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 MIL-HDBK-419A
2.9.5 Salting Methods. The trench method for treating the earth around a driven electrode is illustrated in
Figure 2-26. A circular trench is dug about one foot deep around the electrode. This trench is filled with the
soil treating material and then covered with earth. The material should not actually touch the rod in order to
provide the best distribution of the treating material with the least corrosive effect.
Another method for treating the earth around a driven electrode, using magnesium sulphate and water, is
illustrated in Figure 2-27. A 2-foot length (approximately) of 8-inch diameter tile pipe is buried in the ground
surrounding the ground electrode. This pipe is then filled with magnesium sulphate to within one foot of grade
level and watered thoroughly. The 8-inch tile pipe should have a wooden cover with holes and be located at
ground level.
None of the aforementioned chemical treatments permanently improve earth electrode resistance. The
chemicals are gradually washed away by rainfall and through natural drainage.Depending upon the porosity of
the soil and the amount of rainfall, the period for replacement varies. Forty to ninety pounds of chemical will
initially be required to maintain effectiveness for two or three years. Each replenishment of chemical will
extend the effectiveness for a longer period so that the future treatments have to be done less and less
frequently.
Another method of soil treatment or electrode enhancement involves the use of hollow made electrodes which
are filled with materials/salts which absorb external atmospheric moisture. These electrodes (generally 8-feet
long) must be placed in holes drilled by an earth auger making sure the breather holes at the top are above
grade level. Moisture from the atmosphere is converted to an electrolyte which in turn seeps through holes in
the electrode into the surrounding soil. This keeps the soil moist and thereby reduces the resistance of the
electrode to earth. These electrodes should be checked annually to ensure sufficient quantities of
materials/salts are available and that good continuity exists between the rod and interconnecting cable.
2.10 CATHODIC PROTECTION.
2.10.1 Introduction. When two metals of different types are immersed in wet or damp soil, a basic electrolytic
cell is formed. A voltage equal to the difference of the oxidation potentials of the metals will be developed
between the two electrodes of the cell. If these electrodes are connected together through a low resistance
path, current will flow through the electrolyte with resultant erosion of the anodic member of the pair.
Unfortunately, those factors that aid in the establishment of low resistance to earth also foster corrosion. Low
resistance soils with a high moisture level and a high mineral salt content provide an efficient electrolytic cell
with low internal resistance. Relatively large currents can flow between short-circuited electrodes (such as
copper ground rods connected to steel footings or reinforcing rods in buildings) and quickly erode away the more
active metal (see Section 7.8.1.2) of the cell. In high-resistance cells, the current flow is less and the erosion is
less severe than in low-resistance cells.
2-63
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