A Guide To Water Well Casing and Screen Selection   -   12

5.3  Corrosion Resistance

As a component of well design, the effects of corrosion under the anticipated operating conditions should be examined. Metallic corrosion is a complex subject, more throughly discussed in the Roscoe Moss publication "Fundamentals of Metallic Corrosion in Fresh Water", by J.R. Rossum. Some generalizations are made here which may be helpful. First, well casings and screens are subject to different types and degrees of attack according to their location in the well. Since many ground waters are saturated with calcium carbonate, the screen section will tend to encrust rather than corrode, which promotes longevity but frequently requires periodic removal of buildups to restore production. In most wells the greatest corrosion attack occurs in and just above the "splash zone" between the static water level and pumping level. The casing is exposed to a humid atmosphere or an alternate wetting and drying condition which accelerates attack on steel. The pump column is similarly affected.

With respect to well waters themselves, it is generally accepted that the presence of bicarbonate retards corrosion, sulfate and nitrate are neutral and chloride accelerates corrosion. Concentrations of carbon dioxide of over 50 mg/liter are usually corrosive. Water containing hydrogen sulphide should be checked for the presence of sulphate-reducing bacteria which are highly detrimental to well screens. Acidic water may be severely corrosive , but the corrosion is usually uniform, rather than pitting. Consequently, well life may be greater than normally expected.

It is unfortunate that no one has developed a method of determining the life of material given the constituents of the water. Again, the best guide is experience. Throughout the United States, and in most regions of the world, there are ample well histories giving clear directions. There is no condition, when potable waters are produced, where adequate well life cannot be obtained through the use of proper well design and material selection.

Required well life depends upon a host of economic considerations. If one designs a municipal well for 50 or more years operation, the use of stainless steel material, shoe corrosion rate is negligible in potable waters, is mandated. In most areas of the United States and elsewhere where Waters are generally alkaline, 40-year life is non uncommon. Generally, 30-year life is a reasonable expectation.

With respect to steel casing as noted, greater than average corrosion occurs in the most humid or alternating wet and dry zone. This is a form of atmospheric condition, and the use of heavier wall materials extends life considerably. Doubling wall thickness should extend life four or more times. Increasing wall thickness, however, has not proven to be much help with severely corrosive waters.

The addition of copper to steel so that its content reaches a minimum of .20%, increases its atmospheric corrosion resistance by approximately two times. Specification of this quality material is shown as an alternative in all standard ASTM specifications for structural grade steels. Copper-bearing steel has been used as casing material in the Southwestern United States for over 90 years. Culvert steel is also copper-bearing. A further encouragement to the use of copper-bearing steel is that it results in an increase in casing cost of only about 20% and a percentage increase in total job cost much less than that.

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