Long-term electrode polarization

Long-Term Electrode Polarization of Electrochemically Active Biofilms... 

In a long-term electrode polarization experiment, a selected polarization potential is applied to an electrode with an EAB grown on it and the current is measured. In this way, the total charge transferred in a batch system or the steady state current produced by the EABs in a continuous system can be measured. A long-term electrode polarization experiment identifies sustainable current generation that can be systematically related to controlled parameters such as polarization potential. While the technique appears similar to controlled electrode potential acclimatization, in which EABs are grown on polarized electrodes, the intents of the two are distinct and should be distinguished. For example, G. sulfurreducens DL-1 was allowed to acclimatize on an electrode for 5 months and resulted in selection for a new strain, G. sulfurreducens... 

KN400 [97]. Subsequently, the sustainable current productions of the two strains were measured using long-term electrode polarization experiments. The key value of performing electrode polarization experiments is to observe the electrochemical response of an EAB to a change in a controlled experimental parameter or the effects of acclimatization methods. 

Even though the above work is providing a stable, non-sintering, creep-resistant anode, electrodes made with Ni are relatively high in cost. Work is in progress to determine whether a cheaper material, particularly Cu, can be substituted for Ni to lower the cost while retaining stability. A complete substitution of Cu for Ni is not feasible because Cu would exhibit more creep than Ni. It has been found that anodes made of a Cu - 50% Ni - 5% A1 alloy will provide long-term creep resistance (36). Another approach tested at IGT showed that an "IGT" stabilized Cu anode had a lower percent creep than a 10% Cr - Ni anode. Its performance was about 40 to 50 mV lower than the standard cell at 160 mA/cm. An analysis hypothesized that the polarization difference could be reduced to 32 mV at most by pore structure optimization (37). 

The long-term stability of the material is an essential aspect, particularly when a periodical polarity reversal is applied. Using metallic or nonstable materials which could be corroded (mainly under oxygen evolution conditions) induce undesired long-term pollutions in treated waters and moreover generate additional costs linked to frequent replacement of the electrodes. 

Gowers et al. (1992) have used the linear polarization technique with embedded probes (reference electrode and a simple counter electrode) to monitor the corrosion of marine concrete structures. This technique was described previously without reference to isolating the section of bar to be measured (Langford and Broomfield, 1987). By repeating the measurement in the same location on an isolated section of steel of known surface area the corrosion rate of the actual rebar can be inferred. The main problem is the long-term durability of electrical connections in marine conditions. Also the probes should ideally be built into the structure during construction. A desirable but rare occurrence. Recent developments in corrosion monitoring are described in Chapter 5. 

The most frequently used materials for buried metal structures are the carbon steels. For prevention of their corrosion the most recommended, economical, and effective method is cathodic protection (CP). The use of CP is now standard procedm-e for long-term corrosion protection of imderground pipelines, oil and gasoline tanks, and other structures. With a shift of the metal potential to more of a negative value of -0.85 V versus a C11/CUSO4 reference electrode, it is possible to make the metal surface a cathode, which ensures an immune (no corrosion) state of the carbon steel. Cathodic polarization is achieved by direct current, which can be supplied either by sacrificial anodes in galvanic contact with the steel structure, or by impressed current from a rectifier. 

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