Effects of chunk breakage and surface protective film on negative difference effect of magnesium alloys

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, Delta. The main cause for the difference effect of Mg-alloy which reveals a negative sip 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.