Among the specialized electrical surveys to search for coating faults, assess possible external
corrosion conditions and verify Cathodic Protection effectiveness, DCVG (Direct Current Voltage
Gradient) survey, and in particular the Transverse DCVG is usually considered the most effective
technique for finding coating faults on transport pipelines for oil, gas, and water.
In this paper some preliminary results are reported that show that using a.c. current instead of direct
current the localization of the coating faults can be much more precise and simple to be performed
at the same time. Compared with the DCVG, the ACVG has many advantages among which: it does
not need ON/OFF cycles since the OFF conditions are almost constant in the absence of alternating
stray currents, it can be carried out with economic and simple instrument, the use of unpolarizable
reference electrodes (SCE) is not necessary and furthermore, it locates the faults and quantifies their
importance with much more accuracy.
Besides, the method needs shorter times and less manpower: one only operator is sufficient to
perform the survey and it is therefore cheaper than performing DCVG.
The results of the experiments carried out at the Training and Research Centre of Riyadh by
comparing the results of the two methods using the same current densities, clearly show the
different behaviour of the two techniques, in the presence of simulated / calibrated coating faults.
Further researches are in course with this quite new technique as the localisation of coating faults is
being made in terms of impedance rather than in terms of resistance. This has quite a huge
implication when corrosion conditions of real coating faults are concerned. In case of a.c.
interference (e.g. due to a.c. fed railways or electricity power lines) this could be a unique method
able to locate corroding coating faults on a buried, interfered metallic pipeline.
The method is being patented not only as a new technique for coating fault location, but also for its
promising further developments still under investigation, as capable of finding corroding faults in
case of a.c. interference.
A New Technique for Location of Fault Location on Transmission Lines