Magnesium anodes are provided to the corrosion control industry by a number of domestic and international manufacturers and distributors. Due to the difficulty and time involved in performance testing, anode composition is commonly the only criterion used for quality control by the end users. Many of the anodes, however, have been shown to meet the compositional and potential specifications, and yet have had measured efficiencies as low as 7%.
The paper analyzes multiple ‘high potential’ anodes obtained from a variety of sources (manufacturers, distributors, and end users). The primary focus of the analysis was to establish whether any of the parameters could be used as a reliable predictor of the anode efficiency. The scope of the analysis included such characteristics as anode source, manufacturing process, macrostructure, chemical composition, and microstructure. The tested anodes exhibited efficiency range between 7 and 61 percent based on the ASTM G97 Standard Test Method. It was shown that compliance with the ASTM B843 can not serve as a predictor of the anode performance in the laboratory tests or field trial; electrochemical potentials are not an accurate predictor of the efficiency values. The primary finding of the study was that the efficiency of the high potential anodes is controlled by the presence of the iron-rich secondary phases in the grain boundaries. A correlation was determined between the extent to which the grain boundaries are decorated and the resulting ASTM G97 Standard Test Method efficiency.
Keywords: magnesium anodes, efficiency, ASTM G97