“Corrosion due to different soil conditions is a general corrosion problem, especially prevalent in highly alkaline areas. Corrosion currents leave the pipe wall and pass into compact soils and enter the pipe wall from light sandy soils. The intensity of the corrosion currents and the resulting rate of corrosion at the anodic areas of the pipe are directly proportional to the conductivity of the soil. Soil resistivity has a strong influence on the corrosion rate. Generally, the higher the resistivity (measured in ohm-cm), the lower the corrosion rate. Soil resistivity arises from a number of factors, but fine-grained soils (silts, loams, clays, and peats) have the lowest resistivities and thus the greatest corrosion susceptibility. In-site field measurements of soil resistivity are the most accurate way to determine resistivities. The recommended technique is to use the 4-pin resistivity test according to ASTM G57-78 standard specification. Clay soils with PH higher than 9 and lower than 14 are the most prone to carbonate cracking corrosion.” Maybe the sites bellow will help on your posting http://ops.dot.gov/regs/small_ng/Chapter3.htm http://www.angelfire.com/pop/myfile/EXTDIPhtml.htm http://www.asminternational.org/Template.cfm?Section=ShopASM&template=Ecommerce/FileDisplay.cfm&file=ACF7AC.pdf Regards Luis Marques