Study On Inhibition Mechanism Of Galvanic Corrosion Using Inhibitors In CO₂ Containing Environment

It is unavoidable to use noble anticorrosion alloys in huge complicated systems such as offshore drilling & recovery platform and oil & gas system in rigorous complex environment. The whole reliability and economy are improved by assembling the noble anticorrosion alloys and carbon steel. The safety problem arising from galvanic corrosion has been the weakest portion for corrosion protection in the systems. Most common corrosion inhibitors have poor inhibition on galvanic corrosion. Furthermore, reports of inhibition mechanism on controlling galvanic corrosion by inhibitors are rather few. Therefore, it is significant for both theoretical and practical aspects to investigate galvanic corrosion and inhibition mechanism of inhibitors.In this paper, galvanic corrosion of N80 carbon steel coupled with stainless steel in CO₂-containing corrosion media was investigated by various experimental methods including weight loss, polarization curve, electrochemical impedance spectrum, galvanic current, atomic force microscopy and Fourier transform infrared spectroscopy. By studying the distribution state of surface excess charge, the galvanic corrosion behavior and effect of surface excess charge on adsorption are stressed. The mechanism of inhibitor adsorption on both cathodic and anodic areas of galvanic electrode is proposed. Based on the charge distribution state of the galvanic electrode, special studies are carried out to make it clear the inhibition and synergistic effect on galvanic corrosion. The main results are listed blow.The inhibitor, imidazoline amide, is of anodic adsorption type and mainly inhibits anodic process of carbon steel corrosion in CO₂-saturated solution containing Cl-. It is also an effective corrosion inhibitor for carbon steel in this solution. The adsorption meets the El-Awady thermodynamic kinetic model, and is in line with the Flory-Huggins isotherm. According to the model, at relative lower temperature, an inhibitor molecule replaces one or two water molecules, and it occupies one or two active spots accordingly; at relative higher temperature, an inhibitor molecule replaces at least two water molecules, and it occupies more than two active spots accordingly.Although imidazoline amide has high inhibition efficiency for carbon steel in corrosion environment containing carbon dioxide, like some other corrosion inhibitors, it is non-effective for galvanic corrosion either. In CO₂-saturated solution containing Cl-, at their corrosion potentials, stainless steel and carbon steel carry positive and negative excess charges. However, when stainless steel and carbon steel is coupled in the same solution, at the mixed potential, stainless steel and carbon steel carry negative and positive excess charges respectively. The surface charges on cathodic and anodic areas of galvanic couple are much different from those on single metals.Sodium laurate is not effective for carbon steel corrosion in CO₂-saturated solution containing Cl-, but it is effective for galvanic corrosion of galvanic couple (stainless steel/carbon steel). In the presence of laurate inhibitor, positive excess charges would be carried on both stainless steel and carbon steel. When stainless steel and carbon steel is coupled in the inhibitor solution, at the mixed potential, stainless steel and carbon steel carry negative and positive excess charges respectively. The inhibitor can adsorb to the surface of both carbon steel and stainless steel whether they are alone or coupled. Comparing to the adsorption on carbon steel uncoupled, the adsorbability and coverage of the inhibitor film on the carbon steel of the couple is bigger and larger.There exists synergistic inhibition on galvanic corrosion for the couple (stainless steel/carbon steel) between imidazoline quaternary compound and inorganic ion I-, they remarkbly inhibit the cathodic process of galvanic corrosion. Ion I- can improve the metal surface excess charge on galvanic electrode significantly. The adsorption is strengthened by the synergistic effect of quaternary compound and inorganic ion I-, thus the galvanic corrosion is well controlled.Surface excess charge on metals can significantly affect the adsorption behavior of inhibitors. In CO₂-saturated solution containing Cl-, at their corrosion potentials, stainless steel and carbon steel carry positive and negative excess charges respectively. However, charges change to be inverted when two metals are electrically coupled. Cationic inhibitors can adsorb on the metal carrying negative excess charge, thus the corrosion is inhibited; Cationic inhibitors are difficult to adsorb on the anode of galvanic couple carrying positive excess charge, thus is non-effective to galvanic corrosion. However, high inhibition efficiency for galvanic corrosion can be obtained by using anionic inhibitors. Inorganic anion can change positive charges into negative charges on the anode of the galvanic couple, thus helps cationic inhibitors to be easily adsorbed on it. The adsorption is strengthened remarkably by the synergistic inhibition of cationic inhibitor and inorganic anion, thus the galvanic would be better controlled.