The negative difference effect of magnesium and of the AZ91 alloy in chloride and stannate-containing solutions

The negative difference effect of pure magnesium and of the alloy AZ91 was investigated by volumetric tests in NaCl with and without addition of Na2SnO3. Hydrogen comes from two sources: H2 which accompanies localized magnesium dissolution inside the pits and H2 from H2O reduction at the passive surface outside the pits. By separating the two parts it could be shown that the rate of hydrogen evolution inside the pits is quantitatively consistent with a two-step EC-mechanism of magnesium dissolution with hydrogen evolution coupled to the second dissolution step. Addition of Na2SnO3 does not influence the second step.