Excerpt from our research paper:
Metallurgical development and product evaluation of high silicon cast iron tube anodes
Impressed Current Cathodic Protection (ICCP) plays a crucial role in the protection of vital infrastructure such as pipelines, storage tanks and structures from corrosive destruction. A popular cost effective type of semi0consumable anode used for ICCP is manufactured from high silicon cast iron (HSCI). HSCI is an acid resistant cast iron that contains 14 to 16% silicon and less than 1% carbon that has been used for over 100 years in the Chemical Industry for acid containment and processing. The initial development of high silicon cast iron dates back to 1908 (ref 1) and although the alloys were characterized by excellent corrosion resistance, these alloys still have known weaknesses such as low strength and toughness, high hardness and associated brittleness and cannot be hot worked or machined. Parts made from the alloy have to be made by a casting process and therefore have the associated imperfections of cast metal such as porosity and shrinkage defects.
To manufacture an anode with a high level of integrity and durability there have been several developments and innovations in the cast metal technology. These involve methods to reduce the gas content and to refine the microstructure and to refine and compact the graphite shape by the micro additions of boron and rare earth additions. In addition the application of centrifugal casting has been widely used to ensure required metal feeding and avoidance of inter0dendritic shrinkage porosity.
Since anodes are buried underground and become inaccessible for maintenance and are required to have an operational life in excess of 20 years, in addition to the developing the appropriate mechanical integrity, there is an essential requirement to validate that the anodes will have an appropriate level of corrosion resistance. A review was carried out of the various methods that have been used to assess and to validate the corrosion resistance of anodes. The main sources of information technology for this type of review are NIST, NACE International and ASTM sources of information and guidance. One of the earliest studies on Duriron (an old proprietary name for high silicon cast iron) in 1957 by the National Bureau of Standards demonstrated the successful application of a potentiodynamic method. (ref 2) In addition there were other approaches based on accelerated corrosion and weight loss methods. (ref 3)
This project was carried out to compare the corrosion performance of centrifugally cast anodes made by Specialist Castings and chill cast anodes with a sand core made by Anotec.
Potentio-dynamic test were carried out in accordance with ASTM 59 and ASTM G5. In addition three corrosion tests have been examined to assess their performance as accelerated tests to evaluate the anode quality and consistency.
Based on the results of the corrosion test work the centrifugally cast anode material was found to have superior corrosion resistance. The improved corrosion resistance determined by four different methods was of the order of 20% for the hydrochloric acid test, 30% for the nitric acid electrolytic etch method and 20% for the electrolytic 3% sodium chloride test. The improvements were attributable to higher integrity cast product and lower inter0dendritic porosity. This was confirmed by metallography and measurement of relative density. The measurement of the relative density of the samples confirmed the lower levels of porosity in the centrifugally cast anode material.
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