Strategy for Eliminating Risks of Corrosion and Overprotection for Buried Modern Pipelines

The driving force that causes metals to corrode is a natural consequence of their temporary existence in metallic form. Application of cathodic protection (CP) as measures to stop corrosion has a very long history. The first application of impressed current system for protection of underground structures took place in England and in the United States, about 1910-1912. The first criterion for CP of steel pipelines, that is, −0.850V (vs. copper sulfate electrode) with the CP applied, was proposed by Kuhn in 1933 and has since been accepted and used worldwide on steel pipelines and structures in various soils and water. Kuhn‟s criterion has facilitated the application of CP for extending buried pipelines with the economic growth. Kuhn‟s criterion is referred to the protection potential criterion and in the scope of direct current (DC) corrosion protection. In the early 1900s, the effect of AC interference current on a metallic structure was known and to some degree had been quantified. In 1906, Hayden investigated to determine whether, and to what extent, AC currents passing between any metallic conductors (gas and water pipes, lead cables, etc.) and the ground would produce AC corrosion, due to the introduction of grounded AC systems using the developed single-phase railway motor. Hayden concluded iron is attacked less than lead. Up until the mid-1980s, the prevailing opinion was that, although AC current could cause corrosion of steel, the corrosion rate was a small percentage of an equivalent amount of DC and furthermore could be controlled by the application of CP in accordance with the protection potential criterion. Up to the mid1980s, corrosion failure on a pipeline was not attributed to AC corrosion, probably because the pipelines were bare or less well coated having sufficient grounding, such that induced voltages were not a practical problem. In 1986, corrosion failure on a polyethylene coated pipeline caused by induced AC interference currents resulting from AC powered rail transit system was first reported in Europe despite satisfying the protection potential criterion. Since then, pipeline failures caused by AC corrosion have been reported not only in Europe but in North America. Factors that contribute to AC corrosion include (1) high resistivity of pipe coating, and (2) the increased tendency to locate pipelines paralleling high voltage (HV) AC electric power lines and/or AC powered rail transit systems. It has been definitely shown by the occurrence of AC corrosion on a cathodically protected pipeline that AC corrosion cannot be prevented by CP in the presence of very high AC voltage of a pipeline. The necessity for establishment of criterion for AC corrosion protection has been realized ever since. However, there are no agreed-on criteria for AC corrosion protection. Recently, pipelines are being constructed in parallel with HVAC electric power lines and/or AC powered rail transit systems with thinner, high strength steel-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. The authors have developed an advanced instrumentation for assessing the AC corrosion risk of buried pipelines, and established the new CP criterion based on coupon DC and AC current densities (coupon current density-based criterion). The most distinguished feature of the instrumentation is the simultaneous computation in a measuring unit of 20 ms regarding coupon DC current density and coupon AC current density corresponding to the commercial frequency of 50 Hz. The criterion eliminates all corrosion risks such as AC corrosion, DC stray current corrosion, microbiologically influenced corrosion etc., and overprotection risk. This paper details how the developed instrumentation enabled understanding of AC corrosion or protection level of a cathodically protected pipeline, and establishment of the new CP criterion. Particular emphasis in the presence of AC is placed on the necessity for understanding of (1) limitations of the protection potential criterion and CP, and (2) adverse effects leading to possible AC corrosion and hydrogen embrittlement under overprotection circumstances caused by increasing the CP level as protection measures against AC corrosion. Furthermore, the features of an advanced instrumentation for assessing the AC corrosion risk of buried pipelines are also described with an example of measured data.