ABSTRACT
Tanks made from steel buried underground and used for storage of liquefied gas are protected against corrosion by epoxy coatings. Regular inspection of the integrity is mandated by law in most countries, details pertaining to regulations in e.g. Germany are provided in various texts (e.g. DIN 4681-1; DIN 4681-3). Currently an inspection of tanks with <3 to capacity is obligatory every ten years provided the tank is not exposed to particularly destructive or otherwise challenging environmental conditions. This examination has been performed in the past predominantly by inspection of the inner surface of the steel tank. This procedure is cumbersome, it requires transfer of the liquefied gas into a separate container, generous ventilation o f the emptied container and visual surface inspection by a person who has to climb into the tank. Obviously this procedure is a risky one. In addition it is rather dubious how corrosion-which will proceed on the outer surface in case the coating is defectiveshall be detected. From the content of the tank hardly any corrosive attack can be expected, all typical environmental conditions (humidity, electrolyte, oxygen or protons) necessary for corrosion are completely absent. On the contrary the major risk of corrosion is present on the outer surface. Any damage o f the coating may permit access o f exactly these corrosionenabling substances to the steel surface. Thus any method capable of detecting reliably dam ages of the epoxy coating are welcome. When they can be employed without removing the tank from its site they will be particularly helpful because unnecessary expenses and risks can be possibly completely avoided. TUV SUD Industrie Service GmbH, Munich, Germany, has developed a method based on a DC-measurement of the electric resistance between the steel of the tank and a probe (spear) placed in the ground close to the tank (Registered design/trademark 20 2005 009 752.7). A flow of electric current easily indicates damage to the integrity of the coating. As shown in [FIG: 1] in case of a perfect coating the resistance should be infinite, current should be zero. Actually a small residual current because of leak age at cable insulations etc. may be expected. There are two interfaces to be considered. The interface between the spear and the (wet) soil ( 1 ) exhibits possibly typical properties o f an electrochemical double layer. The second interface (2) is a completely insulating one pro vided the epoxy coating is perfect. At first glance-because of thenon-Ohmic behaviour of interface ( 1 )—application o f an AC-voltage as widely done in measurements o f electrolytic conductivity seems to be the suitable approach. Because of the very large area o f interface (2) (depending on size and shape of the tank several square meters) a considerable capacitance
Figure 1. Simplified equivalent circuit of the arrangement for measuring the integrity of a tank coating.
