Recently, in Japan like other countries in the world, pipelines are being constructed in parallel with HVAC electrical power lines and/or AC traction railroads with thinner, high strength walls, high resistivity coating with little or no corrosion allowance. This means that more attention must be paid to new threats, that is, AC corrosion and overprotection on modern pipelines. To determine the AC corrosion risk, coupons should be installed where the AC current density reaches its maximum. The AC current density IAC together with DC current density IDC of a coupon are the primary determining factors in assessing the AC corrosion risk.
The authors have developed an advanced instrumentation for assessing the AC corrosion risk of buried pipelines. The most distinguished feature is the simultaneous computation in a measuring unit of 20 ms regarding coupon DC current density IDC and coupon AC current density IAC (50 Hz) corresponding to the commercial frequency of 50 Hz that is the predominant frequency component on AC corrosion of buried pipelines. Immediately after computing the averaged IDC and IAC (50 Hz) in measuring time, the results are referred to the coupon current density-based cathodic protection (CP) criterion established by the authors.
In 1986, corrosion failure on a pipeline caused by induced AC interference currents was first reported in Europe despite satisfying the protection potential criterion1. The pipeline was installed in 1980 paralleling a 15 KV AC powered railroads operated at frequency of 16-2/3 Hz. Since then pipeline failures caused by AC corrosion have been reported not only in Europe but in North America2-4. Based on the experience of AC corrosion, recognition of AC corrosion and its mitigation measures has been gained worldwide by pipeline engineers. As a parallel length of a pipeline to HVAC power lines and/or AC powered railroads, an induced AC voltage on a pipeline becomes higher. If there is a holiday in very high resistivity coating, this area will be subjected to severe AC corrosion. Today it is acknowledged that, the AC corrosion risk of pipelines with high resistivity coating must be evaluated by installing steel coupons at pipe depth and measuring the DC and AC current densities when a coupon is connected to the pipe. However, no measuring technique for the evaluation of AC corrosion risk in the field is recognized. This paper focuses on the measuring technique using an advanced instrumentation in the field to assess the AC corrosion risk of a cathodically protected pipeline paralleling an electric power transmission line and/or an AC powered railroad.
for AC current densities greater than 30 A/m2the maximum corrosion rate is in excess of 0.1 mm/y despite a constant cathodic protection current density of 2 A/m2. In this case, the protection potential criterion is not applicable.
A 1992 report by Funk, Prinz and Schöneich described the test results of steel coupons with respect to corrosion versus AC and DC (cathodic protection) current densities as follows5: