(92a) Corrosion Modeling Using Electrochemistry and Computational Fluid Dynamics

Corrosion
modeling using electrochemistry and computational fluid dynamics

The Butler-Volmer equation for
surface electrochemistry potential and Laplace equation for electrolyte
solutions have been available in CFD code for a while, but the challenge
remains to find a numerically affordable way to model the mass-transfer limited
corrosion rates. Mass-transfer coefficient using the Sherwood number (Nesic et al., Corr Sci., 1996) approach seems
promising, but it is limited by the use of average pipe velocity values, which
is unable to differentiate local flow changes in geometries like pipe bend and
welding locations. Conventional methods use the wall shear stress as the
surface mass transfer parameter, but unfortunately it is extraordinarily
expensive to solve wall shear stress accurately even for flows in a straight
pipe.  A better method is proposed to use
the free stream velocity as the input parameter to the Sherwood number. However,
the free stream velocity value requires the estimation of the boundary layer
thickness in turbulent flows and its accuracy depends strongly on the
turbulence models. Interestingly enough it was found that the free stream
velocity value is not sensitive to the accuracy of boundary-layer thickness predictions
and can be reliably used to identify flow velocity variations due to local
geometry changes. This method effectively addresses the drawbacks of the
Sherwood number approach, and avoids the costly computation needed to
accurately predict the wall shear stress. The proposed model was applied to a
pipe bend experiment conducted by Zhang et
al. (Corr. Sci., 2013) and the results show that the predicted variations
of corrosion rate are with 10 % range from the experimental results.