This paper is about a study on the impact(s) of elemental and ionic mercury (Hg) to carbon steel corrosion in sweet and sour environments with and without Corrosion Inhibitor (CI) injection. Evaluation on the effect of mercury (Hg) deposition and the corrosion behaviour via Corrosion Rate (CR) using Linear Polarization Resistance (LPR), solution chemical properties, surface profile and corrosion product analysis are performed. Bubble test was carried out in glass cells and high-pressure rotating cylinder electrode (HPRCE) autoclaves test under multiple variables to study the CR behaviour. Fluid Analysis is performed using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) and XRD to analyse the chemical reaction of the fluid. Surface analysis is conducted using visual inspection and XRF.

The bubble test analysis shows that elemental and ionic Hg do not affect the CR under CO2 environment. Likewise, in H2S-containing environment, elemental Hg does not affect H2S corrosion evidence by the low pre-corroding CR. However, ionic Hg is observed to influence H2S corrosion where higher CR is measured. This is potentially due to the reaction between ionic Hg and H2S lead to the formation of HgS, depriving carbon steel inhibitive effect by H2S. It was observed that the addition of Corrosion Inhibitors does not reduce the CR probably due to the reaction between the ionic Hg and the sulphur-based compound from the CI, resulting in the decrease of film forming tendency. HPRCE autoclave result shows that CI had successfully reduced the corrosion rate to the targeted CR provided higher CI injection dose is introduced. Significant HgS spectrum observed during X-Ray Diffraction (XRD) analysis suggested that ionic Hg might have reacted with H2S and CI. This was supported by the presence of ionic Hg which might have interfered with the inhibitive mechanism of the CI and delayed the formation of passive layer on the metal. ICP-OES and X-Ray Fluorescence (XRF) indicated the presence of HgS traces potentially due to high reaction rate between ionic Hg and injected CI, thus accelerating the formation of HgS.

Sulphide-free CI formulation should be used if ionic Hg is present to ensure the CI effectiveness and its consumption during operations. In the event where the CI formulation cannot be changed, an enhanced pre-qualification evaluation as conducted in this study is to be carried out if ionic Hg is detected in the system to ensure the CI is effective and compatible with the existing process fluid.