In the present study, the corrosion behavior of as-cast Mg-AI and Mg-AI-Zn alloys was studied as a function of the A1 content in the matrix. Corrosion properties such as the corrosion rate, corrosion potential, and repassivation tendency were estimated through immersion and electrochemical tests. The corrosion potential and corrosion rate of a solutionized alloy depend mainly on the A1 content of the as-cast alloy. The variation of A1 content in the Mg-rich matrix influences the stability of the passive film and the repassivation tendency, i.e., as the A1 content of the matrix increases, the repassivation tendency of the surface protective film after its breakage deteriorates. Also, it was proven that the enhancement of corrosion resistance by heat treatment, as in T6, is due to the decrease of solute concentration in the matrix, in addition to the effect of the precipitate, which plays the role of a barrier against corrosion.

In the present study, the factors having an affect on the difference effect of Mg-alloy, such as the role of monovalent ion, chunk breakage and surface protective film are discussed in detail. The degree of difference effect of as-cast magnesium alloy increases with Al content and applied external potential. The chunk breakage of precipitate was observed under open circuit potential, as well as applied external potential. Also, the precipitate which is disrupted as chunk type maintained its original morphology as that of as-cast alloy among the corrosion products after a corrosion test. It is thought that chunk breakage is a secondary factor bearing on the difference effect, which varies only the degree of the difference effect, Δ. The main cause for the difference effect of Mg-alloy which reveals a negative sign is due to the corrosion behavior of the surface protective film which is influenced on the Al content of the Mg-rich matrix, e.g., the repassivation tendency.