Fixed sign error on the last term of the equation for Phi in the corr…

…osion app

applications/corrosion/tex_files/corrosion.tex

@@ -239,7 +239,7 @@ \section{Governing Equations}
 \end{equation}
 \noindent where $c_i =c_M, c_+$ and $\Phi$ is the electrostatic potential.  The last term of the right hand side of Eq. \eqref{conc_dynamics} is zero for $c_i =c_+$  because $i_{rxn}$ is the reaction current density for the dissolution of the metal (M), which does not involve the supporting cation (+) and anion (-). The SBM reformulated governing equation for the potential is
 \begin{equation}
-\nabla \cdot (\psi \kappa \nabla \Phi) = F \nabla \cdot \left[ \psi \left( z_M (D_- - D_M) \nabla c_M + z_+ (D_- - D_+)  \nabla c_+ \right) \right] + |\nabla \psi | i_{rxn},
+\nabla \cdot (\psi \kappa \nabla \Phi) = F \nabla \cdot \left[ \psi \left( z_M (D_- - D_M) \nabla c_M + z_+ (D_- - D_+)  \nabla c_+ \right) \right] - |\nabla \psi | i_{rxn},
 \end{equation}
 where 
 \begin{equation}