Temperature plays a significant role in the formation of protective iron carbonate films in CO2 corrosion as it increases the rate of chemical reaction, transport of chemical species to and from the bulk solution and the electrochemical reaction rate at the metal-solution interface. It has been reported in the literature that to form protective iron carbonate in a short time period, either the temperature or the pH has to be high. Increasing the temperature initially increases the rate of corrosion until a critical temperature is reached30. Beyond the critical temperature, the precipitation of an iron carbonates film starts which reduce the corrosion rate of the metal by acting as a diffusion barrier. The critical temperature also varies with pH and Fe2+ ion concentration in the bulk solution as shown in Eqn (1).
The rate of precipitation of iron carbonate (RFeCO3) can be expressed as a function of
Temperature (T), supersaturation (S), the solubility limits (Ksp) and surface area to volume ratio (A/V).
RFeCO3 = (A/V) ∗ f(T ) ∗(Ksp) ∗f(S ) --------------- (1)
Temperature can also change the molecular form of surface films by increasing the nucleation rate and a subsequent enhanced growth rate. When the precipitation rate is controlled by the crystal growth rate, the crystal growth rate can be expressed in terms of super saturation as
Rgr = Kgr (S – 1)2 ---------------- (2)
where, Rgr is the growth rate and kgr is the growth rate constant. To get a considerable precipitation rate, super saturation (defined as the ratio of the product of concentration of Fe2+ and CO32- ions to the solubility product (Ksp) has to be larger than unity and the kinetic growth rate constant has to be large. Increasing the temperature helps in attaining the super saturation state by increasing this kinetic constant (kgr) factor. Hunnik et al., developed a mechanistic model that predicts the kinetic constant (kgr) to increase by several orders when the temperature is increased from room temperature to 50°C. Nesic et al., proved experimentally that the precipitation rate constant increases with increasing temperature.
Increasing the temperature can either increase or decrease the corrosion rate depending on whether the solubility product of iron carbonate is exceeded. At low pH when the protective film does not form, the corrosion rate increases with increasing temperature; however at high pH, when the concentration of Fe2+ and CO32- exceeds the solubility limit, increasing the temperature will increase the rate of precipitation and enables the formation of protective iron carbonate films and decreases the corrosion rate. Surface films formed at high temperatures are continuous, compact and more stable than those formed at low temperature. The protective nature of iron carbonate films can be increased at elevated temperature.
See Also:
Effect of hydrodynamics on Carbon Dioxide CO2 corrosion on carbon steel pipe lines
Effect of CO2 partial pressure on CO2 corrosion on carbon steel pipe lines
Effect of Fe2+ concentration on Carbon Dioxide CO2 corrosion on carbon steel pipe lines
No comments:
Post a Comment