Effect Of Corrosion Inhibitor Active Components On The Growth Of Iron Carbonate Scale Under CO2 Conditions

The possible mechanism for inhibition has been debated on whether or not an existing corrosion scale is required for inhibitors to adsorb onto the surface. Previous work1-3 has shown both conditions to be true under certain testing conditions. This paper will demonstrate there is a synergistic relationship between iron carbonate and a quaternized amine corrosion inhibitor to form a tighter more corrosion resistant scale. This paper will explore the adsorption of quaternized amine during iron carbonate scale formation through electrochemical and post-test microscopic analysis. There is evidence that quaternized amines increase the precipitation rate of iron carbonate to enhance the already protective passivation layer, which is the opposite that was observed in previous work where the inhibitor prevented further iron carbonate from forming.

Corrosion inhibitors are one of the numerous methods to protect against the corrosion process so the oil industry can maintain the demand of production. Researchers4-9 have carried out studies on nitrogen based corrosion inhibitor compounds; such as, imidazolines, amines, quaternized pyridines, etc in order to find the ideal corrosion inhibitor compound. Work has been carried out focusing on understanding how the inhibitor interacts with the surface and how the structure would improve that interaction. Although the ultimate goal of a corrosion inhibitor is to decrease the corrosion rate in comparison to untreated conditions, the mechanism for optimal corrosion mitigation may not be thoroughly understood. Depending on the corrosion scale produced, it may be desired to either slow down the formation of the corrosion products by forming a molecular film on the metal or the inhibitor works cooperatively with the corrosion scale to form a passivation layer and prevent further corrosion from occurring. Bilkova¹ showed imidazoline inhibitors did not have an effect on the corrosion rate when applied to an existing iron carbonate scale which had been previously grown in CO₂ solutions with a pH greater than 6.0. The same inhibitor was shown to decrease the corrosion rate dramatically when it was added to a system where the iron carbonate scale was still forming and halted any further growth of iron carbonate. Chokshi² et al also observed the prevention of iron carbonate growth due to imidazoline inhibitors. They used various levels of supersaturation of iron (eq. 1) to observe the effect imidazoline inhibitors had on iron carbonate during scale formation and after the scale had formed. Supersaturation is defined as:

(mathematical equation available in full paper)

Where SS is the supersaturation, c is concentration and K_sp is the solubility constant. At a supersaturation of 150, the inhibitor prevented further growth of iron carbonate even after it had begun to scale causing a decrease in the corrosion rate. At a supersaturation level of 30 or less, the addition of inhibitor to an existing scale decreased the corrosion rate further indicating the inhibitor does interact with the corrosion scale. Gulbrandsen³ et al observed the effect imidazoline and amine inhibitors had on the corrosion rate after being applied to an existing iron carbonate scale. While the corrosion rate dropped after inhibitor treatment, the longer the iron carbonate scale was allowed to grow the inhibitor had less impact on the decrease of the corrosion rate.