Numerical analysis model of galvanic corrosion in consideration of ion movement and reactions

Abstract

We have developed a new numerical analysis model for galvanic corrosion that can calculate ions movement,
reactions and electroneutrality. In this model, major species and reactions were considered, and ion density distribution
is corrected by solving Poisson's equation to satisfy electroneutrality. Reactions are calculated based
on chemical equilibrium. Galvanic corrosion of a Fe/Zn couple in a NaCl, MgCl2 solutions and artificial sea
water was calculated by this model. The distributions of pH, ion density and corrosion products were discussed
in these solutions. The distributions of corrosion product obtained by the numerical analysis model agreed
well with those measured by FT-IR method qualitatively. In 500ppm NaCl solution, Zn(OH)2, ZnCl2/6Zn(OH)2
and ZnCO3 were precipitated on the Zn and near the Fe/Zn interface, while there were little precipitations on
the Fe far from the Fe/Zn interface. On the other hand, Mg(OH)2 was precipitated on the Fe in 500ppm MgCl2
solution and 1/100 artificial sea water. Mg(OH)2 may contribute to corrosion protection of the Fe in the solution
including Mg2+. It is noted that Mg(OH)2 can be precipitated only in the cathodically protected area because
the solubility product constant of Mg(OH)2 is larger than that of Fe(OH)2.