Erosion-corrosion deterioration of carbon steel in CO2-saturated systems with sand is a problem in the oil and gas industry because the combined effects of erosion and corrosion can reduce the protection provided by iron-carbonate scale formation or inhibitors. Oil and gas production can be accompanied by the formation water (typically chloride containing brine). Some effects of chloride concentration on corrosion are not widely known, and this can result in misleading conclusions. The goal of this paper is to contribute to a better understanding of the effects of chloride concentration in CO2 corrosion. Previous studies reported in the literature and experimental and theoretical studies conducted in the present work have shown that increasing the NaCl concentration in solution has three important effects on corrosion results. First, standard pH meter readings in high NaCl concentration solutions require corrections. Second, increasing the NaCl concentration decreases the CO² concentration in solution and therefore contributes to a decrease in the corrosion rate. Third, increasing the NaCl concentration increases the solubility of FeCO³ and therefore reduces the likelihood of forming an iron carbonate scale. High NaCl concentration also decreases the sand erosion rate of the metal slightly by increasing the density and viscosity of the liquid.
Over the past 20 years¹-¹¹, the importance of mitigating erosion-corrosion in the oil and gas industry has led to research which has contributed to a better understanding of the effects of sand on erosion-corrosion. As used in this paper, the term “erosion-corrosion” refers to the erosion of protective iron carbonate scales or oxide layers by solid particle impingements, thereby allowing corrosion to take place at (higher) bare metal rates. Protective layers can be removed partially or completely. Similarly, some inhibitor films could be removed from surfaces by solid particle impingement¹0. The objective of the research presented here is to examine the effect of high NaCl concentrations on uniform corrosion, erosion, and erosion-corrosion behavior of carbon steels in CO² saturated systems. Oil and gas formations usually contain large amount of brine (altered seawater trapped in sediment pores). Chloride concentration can vary significantly, on average, produced brine during oil and gas production contains 50,000 mg/L chloride (Texas Water Commission (TWC))¹², which is corrosive to metals. Previous work by Uhlig et al. (1985)¹³ reported decreased oxygen corrosion with increased NaCl concentration in solution¹³. According to their investigation, increasing the amount of chloride in solution increases the conductivity and consequently the corrosion rate until a critical amount of NaCl (3 wt% NaCl) and, after that, increasing the chloride content will decrease the corrosion rate by decreasing the solubility of the corrosive agent (oxygen in their investigation). Nesic et al.14,15 also reported a considerable amount of change of corrosion rate by increasing the NaCl concentration in solution. At 20ºC, the corrosion mechanism gradually changes from mixed charge transfer/limiting current control to pure charge transfer control with the increase of salt concentration. An increase in salt concentration retards the heterogeneous charge transfer reactions, as well as the homogenous chemical reactions and the mass transfer15.
