Corrosion influenced by alternating current (a.c.)
The pick up of alternating current (AC) by buried pipelines is a well known
phenomenon, in particular in areas of dense infrastructure. Inductive,
capacitive, or ohmic coupling to electrical systems like power transmission
lines, railways, or grounding systems of power stations may introduce high AC
voltages of technical frequencies (50 or 162/3 Hz in Europe). Voltages of more
than 100 VAC versus remote earth are reported, which do not only pose a
hazardous situation to humans getting in direct contact, but which may also
cause high AC currents across the interface metal/soil at coating defects. Such
currents were found to be potentially corrosive, also for cathodically protected
One of such cases on a gas transmission line had occurred in Austria and was investigated by the TVFA. The attack is characterized by localized corrosion, leading to a leak, finally. The bitumenous coating above the corrosion site had a bubble-like structure, filled with finely dispersed magnetite and a mixture of sodium carbonate / bicarbonate. Moreover, the bitumen around this site had a shiny, remelted appearance indicating excursions to elevated temperatures.
These findings are consistent with the findings we derived from laboratory experiments. This project for the Austrian Association for Gas and Water (ÖVGW) was made in cooperation with the Institute of Electrical Power Systems of the TU Graz. In these soil box experiments, several side effects of AC induced corrosion were identified:
(i) increased conductivity of the soil close to the metal surface under test due to ion enrichment.
(ii) formation of hydrogen gas, leading temporarily to full or partial insulation of the sample.
(iii) temperature increase due to electrical heat dissipation in the electrolyte.
(iv) dependence of corrosion on crevice conditions at the metals surface.
These effects are considered responsible for the highly nonstationary behaviour of AC corrosion.
The electrochemical data received from these experiments indicate that the negative cycle of AC leads to water electrolysis. The consequence of such a process should be strong alkalization of the metals surface. This finding supports the hypothesis of AC corrosion occurring under strongly alkaline conditions where iron is known to be unstable.
Future work will address the assessment of this corrosion phenomenon by electrochemical measurements. The analysis of the nonlinear distortions in potential caused by the large signal excitation of AC was found to be a promising method.