MASS TRANSFER IN A CREVICE

The molar flux in this type of corrosion is mainly due to the mass transfer of Na+, Cl, and OH ions and dissolved molecular oxygen (O2) in the seal. According to the stoichiometry in eq. (8.4), the hydroxyl ions (OH ) form in the presence of water and oxygen and react with ferrous ions (Fe ) to form ferrous hydroxide, Fe OH), as the the crevice corrosion product, eq. (8.6). However, crevice corrosion can be avoided by cathodic protection of hoUdays by controlling current leakage and related local potentials. 

This model has been successfully used by Chin and Sabde [25] for crevice cathodic protection using numerical analysis based on the dilute solution theory and reduction reaction of dissolved oxygen and Aa+, Cl, and OH ions at the crevice surface. Hence, the Nemst-Plank equation, eq. (4.2), can be generalized as a differentiable and continues scalar diffusion molar flux function 

The ferrous Fe+ ions are not included in the analysis since the crevice cathodic protection is for preventing the formation of this t5 e ion. For a coupled diffusion and migration molar flux under steady-state conditions, dCjIddt = 0, the molar flux becomes the continuity equation for mass transfer under steady state conditions 

Expanding eq. (8.44) into two-dimensional cylindrical coordinates gives a generalized expression that is solved numerically for particular species involved in the electrochemical process of crevice cathode protection 

Electric neutrality of the electrol5de dictates that V = 0 and eq. (8.42) along with y = 0 gives 

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