Understanding the Scales Formation Inhibition Mechanism on C-Steel in a Sour Media

Corrosion control in sour environments is a severe challenge for the oil and gas industries with respect to sour media. One of the approaches to mitigate such a problem is to use corrosion inhibitors (CI). The selection of a single (or a mixture of) CIs for a particular oil or gas production, transportation system or storage facility depends on its condition. In sour oil or gas media, depending on the ratio between carbon dioxide and hydrogen sulfide in the gas, pH, pressure, temperature, and brine chemistry, various kinds of iron sulfide and iron carbonate product layers can be formed. To control such formation scales of iron sulfides, it is essential to understand the mechanisms of corrosion inhibition in H2S systems. In this work, the electrochemical behaviour of C-steel in sour (i) de-ionised water (DIW) and (ii) in different NaCl solutions were examined in an H2S system. A series of experiments were conducted at ambient (RT), 50 oC and at 80 oC using different combinations of inhibitors (A and B). The corrosive behaviour of C-steel was investigated by measuring the open circuit potential (OCP), linear polarisation resistance (LPR), and potentiodynamic polarisation (PDP) in addition to scanning electron microscopy (SEM) coupled with an energy dispersive X-ray (EDX) and Raman spectroscopy (RS). The outcome has shown that the corrosion rate of C-steel initially decreased and then stabilised with time in the CI solution that might be linked to the inhibition effect of the corrosion product layers on the iron surface. SEM analysis shows that no film of corrosion products existed. CIs control the corrosion process and prevent passive film formation (iron sulfide), even with the presence of H2S in all tests.