Accurate monitoring of cathodic protection relies upon the proficiency of the test personnel and the proper operation of the required equipment. The equipment consists of properly insulated test leads, an electronic voltmeter with a known internal circuit resistance, and a reference electrode. The proper operation of each piece of equipment is critical to the accuracy of the cathodic protection test results. However, the reference electrode is not often suspected of being responsible for unexpected cathodic protection readings. This paper will provide background on the use of portable reference electrodes and data used to evaluate the performance of buried reference electrodes installed adjacent to a single, cathodically protected, underground storage tank. Data accumulated at the test site clearly indicates that not all buried reference electrodes provide identical results.
Cathodic protection (CP) testing for steel underground structures has typically been accomplished using a volt meter suitable for the test, a set of test leads, and a standard half-cell. Generally, this half cell is referred to as a reference electrode, is commonly of the copper/copper sulfate (CSE) variety, and portable in nature. The standard methodology for the use of this equipment has always been; 1) make good electrical connection to the structure to be tested, 2) connect test leads to the meter observing correct polarity, and 3) place the CSE in the electrolyte (soil) directly above the structure.
Over time, test personnel have noted various difficulties in accomplishing step number 3) when attempting to check the CP for steel UST's. Among the problems encountered were contaminated soil, frozen soil, limited or no access to the electrolyte, and excessively dry soil conditions at the available CSE placement location(s). In an effort to control these factors, many of which can lead to inaccurate half-cell potential measurements if not avoided, the buried reference electrode (BRC) was studied by the Steel Tank Institute.
This study was undertaken with the aid of third party corrosion consulting companies. Previously, a third party investigation of nine (9) locations with steel UST's under CP indicated a direct relationship between half-cell potential level, soil resistance, and the depth at which the half-cell potential is obtained by CSE placement I as shown in Table I. With typical UST installations, the portable reference electrode is placed as much as 4-5 feet above the structure and in high resistance, non-conductive soil or backfill. With many cathodically protected UST's the anodes are placed near the bottom of the tank, at each end (head) 2. This is done to assure full protection to the portion of the structure which is generally in the most corrosive environment, and most likely to have a corrosion failure, the tank bottom. Thus, half-cell potential measurements taken at the surface using a portable reference electrode are not necessarily providing useful data.
