There are several techniques for measuring soil resistivity. A common method is described in ASTM G 57. It should be noted that soil resistivity alone should not be used to determine soil corrosivity. The resistivity of the pad material may be higher than the existing surrounding soil. Corrosive soil beneath the higher resistivity pad material may contaminate the pad fill by capillary action and should be a consideration when determining sand pad thickness. Thus, resistivity of surrounding soil may be used to help determine the probability of corrosion on the tank bottom. The results of soil resistivity surveys should be considered and used to help determine the need for cathodic protection. However, other properties such as chlorides sulfides and sulfates of the soil should also be considered.
We provide corrosion risk assessment and corrosion mitigation of bottom plates on aboveground tanks used for the storage. The standard method of determining the corrosiveness or the effectiveness of cathodic protection on a tank bottom is the tank-to-soil potential measurement. One of the problems associated with monitoring cathodic protection systems on tank bottoms is the inability to assess empty space under the tank or to place a reference electrode in close proximity to the underside of the tank resulting in measurements that may not represent the tank-to-soil potential at specific areas or at the center of the tank bottom. When utilizing earthen foundations, soil analysis is useful to help determine whether the potential corrosion activity will be high enough to make cathodic protection necessary and whether cathodic protection will be a practical application to prevent corrosion. The advantages and disadvantages of each will be discussed. Determination of aggressive ions such as chlorides and sulfates along with measurement of moisture, pH and resistivity at shallow and deep burial are helpful for further corrosion analysis. Predictive modeling based on soil corrosivity data will provide life expectancy or remaining life with moderate to high confidence layer. Corrosion mitigation techniques such as cathodic protection, concrete foundations and VCI will discussed briefly.