Despite extensive use of corrosion inhibitors (CIs) to mitigate internal pipeline corrosion, few studies
address how corrosion residue or products affect their performance. In the present work, in-house
synthesized CI model compound, tetradecyl phosphate ester (PE-C14), was used to investigate the effect
of cementite (Fe3C) on corrosion inhibition performance on C1018 steel with ferritic-pearlitic
microstructure, on which residual Fe3C skeleton tended to stay on the surface during pre-corrosion. A 40
μm Fe3C skeleton was exposed and maintained on specimen surface after 2-day pre-corrosion under
studied condition. The corrosion rata (CR) evolution results indicated that the residual Fe3C skeleton
decreased significantly the inhibition efficiency (IE) of PE-C14, with 300 ppmw being insufficient to obtain
an acceptable IE. Some caveats related to the testing protocols are discussed. Under condition of
maintained Fe3C, from the polarization curves, corrosion rate and limiting current of H+ reduction reaction
were significantly accelerated due to the Fe3C layer supporting excessive cathodic reaction. With a ca.
40 μm Fe3C layer, the retardation on anodic reaction with 300ppmw PE-C14 is very limited compared with
that on bare surface. After adding PE-C14, a non-uniform corrosion morphology was observed on
specimens with 2-day pre-corrosion, which could be postulated to promote localized attack. Two main
limits to the above results are identified: the experiments were conducted in close medium where the
inhibitor concentration of the bulk could easily be depleted and the obtained data can only be applied in
case a 40 μm thick layer of cementite is formed. These two factors have a significant effect on the
outcome and further tests and work are ongoing to address these limitations.