EARTH RODS - Service Life

1-National Bureau of Standards USA
From 1910 to 1955, the National Bureau of Standards conducted an extensive underground corrosion study in which 36,500 specimens, representing 333 varieties of ferrous, nonferrous, and protective coating materials were exposed in 128 test locations throughout the United States. It is widely regarded as one of the most comprehensive corrosion studies ever conducted.
The time required to remove 254μm of copper can be used to establish a ground rod’s nominal service life. Although different soils were used, the data from 8 to 13 years shows that the rate of corrosion seems to lessen with time. This is probably due to the protective copper oxide film that develops on the rod. It can be conservatively assumed that in the vast majority of soils tested, a service life of far in excess of 40 years can be expected.
“An analysis of tests of hot dipped zinc showed that in most of the soils, zinc coatings of 89μm or less were destroyed
during the 10 year exposure period, and pitting of the underlying steel occurred.”
Additionally, an analysis of galvanized pipe shows an average penetration of roughly 64μm after 10 years. After 13 years; the average penetration of copper was 0.011938μm.
This study definitively shows the difference in corrosion resistance between copper and zinc. Copper provides protection that is several orders of magnitude greater than
zinc.

2-Naval Civil Engineering Laboratory
One of the most useful ‘real time tests’ was conducted in 1960 by the Naval Civil Engineering Laboratory (NCEL) in the USA on a 7-year program of testing metal rods for electrical earthing/grounding. Copper-bonded, stainless clad, and galvanized steel rods were included among other materials. Samples were buried individually and connected in pairs to determine the galvanic corrosion effect each rod had
on other metals. Although this study is not nearly as rigorous as some others undertaken, it does provide an independent
source of information specifically relating to earth rods.

The study set out to establish the best type of electrode as determined by the following:
1. It should be easy to drive
2. It should be resistant to corrosion
3. It should not cause galvanic corrosion to nearby
metal.Regarding single rods, these observations were made
after samples were removed following 7 years of exposure:
Galvanized steel rod: “Most of the galvanizing had been lost. Rusting of the steel was greatest near the surface of the ground. Pitting was worst here and near the tip.”
Stainless-clad steel rod: “The cladding was free of corrosion, but at the tip the steel core had corroded to a point about 25mm inside the cladding.”
Copper-clad steel rod: “The copper cladding was virtually free of corrosion, but the steel core had corroded at the tip to a point 50mm inside the cladding.”
The study concluded, “Magnesium, aluminum, zinc, mild steel, and galvanized steel rods did not have the desired corrosion resistance.” There was a concern about the copper
clad rod causing galvanic-corrosion to coupled mild steel.
The amount of galvanic corrosion caused by coupling copper and steel will vary depending on their ratio. Since the ratio of steel to copper in most grounding systems is usually large, the amount of steel corrosion is negligible. Additionally, there are often other sources of copper in the ground, which can
add to galvanic corrosion concerns.
3-National Electrical Grounding Research Project – Pawnee Site Exhumation
The NEGRP was started in 1992 to compare the long-term performance of different types of earth/grounding electrodes.
In 2003, one of the original sites (Pawnee) was excavated and the rod electrodes removed. 5/8" (16mm dia) copper-
bonded and 3/4" (19mm dia) galvanized rod samples were exhumed. The results were definitive. The 5/8" (16mm dia) copper-bonded rod was virtually free of corrosion while the 3/4" (19mm dia) galvanized rod showed significant deterioration.