Mitigating Corrosion Risks in Oil and Gas Equipment by Electrochemical Protection: Top of the Line Corrosion

  • Fredric Ajayi

Student thesis: Phd


This study investigated the corrosion processes at the top and bottom of carbon steel pipelines transporting wet gases, and studied possible chemical mitigation strategies. First, immersion tests were carried out using carbon steel to study the effects of de-aeration with high purity nitrogen gas on the corrosion rate. Secondly, the corrosion rate was assessed for varying chloride ion concentrations in an aerated environment. In general, increasing de-aeration time changes the corrosion mechanism from mass transfer oxygen reduction to water reduction reaction. However, oxygen solubility controlled the corrosion process in aerated solution containing different chloride ion concentrations. A special two-electrode cell was designed for the top of the line corrosion (TLC) electrochemical measurements but a conventional three electrode cell was used for the bottom of the line corrosion (BLC) measurements. The TLC rate increases with temperature, and X-Ray Diffraction (XRD) confirmed the presence of chukanovite {Fe2(CO3)(OH)2}and possibly ferrous carbonate corrosion products at 40oC and 60oC respectively. However, for the BLC, the cementite phase remained on the metal surface after preferential dissolution of the ferrite phase in the carbon steel. Addition of acetic acid (HAc) locally dissolved the initially FeCO3 film formed on the metal surface, causing local corrosion damage. Addition of methyl di ethanol amine (MDEA) as a pH stabiliser reduced TLC and BLC rates due to enhanced stability of FeCO3 at pH 5.7-6.3. When Zn2+ ions were added as ZnCl2, both Fe2O3 and ZnCO3 were formed at reduced corrosion rate. Whenever FeCO3 film was damaged/dissolved by HAc addition of neither pH stabiliser; MDEA nor hydrate preventer; mono ethylene glycol (MEG) could not re-establish a protective film on the metal surface. The following organic inhibitors were investigated as potential mitigators of TLC: 2-mercaptobenzimidazole (2MBI), 2-amino-5-ethyl-1,3,4-thiodiazole (AETDA), 2-phenyl-2-imidazoline (2PI), dicyclohexylamine (DHA), and a commercial inhibitor formulation (CI-A). The inhibition efficiency (IE%) was found to increase in the order CI-A>2MBI>AETDA>DHA. Their efficiency increases (except DHA) with inhibitor concentrations both at top and bottom of the line. 2MBI and CI-A behaved as mixed inhibitors but AETDA behaved as cathodic inhibitor, all were best fitted to a simple Langmuir adsorption isotherm. However, IE% of DHA decreased at higher inhibitor concentrations. Surprisingly, 2PI inhibitor increased the corrosion rate, and the corrosion rate further increased with increase inhibitor concentrations. Weight loss measurements results of TLC are also presented which showed lower inhibition efficiency for all the inhibitors investigated compared with electrochemical measurements in similar environments. The free energy of adsorption (∆Goads values for 2MBI and AETDA are around -36kJ.mol-1 while for CI-A the value was -15kJ.mol-1 (-7kJ.mol-1 in the presence of HAc). This is evidence for adsorption of 2MBI and AETDA on the metal surface by chemisorption with CI-A by physisorption. XPS analysis confirmed the presence of FeCO3 and FeOOH as corrosion products in the brine solution in the absence and presence of HAc containing different corrosion inhibitors.
Date of Award31 Dec 2015
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorStuart Lyon (Supervisor) & Rob Lindsay (Supervisor)


  • Acetic acid, adsorption, condensation, inhibitor, top of the line corrosion

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