Characterization of Biofilm Formation and Coating Degradation By Electrochemical Impedance Spectroscopy
Authors: Permeh Samanbar , Lau Kingsley , Duncan Matthew 
Published: February 2020

Recent findings showed severe localized corrosion of submerged steel bridge piles in a Florida bridge and was associated with microbial activity in presence of marine foulers (Permeh et al, 2017;2018). Microbiologically Influenced Corrosion (MIC) can cause severe degradation of civil infrastructure. MIC has often associated with the formation of biofilm (including the complex interaction of microbe communities and development of extracellular polymeric substances (EPS)) that can influence the corrosion process either by creating oxygen differential aeration cells or generating acidic substances and cathodic reactant depending on the type of bacteria. Sulfate reducing bacteria (SRB) has received much attention in the study of MIC of steel developed in natural waters (Melchers and Wells, 2006; Castaneda and Benetton, 2008). Reactions associated with SRB causes cathodic depolarization of steel leading to the reduction of sulfates to sulfides and promoting steel corrosion (Borenstein,1994).
Coatings have been developed to mitigate MIC and marine fouling. Studies have shown that coating blistering and disbondment can occur as a result of microbial attack due to the production of metabolites that degrade coating chemical and physical properties (Mansfield et al.,1998; Muntasser et al.,2002). Recent work evaluated the corrosion mitigation properties of a commercially-available, water-based, self-polishing, antifouling coating in environments that supported SRB as part of a larger research program to identify susceptibility of steel degradation in natural waters due to MIC (Permeh et al.,2019). In the work described here electrochemical impedance spectroscopy (EIS) was conducted to identify microbial activity and degradation of an antifouling coating.
EIS was conducted on steel coupons coated with an antifouling coating (with and without coating defects) exposed to SRB inoculated modified Postgate B solution. The exposed coating surface area was ~19.6 cm2 and the coating thickness was ~0.17-0.35 mm. A three-electrode configuration was used for EIS measurements where the coated steel plate was the working electrode, an activated titanium rod was used as a reference electrode, and an activated titanium mesh was used as a counter electrode. EIS measurements were made at OCP condition with 10 mV a.c. perturbation voltage and at frequencies 1Mhz>f>1Hz.
The measurements resulted in impedance with multiple loops in the Nyquist diagram (as shown in Figure 1) associated with processes relating to the polymeric coating, development of surface layers (biofilm), and the steel interface. Fitting of the impedance response to equivalent circuit analogs were made to identify coating characteristics and surface layer formation during the experimental exposure in inoculated and non-inoculated solutions. The results revealed near-ideal capacitance for the polymeric coating and a decrease in coating pore resistance. In samples inoculated with SRB, a second high frequency loop developed indicating formation of surface films. The results from EIS indicated degradation of the coating due to its self-polishing characteristics and that formation of surface layers associated with SRB can form as biocide components of the coating become depleted. Continued SRB growth may allow development of marine fouling if the antifouling coating components become less effective and allow possible shelter for SRB to promote MIC. Further work to verify these characteristics in field settings are in progress.
References

Borenstein, S. (1994). Microbiologically influenced corrosion handbook. Elsevier.

Castaneda, H., & Benetton, X. D. (2008). SRB-biofilm influence in active corrosion sites formed at the steel-electrolyte interface when exposed to artificial seawater conditions. Corrosion Science, 50(4), 1169-1183.

Mansfield, F., Lee, C. C., Han, L. T., Zhang, G., & Little, B. (1998). The Impact of Microbiologically Influenced Corrosion on Protective Polymer Coatings. University of Southern California Los Angeles Dept of Materials Science And Engineering.

Melchers, R. E., & Wells, T. (2006). Models for the anaerobic phases of marine immersion corrosion. Corrosion Science, 48(7), 1791-1811.

Muntasser, Z., Al-Darbi, M., Tango, M., & Islam, M. R. (2002, January). Prevention of microbiologically influenced corrosion using coatings. In CORROSION 2002. NACE International.

Permeh, S., Boan, M. E., Tansel, B., Lau, K., & Duncan, M. (2019, February). Update on Mitigation of MIC of Steel in a Marine Environment with Coatings. In Coatings+, SSPC 2019.

Permeh, S., Li, B., Boan, M. E., Tansel, B., Lau, K., & Duncan, M. (2018, July). Microbially Influenced Steel Corrosion with Crevice Conditions in Natural Water. In CORROSION 2018. NACE International.

Permeh, S., Reid, C., Echeverría Boan, M., Lau, K., Tansel, B., Duncan, M., & Lasa, I. (2017, April). Microbiological Influenced Corrosion (MIC) In Florida Marine Environment: A Case Study. In CORROSION 2017. NACE International.

Figure 1