INSTALLATION OF AN INSTRUMENTED CATHODIC PROTECTION SYSTEM ON A LARGE DIAMETER AST

AUTHORS

ABSTRACT

Paper No. 03200 presented at Corrosion 2003, March 16 &45; 20, 2003, San Diego, California
Published by NACE International, Houston, TX www.nace.org © NACE, 2005 Corrosion on tank bottoms has been a problem for storage facilities that has led to many solutions. A common practice is to use secondary containment, or double bottom tanks as a means of ensuring that any leaks in the primary bottom would not lead to environmental contamination. These systems have been installed for several years, yet there has been little in the literature to document the effectiveness of the cathodic protection (CP) systems installed in the shallow spaces between the two bottoms. This paper describes the installation of a highly instrumented CP system on a large above ground storage tank bottom and provides data demonstrating the effectiveness of the system.

CONCLUSIONS

Terry Wilken was President and Chief Engineer at Schmoldt Engineering, which designs and installs cathodic protection systems, at the time the paper was written. Terry has since retired. When planning and installing cathodic protection on a double bottom tank, it is important to design and install sufficient monitoring capability into the system.

Reference electrodes must be placed exactly mid-way between anodes to minimize IR drop error in potential measurements. More precise potential measurements can be obtained by either interrupting the rectifier or using cathodic protection coupons placed very close to the reference electrodes.

Jim Dimond is President of Dimondale Co., Inc which designs and manufactures cathodic protection products. He has over 30 years experience in cathodic protection including product design, manufacturing and field engineering. Jim may be contacted at PO Box 838, Middlefield, OH 44062 or In this particular installation where the anodes were placed eight inches (20 cm) below the primary bottom, a five foot (1 ½ m) inter-anode spacing was adequate to obtain cathodic protection out to the mid-point between anodes.

Frank Ansuini is Technical Director of EDI which specializes in the development and marketing of corrosion control products. He has over 30 years experience in research and product development in corrosion. Frank may be contacted at PO Box 31, Albion, RI 02802 or Polarization or depolarization takes time on any uncoated structures. The structure should be regularly monitored after the system is energized and applied current periodically adjusted in order to obtain the desired results.

For this application, it is best to use a constant current rectifier to have complete control over the applied current levels.

Click to download paper in PDF format (265kb) Anode life can be maximized and electrolyte dry out minimized by using the lowest level of current necessary to meet NACE criteria.