Voltammetry at rotating disc electrodes and scanning electron microscopy are used to compare the corrosion of a 13 % chromium steel (API 5CT L80-13Cr) and carbon steel (X65) in simulated oilfield brines based on 3 % NaC1 saturated with CO2 and with various additions of acetate, acetic acid and bicarbonate. It is confirmed that the 13 % Cr steel is significantly less prone to corrosion and the rate is determined by the properties of the passivating film formed on its surface. In sharp contrast, the carbon steel remains active for corrosion and its rate is determined by the concentration of reactants for the cathodic reactions contributing to corrosion
During the recovery of oil, an aqueous brine saturated with carbon dioxide is usually one component of a multiphase flow stream. Wherever possible, cost considerations determine that the pipework is constructed from carbon steel. Carbon steel, however, does not always have the essential resistance to corrosion and it is widely recognized in the oil industry that 13 % chromium steels give significantly longer lifetimes 1. This was recently confirmed by extended laboratory testing
2. One situation where carbon steels are found to undergo enhanced corrosion is when the oilfield brine contains the anions of weak acids, most commonly acetate ion (OAc-). A number of papers 3-9 have addressed the reasons for the enhanced corrosion. In recent papers 69, we have developed the concept that the corrosion rates of carbon steels in such brines containing acetate/acetic acid can be understood if it is recognized that the cathodic current in the corrosion process arises from the parallel reduction of several species independently transported to the steel surface, namely (a) proton (b) carbonic acid (c) acetic acid and (d) carbon dioxide though a mechanism involving its initial hydration; an increase in the concentration of any of these species will accelerate corrosion. This model requires that the equilibrium speciation of the brine can be calculated and this depends on temperature and pressure as well as the concentrations ofNaC1, HzCO3/HCO3-/CO3:2-/CO2, and HOAc/OAc.
Although the use of 13 % Cr steels has been recommended for such acetate containing brines, there have been no reported studies of this steel in the brine compositions met in oil pipelines. Ueda and Takabe 1° have reported that the addition of acetic acid (0.5 %) to 5 % NaC|/CO2 increased the rate of corrosion of 13 % Cr steel but in these experiments there was no acetate or bicarbonate present so that the pH was always ~ 4. Linter and Burstein 11 have described a detailed voltammetric study of 13 % Cr steel in NaC1/CO2 but enforce a constant pH of 4 by using a buffer. A phthalate buffer was used and this introduces the chemistry and electrochemistry of phthalic acid, H2Phth, and the HPhth- anion (proton donors similar to HOAc). Hence, in both these papers, the pH was not allowed to float and hence the conditions are not the same as in the oil/brine stream in the oi|field where the pH of the brine is generally in the range 4.5 - 6.0, especially when significant concentrations of acetate or bicarbonate are present. This paper compares the voltammetry and scanning electron microscopy of a 13 % chromium stainless steel (API 5CT L80-13Cr) and a carbon steel (X65) in simulated oilfield brines based on 3 % NaC1 saturated with CO2 and with various additions of acetate, acetic acid and bicarbonate. It will be shown that the behaviour of the two steels is completely different.
