Abstract:
The aim of this research was to investigate the cause of the severe localised
corrosion that sometimes occurs at welds in carbon steel pipelines carrying
hydrocarbons and inhibited brines saturated with carbon dioxide. A rotating
cylinder electrode (RCE) apparatus was designed so that electrodes machined from
the weld metal, heat affected zone (HAZ) and parent material of welded X65
pipeline steel could be galvanically coupled and tested in high shear stress
conditions. The galvanic currents flowing between the weld regions were recorded
using zero-resistance ammeters and their self-corrosion rates were found by
polarisation resistance measurements. The total corrosion rate of each weld
region was obtained from the sum of the self-corrosion and galvanic
contributions. In uninhibited conditions, the weld metal and HAZ were both
cathodic to the parent material and localised corrosion was prevented. However,
when an oilfield corrosion inhibitor was present a current reversal took place,
which resulted in accelerated weld corrosion. Electrochemical impedance
spectroscopy (EIS) showed that the inhibitor film had lower electrical
resistance and was less protective on the weld metal than on the parent
material. At the highest shear stress, a second current reversal could occur
when the inhibitor was removed from all regions of the weld and there was a
return to the original galvanic behaviour. It was concluded that preferential
weld corrosion was caused by unstable conditions in which the inhibitor film was
selectively disrupted on the weld metal but remained effective on the other weld
regions.
