Graduate Thesis Or Dissertation
 

Determining localized anode condition to maintain effective corrosion protection

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  • The main objective of the research is to develop a prototype device that can determine the condition of the thermally sprayed zinc anode in cathodic protection systems. The device needs to meet the following requirements: applicability in the field, non-destructiveness or minimal destructiveness of the zinc anode, ease of use with little to no technical background, and the ability to determine anode condition. Two different technologies, ultrasound and resistance/permeability, were explored to examine their prospective in determining the localized anode condition. Ultrasound testing was performed on three different sets of samples. Analysis using time-of-flight was able to determine the thicknesses of the zinc sheet samples. Calculated thicknesses were essentially identical to the nominal thicknesses. Similar analysis resulted in negative results when applied to electrochemically aged and unaged samples. It is believed that the ultrasonic waves are being scattered by the roughness at the zinc/concrete interface or, perhaps, in the bulk of the thermally sprayed zinc. An alternative ultrasonic configuration using surface (Lamb) waves is recommended for future studies. Cyclic voltammetry was used to measure the resistance of the electrochemically aged samples. Dry resistance measurements were performed on five different electrochemically aged blocks. It was determined that the average resistance of the blocks increased rapidly up to approximately 5 bridge years, which corresponds, generally, to the maximum bond strength, and then gradually increased or leveled off. Individual resistance data was seen to vary with time, but changes were consistent from block to block. The resistance is shown to be inversely related to temperature and believed to be affected by environmental factors. Open circuit potential showed less variation with time when compared to the resistance measurements. Using a three electrode configuration, the potential of Zn generally tracked the resistance and bond strength measurements, while the potential of Fe increased with age. No dynamic changes in resistance were noticed with the addition of water. It is believed that the water was impeded due to lack of permeability in the samples. A noticeable decrease in resistance was observed when the water had a path to the zinc/concrete interface, either by a pin hole or bare concrete adjacent to the zinc. It is recommended that a simple field test be developed to monitor the response of a CP system for an applied voltage as a zone is sprayed with water, and that such a test may be aided by systematic creation of pin holes in the thermally sprayed zinc anode. It is therefore, recommended, that the created pin holes be studied in an accelerated aging environment. Preliminary data from this study suggest that not only will the creation of pin holes assist characterization of anode condition through electrical response, but may increase the service life of the zinc anode by decreasing circuit resistance of the electrochemical cell.
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