The Effect of Bentonite on External Corrosion of Well Casings

  • Mohammed Orayith

Student thesis: Phd


ABSTRACTThe overall goal of this research is concerned with understanding the effects of bentonite on the external corrosion of bare mild steel well casing. Na-bentonite is mainly used in enormous amounts in drilling processes, so it used as the main electrochemical environment surrounding the casing at different condition. The major part of the current study was divided into 3 stages; the first stage is constant current cathodic protection (CP) with a range of 0.0 (Open Circuit Potential) to 200mA.m-2 was applied respectively to protect bare mild steel buried vertically inside the bentonite layer contains 45% (w/w) 0.5 M NaCl Solution. This study was attempted to investigate the polarisation potential distribution over depth. The second stage is polarisation potential with a range of OCP to -1.15V/SCE was applied to protect mild steel exposed to 0.5M sodium chloride solution containing different concentration of bentonite, namely 0.0%, 1.0 and 10.0% (w/w). The third stage was concerning with the ZnCl2 added at 500ppm and 1000ppm to the bentonite as cathodic inhibitor to investigate its effects on the corrosion process of mild steel. CP at 0.0mV (OCP) and -1.15 V against saturated calomel electrode (SCE) were applied for this experiment.Weight loss, visual observation, Open Circuit Potential (OCP), cathodic protection and linear polarisation resistance (LPR) techniques were employed for this study. Optical microscope, Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray analysis (EDX), and X-Ray diffraction (XRD) analysis are illustrated to study, examine and analyse the films that formed on the metal surface.It is demonstrated that the corrosion rate produced from LPR measurement data was fairly lower than that obtained by experimental weight loss data. Low corrosion rate was recorded for the specimens immersed in 0.5NaCl solution, containing bentonite compared to that were obtained for solution free of bentonite. Magnesium hydroxide and calcium carbonate were the main chemical compounds that detected on the metal surfaces when cathodic protection (CP) was applied at -1.15V vs. SCE. It was confirmed that, the addition of ZnCl2 at two different concentration, 500ppm and 1000ppm has reduced the current density applied to a considerable value. This was ascribed to the formation of compacted and uniform film on the protected surface. The examination of the specimens using EDX and XRD has shown the formation of zinc containing compounds. Small amount of NaCl in the form of halite for both concentrations was also detected.
Date of Award1 Aug 2012
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorJames Scantlebury (Supervisor)


  • Bentonite, Well CasingCathopdic protection, SEM and XRD

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