Multiple mass transport and reaction type rate processes are involved in high temperature refinery sulfidation corrosion. Corrosion tests provide only aggregate rates of corrosion which results from underlying generally sequential sub-processes. Theoretically, rates of these sub-processes can be calculated using principles of nonequilibrium thermodynamics and kinetics, if associated phenomenological coefficients are known. These coefficients are traditionally obtained by experimentally confirming the governing rate determining step. In complex systems such as refinery sulfidation corrosion, conventional empirical models such as Arrhenius, parabolic, logarithmic, or other popular rate laws cannot be confirmed exclusively. This has been elucidated by high temperature sulfidation tests manipulating concentration, temperature, and duration. Theories of solid-state chemistry and general chemical science are discussed considering the experimental data in order to create a mechanistic model which can simulate trends in corrosion rates.
High temperature sulfidation (or sulfidic) corrosion of steel by sulfur species in crude oil has long been known to damage refinery equipment.1 Corrosion engineers have been using prediction curves derived from field corrosion data2,3 to estimate rates of sulfidation corrosion. However, a significant inaccuracy is often encountered in these estimations because of the extensive diversity in molecular structures of sulfur compounds in crude oils. Additionally, the presence of naphthenic acids complicates the problem further by influencing, physically and chemically, the sulfidation mechanism.4 Hence, a reliable prediction model of sulfidation is sought after, especially in the presence of naphthenic acids.
Several models for refinery sulfidation corrosion have been published.5-11 Some of these models are based on empirical laws and the rest have arbitrary relationships and "fudge factors". An effective prediction model should secure sulfidation rate as a function of system variables such as composition of sulfur compounds, temperature, and pressure, based on a mechanistic description. In general, any specific sub-process or a step of the mechanism is validated by experimentally confirming the rate law associated with the step which relates corrosion rate with one or more system variables. Further manipulation of system variables is sought to confirm the rest of the steps. The methodology is illustrated schematically in Figure 1. When the manipulation range is limited by practicality, the remaining steps must be deduced pragmatically in the absence of other adequate methods. If none of the reaction rate laws can be distinctly identified then the reaction can be considered in "mixed control" of several mechanistic steps, meaning that the unit rates of individual steps are not significantly different. Here, the refinery sulfidation corrosion is analyzed as a mixed controlled reaction to develop useful insights into the mechanism.
