Archive for November, 2012

Principles of Corrosion

Thursday, November 29th, 2012

Corrosion is the disintegration of metal through an unintentional chemical or electro- chemical action, starting at its surface. It is recognized as one of industry’s most difficult problems and the resulting losses each year are in the tens of billions of dollars.

Metals are usually extracted from ores through the application of a considerable amount of energy. Certain environments offer opportunities for these metals to combine chemically with elements to form compounds and return to their lower energy levels. In other words, corrosion can be viewed as the tendency of a metal to revert back to its natural and more stable state as an ore.

Corrosion of a buried metal is greatly influenced by the soil in which it is buried. The major soil conditions which affect corrosion and must be considered are type, drainage, resistivity, acidity, and particle size. The ditch backfill environment is a man made disturbance of the natural environment and can adversely affect these soil conditions.

Soil may consist of sand. loam. clay. mulch. or a combination of any or all of these. The most corrosive is mulch; the least. sand.

Soils with the poorest drainage are the most corrosive while well drained soils are the least corrosive. The exception to this is a soil which contains cinders. Cinder fills are well drained but any moisture present makes it a very corrosive environment.

Low resistivity soils are corrosive and high resistivity soils are the least corrosive. as a rule. However. acidity and oxygen concentration can alter this.

A neutral soil (PH-7.0) is least corrosive. A soil with a PH of 3.0 would be very acidic and corrosive. Alkaline soils (PH above 7.0) contain dissolved salts which can accelerate corrosion. This is especially true in desert soils.

Large soil particles can allow oxygen to penetrate to the buried metal product. If an area with an abundance of oxygen is next to an area of a limited supply of oxygen, a differential aeration corrosion cell is created.