatings and AA2024-T3. Results indicated that the presence or activities of A. niger and bacteria could decrease the effective lifetime of a CrO4-2-containing coating by increasing the rate of release and removing Cr+6 from solution. (C) 2012 The Electrochemical Society. [DOI: 10.1149/ 2.057211jes] All rights reserved.