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#066 Ground Corrosion

Category : Corrosion - Corrosion Protection
December17, 2010

A variety of metallic objects such as gas, water, communication, and power lines are ground embedded. Especially in recent years the oil and natural gas lines are used for large scale long distance distribution of energy sources, and corrosion protection measures for such infrastructure has become of important safety concerns. The corrosions occurring in soil differ slightly from those occurring in atmosphere and water. Soil is comprised of mixtures of soil particles, water, and air. The corrosions in the soil have some similarities with those occurring in air with thin water layer, but different where oxygen supply is much less and wet-dry cycles are less frequent. Additionally, the ground water contains various dissolved elements and have varying water retention characteristics/humidity, causing a wide variety of corrosions due to varying oxygen supplies, diffusion rates, and water/oxygen reactions.

Factors influencing the soil corrosions are pH, electrical conductivity, water content percentage, drainage characteristics, air permeability, and chemical composition (ion types and concentration levels). The electrical conductivity is related to moisture content and ion concentration, water content percentage and drainage characteristics are related to moisture content. Porosity of soil limits the oxygen and water supplies and corrosions of steel in soil would be less compared to being in air or water. If corrosion were to progress uniformly, it is said to be 0.06mm/year at the most.
However, soil is an environment that makes steel corrosions quite non-uniform. The important factor is the amount of oxygen diffusion onto metal surfaces, and it varies depending on porosity and water content of the soil. Corrosions progress more with more oxygen, but at times oxygen concentration differences create concentration differential cells that accelerates corrosions.
An example of soil composition related problem would be a case of soil mixture of sand and clay. If plumbing is to be embedded in such a soil mixture, localization of corrosions would occur due to higher air permeability of sand.

A phenomenon of metal corrosions occurring when DC currents flow from metals to environments were previously discussed. This can also occur even without corrosion battery mechanism. Currents leaking from DC electric trains and metal smelting plants to the soil would enter embedded structures such as pipelines and again exit to the soil. These are called "Stray Currents" and corrosions caused by these currents in the soil are often seen as problems. Similar corrosions can also be seen in fresh and sea water, though not as often as in the soil, and are called "Stray Current Corrosions". Various countermeasures are being considered, but the applications are limited to coating with coal-tar type paints and electrolytic Corrosion protection methods due to cost limitations.