Design of magnesium anode systems requires a thorough evaluation of the application with consideration of the important variables. Once these variables are measured or assumed, the process typically is divided into the following steps.
- Calculate exposed surface area – This requires an assumption of coating quality. A high quality coating can provide protection to over 99% of the total surface area. A conservative assumption of coating efficiency is 95%.
- Calculate total current requirements – This requires an assumption of current density. The typical range of current density for steel in common soils is 1 to 3 milliamperes per square foot of exposed surface. A conservative assumption is 2 milliamperes per square foot.
- Calculate anode current output – Using previous discussed formulas, calculate anode output considering the effect of coating, soil resistivity, alloy selection, and anode spacing. Several anode sizes should be evaluated.
- Select anode size based on life – Once anode current outputs have been calculated, the appropriate anode size can be selected by evaluating the projected life. Typical design lives range from 10 to 20 years.
- Calculate total anode quantity – Divide the total current requirement in amperes by the calculated anode output in amperes. The result is the estimated total number of anodes required.
- Determine anode location and configuration – This is primarily based on local site conditions and an even distribution of current from the anodes.