Abstract
A common method of corrosion control of buried pipeline is that of cathodic protection involving the use of sacrificial anodes. To ensure maximum effectiveness of an anode, a suitable backfill material of Gypsum/Bentonite is required to maintain the necessary current flow. This material is generally inert and moisture retentive. In the River Project of Libya, the bentonite used was collected from two different clay member formations. The destructive inspection of zinc anodes revealed that the backfill material used from one formation (No. 1) has dried quickly and shrunk away from the anode and the other backfill used from formation No. 2 was still moisture retentive and working properly. Therefore a laboratory tests were carried out to determine the causes of the shrinkage of formation 1 and also to identify the characteristics of Bentonite used and its suitability for use in the backfill, and how to delay the moisture content loss with minimum shrinkage. This study has included chemical, physical, mineralogical and geotechnical laboratory tests. The tests revealed that the Bentonite which was in use (formation No.1) was in commercial terms relatively low grade (c. 46% Montmorillonite) with liquid limit values of c. 120%, and the gypsum was relatively pure, being composed of >95% gypsum, and the backfill mixture contained c. 11% Montmorillonite. Engineering testing suggests that if the ground conditions cause the backfill to dry, shrinkage may lead to a loss of anode function. Such shrinkage would be reduced if the backfill was emplaced at lower moisture content and higher density, this is also likely to increase its ability to absorb water. However, if the backfill does dry out, the anode may also not function correctly. Also this test revealed that the Bentonite should be calcium based not sodium based.
