Hydrodynamics affects corrosion rate by changing the rate of mass transfer involved in CO2 corrosion. Due to the flow sensitive nature of the cathodic reactions, at higher velocities H+ reduction is mass transfer controlled (when the overall reaction rate is controlled by the rate of diffusion of H+ to the electrode surface) and H2CO3 reduction is controlled by both mass transfer and chemical reaction, while the latter part comes from the hydration of CO2 to H2CO. For a corrosion form that involves fluid flow, the effects due to mass transfer and momentum transfer has to be considered. The shear stress at the interface between the solid wall and fluid represents the momentum transfer rate, whereas the mass transfer rate constant represents the mass transfer rate. The effect of wall shear stress on mass transfer was explained by Silverman. It was found from experiments that the magnitude of the wall shear stress is too low for hydrodynamic scale destruction. On the other hand, it was found that above a critical wall shear stress flow induced localized corrosion is initiated.
When there is no protective film formed on the surface, increasing the velocity of the
fluid increases the corrosion rate by increasing the mass transfer of species between the electrode surface and the bulk solution. The effect of turbulent flow on corrosion rate when there is no protective film present can be given by the following equation
Corrosion Rate = x ∗ (flowrate) n ---------------------- (3)
Where, x is a constant and n is the exponent factor which depends on the corrosion mechanism involved. The exponent factor has a value of 0.8 for a diffusion controlled reaction in a smooth pipe and varies between 0.4 – 0.7 for those reactions which are partially controlled by chemical reaction and diffusion reaction.
The exponent factor gives an indication of flow sensitive nature of the cathodic reaction, and the higher the value the more flow sensitive is the corrosion rate. When the corrosion reaction is dominated by charge transfer, the increase in velocity has less effect on the corrosion rate. Increase in velocity decreases the precipitation rate and surface saturation of Fe2+ and CO3 2- because of near wall turbulence, which prevents Fe2+ ions from precipitating. On the other hand, at low velocity, the rate of precipitation is higher than the corrosion rate thus enabling protective film formation.
The surface films formed on carbon steels at higher velocity are less protective than those formed at low velocity. The surface super saturation is less than one when the velocity is greater than 3 m/s. After film formation, hydrodynamics can affect the corrosion rate by mechanically removing the film. Based on experimental results, Ruzic et al., assumed that the mechanical removal of film in single phase flow takes place in the following sequential steps.
i. Separation from substrate
ii. Vertical cracking
iii. Crack opening and widening
iv. Film detachment.
After the film gets damaged, it creates a potential difference between the protected and unprotected region on the carbon steel, which increases the corrosion rate by localized corrosion called galvanic corrosion. A galvanic corrosion cell is formed when two similar or dissimilar metals are electrically connected where the metal with more electronegative potential will preferentially corrode
ee Also:
Effect of temperature on Carbon Dioxide CO2 corrosion on carbon steel pipe lines
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
ee 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
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