Anodes need to be in soils or electrolytes in order deliver cathodic protection current to buried structures. HSCI anodes work well in natural waters, but they are seldom buried in the ground without being bedded in high quality carbon fines (coke). Coke increases the area of anodic surface in contact with the earth to minimize resistance, and most importantly, mitigate Groundbed Drying. Calcined petroleum coke breeze also serves to prevent drilled deep-wells from collapsing inwards.
Current Density Limitations: In operating systems, high silicon cast iron anodes are invariably called upon to discharge current well below the limit that HSCI is able to handle without failing catastrophically. Most frequently, for earth or mud groundbeds, current density must be limited to prevent either groundbed drying by electro-osmosis, or gas blockage from electro-chemical processes around the anodes.
How Current Density affects CP groundbed design is explored in: Groundbed Design with HSCI Anodes.
Anode Life depends on the consumption rate of high silicon iron in the environment involved. As rule, HSCI groundbed designs assume that the consumption rate will be about 0.31 kg/Ayr (0.75lb/A-yr) at current discharge density ranging from 5 to 10A/SqM (0.5 to 1 A/sqft). Although rate of consumption depends on many factors; environment and current discharge density are most significant. Accelerated Testing at 25 A/sqM (2.5A/Sqft) indicates that a sulphate environment will increase consumption relative to chloride by roughly 15% to 30%(1). From the same tests, consumption was nearly 20% higher for conventional sand cast silicon iron, than for Anotec Chill Cast Silicon Iron. In a different test, a 50% increase in the consumption rate of sand cast silicon iron corresponded to a 50% increase in current density (2). In contrast, a 50% increase in consumption rate of Anotec Chill Cast material required a 500% increase in current density (3).
Anotec attributes the significantly better performance of Chill Cast anodes, compared to Sand Cast and Centrifugal Cast, to the superior Metallic Structure achieved by Anotec's Chill Casting process. In salt water tests for HVDC groundings (4), Chill Cast HSCI electrodes gave16% longer life than sand castings. It is Interesting that current reversal did not appear to increase consumption for these grounding electrodes.
- Anotec, "Comparative Accelerated Corrosion Tests", October 1989 (L13A)
- Jakobs J. A., "Test of Performance of High Silicon and Graphite Anodes in Various Environments at Different Current Levels", Caproco, 1981. (L01)
- Anotec, "HSCI Anode Life, Consumption, Utilization and Limitations"
- ABB Report: "Corrosion Testing of Silicon Iron electrodes operating as Anodes (and) Cathodes", 1999 (L93)