Kinetics Analysis of X65 Steel corrosion reactions at the simultaneous presence of CO2 and H2S

In this research, the effect of fluid velocity on corrosion kinetics of X65 steel has been investigated in an aqueous solution containing CO2 and H2S according to the steel structures corrosion sensitivity in aqueous environments. The aqueous solution saturated with CO2 and contained 50 ppm H2S was used to perform the corrosion tests. The fluid velocity varied between 0 to 1000 rpm, and the tests were carried out at three different temperatures of 298, 318, and 338 K. During the tests, parameters like the iron ion concentration (iron count) (ICP) and instant corrosion rate were monitored. The corrosion products were characterized using XRD and SEM methods and the kinetics and mechanism of the corrosion process were analyzed. The results showed that the first layer, containing mostly mackinawite, does not have the appropriate density, in such a way that the porosities and micro-cracks could be a suitable path for corrosive ions to penetrate the steel surface. Then, as time passed and the thickness of the corrosion product film increased, the number of porosities decreased; this led to the compactness of the product film and resulted in the decrease of diffusion and ion exchange in the interface, and consequently, the corrosion rate decreased. According to kinetics analysis on the concentration of iron ion variation with time at various temperatures, it was observed that the controlling mechanism of corrosion rate in all the test velocities is of two-dimensional diffusion (g(α)=(1-α)ln(1-α)+α). However, with the increase of fluid velocity, the activation energy of the mentioned diffusion mechanism has increased from 646.47 J/mol to 2743.41 J/mol. In fact, according to the phase analysis and microstructure evaluation of the corrosion products, the reason for this increase could be due to the effect of fluid velocity on the nature and diversity of the corrosion products, especially their continuity and compactness. Therefore, the increase in fluid velocity, at first, resulted in an increase in the corrosion rate, and then, with the formation of a corrosion products layer, its effect has been decreased.