Application of EIS to Assess Microbiologically Influenced Steel Corrosion in Marine Fouling Crevice Conditions

Abstract

Microbiologically influenced corrosion (MIC) can cause severe degradation of civil infrastructure. Recently, severe localized corrosion of steel H-piles submerged in a brackish natural water in a Florida bridge was associated with MIC and fouling (Permeh et al., 2017). The localized corrosion cells/pits were of up to 3" in diameter and penetrated through the steel thickness. Microbiological and chemical analysis of the water samples showed a high population of sulfate reducing bacteria (SRB) and a high concentration of sulfate ions amongst other nutrients. The steel piles had noticeable heavy marine growth which were thought to have an effect on the corrosion process by supporting biofilm development, creating localized corrosion and differential aeration cells (Little et al, 2007). In the work described here electrochemical impedance spectroscopy (EIS) was conducted to characterize the development of biofilm in association with fouling crevice environments.Electrochemical impedance spectroscopy (EIS) has been widely used to assess the electrochemical properties of corrosion systems and has been applied to assess physical biofilm conditions for microbiologically influenced corrosion (MIC) (Mansfeld , 2005). EIS measurements over a wide range of frequencies can be used to identify the electrical characteristics associated with the biofilm and the steel substrate. Both laboratory and field exposure tests were conducted. Field exposure tests included long term exposure (~300 days) of steel coupons at a Florida bridge site that sustained macro- and microfouling organisms. Laboratory tests were conducted in nutrient-rich test solutions inoculated with sulfate reducing bacteria (SRB) in either de-aerated or naturally aerated conditions as well as with idealized crevice geometries. EIS testing was made at the OCP condition with 10 mV AC perturbation voltage from frequencies 100 kHz > f >100 mHz.EIS of the specimens inoculated with SRB and with crevices resulted in impedance responses with multiple loops in the Nyquist diagram. Fitting of the impedance response to equivalent circuit analogs were made to identify biofilm, surface layers, and the steel interfacial characteristics during the experimental exposure (Permeh et al.,2019). For non-crevice specimens without SRB inoculations, the impedance response related to the solution resistance, steel interfacial capacitance and charge transfer resistance, characterized by the Randles circuit. However, for the inoculated specimens, impedance dispersion at high frequencies (sometimes showing a loop in the Nyquist diagram) on first approximation was related to the capacitive and resistive characteristics of the biofilm. In crevice environments, a more complicated behavior arose due to uneven current distribution and heterogeneous impedance within the crevice. The impedance behavior for the porous and laminate crevices can be characterized by multiple time constants associated with the crevice outer material, steel interface, and the current distribution effects in the crevice that can be considered by treatments such as a transmission line. In the inoculated non-crevice specimens, the high frequency impedance dispersion was more readily apparent in the Bode phase angle plot. In that representation, the impedance initially showed a uniform valley with a distinct trough between 10 and 100 mHz. With time, two unique troughs developed at frequencies below 1 Hz (Figure 1) for the inoculated specimens. An increase in capacitance was attributed to biofilm growth.In part to provide a quantitative comparison of the overall impedance of the systems (despite the complicating factors for the crevice specimens), on first approach, the total impedance at 100 mHz (|Z100mHz¬|) was compared. The differences in the total impedance for the various test conditions would reflect the resistive and capacitive electrical behavior of the electrolyte, biofilm, and the steel interface that in turn are directly or indirectly related to SRB activity. EIS identified the development of surface films including by an increase in its capacitive behavior that resulted in lower relative total impedance at a given low frequency (i.e. 100 mHz). Test results confirmed that SRB proliferation and corrosion can occur as the fouling macroorganism can accommodate different environmental condition.