EFFECTIVENESS OF CATHODIC PROTECTION. PHASE 3. FINAL REPORT MARCH 1983-JUNE 1984.
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Cathodic protection is one method used to control corrosion on buried gas distribution piping. Proper interpretation of the level of cathodic protection requires appropriate correction or compensation be made in the pipe-to-soil potential for errors from IR drop in the soil and from stray AC and/or DC interferences. A microprocessor-based prototype instrument to make this correction on piping protected by impressed-current systems was designed, fabricated, and evaluated in the field. The instrument performed as designed, however modification is required before final design. An unexpected spike or transient in the pipe-to-soil potential waveform from some rectifiers interfered with accurate polarized potential measurements in many cases. Future software modifications are expected to identify and accurately treat most of these sistuations and improve AC rejection. Commercialization appears to be economically feasible. Laboratory experiments have demonstrated that the AC-impedance techniques appear to be feasible to assess the corrosion rate of unprotected steel, and the corrosion state of buried steel protected by either impressed-current or sacrificial-anode systems. Two AC impedance parameters that correlate well with the corrosion state have been identified. An instrument design concept embodying this technique for buried piping has been developed and further research and development is planned.