Anode Consumption

The consumption rate of High Silicon Chrome Cast Iron anodes has been found to be between 0.2 and 1.2 pounds per ampere-year. For anodes of the same chemistry and microstructure, variance in consumption is primarily due to the chemical and physical characteristics of the anode environment. The consumption rate does not appear to be significantly affected by current density (amperes per unit area of anode surface). The use of coke breeze around the anode in soil ground beds will tend to lower the consumption rate. A generally accepted design guideline for anodes buried in coke breeze is 0.7 pounds per amp-year.

Current Density Limitations

The maximum stable current density of discharge may be limited by the environment regardless of the anode type. In free flowing water or in very wet soil ground beds, there is very little restriction on current density. However, anodes buried in clay soils tend to suffer "electro osmotic drying", a phenomenon of magnitude directly proportional to current density. For any particular soil with electro osmotic characteristics there will tend to be a critical maximum current density at the anode soil (or coke breeze to soil) interface, above which progressive drying occurs, with corresponding increases in anode-soil resistance. Drying is usually reversible by increasing soil moisture and/or lowering current density.

As a guideline to minimization of electro osmotic drying in groundbeds installed in clay soils, use of the following design maxima has resulted in stability of 90 to 95 percent of beds in areas of high osmotic drying potential.

Average soil resistitivity along groundbed, Ohm-cm	Maximum Amps per anode in a coke breeze column, 12" OD by 60"	Equivalent current density on surface of coke breeze column, Milliamps/sq ft
Less than 1000	2.00	127 (see note)
1000 - 1500	1.75	111 (see note)
1500 - 2000	1.50	96
2000 - 3000	1.25	80
Over 3000	1.00	64

Note: For greater success, limit current density to less than 100 mA/sq ft for soils of less than 1500 ohm cm resistivity.

• Note: Maximum amperage based on maximum value in manufacturer publications, 15-year minimum life criterion (0.7 lb/amp-year of 0.31 kg/amp-year) at 80% utilization, or 1 amp/sq ft (10.8 amp/m²).
• Warning: Reduce current density in clay beds with high osmotic drying potential. Refer to Anode Consumption and Current Density Limitations in this web page.
• Reference: NACE Technical Committee Report "Impressed Current Anodes for Underground Systems" [T10A-10 latest].