Effect of an Mg-rich matrix on the corrosion behavior of As-cast magnesium-aluminum alloys

Abstract

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.

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

“…The corrosion potential of the solutionized samples are shown in Table 2. As the Al content increased to 9 wt.% Al, the corrosion potential decreased from -1.60 V to -1.65 V. When the Al content was 12 wt.% Al, the corrosion potential of the solutionized sam- ple increased again to the cathodic region, -1.63 V. This variation of corrosion potential with Al content is due to the mutual action between the corrosion behavior of the repassivation tendency and the mixed potential theory [16]. Fig.…”

“…Fig. 8 reveals the repassivation tendency with the Al content of the solutionized samples [16]. This indicates that, after the breakage of the surface protective film, the stabilizing time to original corrosion potential, depends on the Al content of the solutionized sample.…”

“…This indicates that, after the breakage of the surface protective film, the stabilizing time to original corrosion potential, depends on the Al content of the solutionized sample. Since the Al in the Mg-rich matrix existed in pure state and acts as a cathodic phase to the surrounding Mg-rich matrix [16], the passive film of the matrix becomes more unstable with the Al content. Therefore, the equilibrium corrosion potential for the redox reaction is lowered to the active region with the Al content.…”

“…It may be thought that this is because the effect of mixed potential is more dominant than the decreasing effect due to the unstability of the passive film. Therefore, as mentioned above, when the Al content is about 9 wt.% Al, the corrosion potential indicates a critical range where the decrease in corrosion potential through the instability of the passive film offsets the effect of the mixed potential theory [16]. Fig.…”

Semi-Solid Processing of Magnesium Alloys

“…At present, die-casting is mainly utilized to produce the Mg alloy products. In recent years, the modification of alloy compositions and/or heat treatments has been attempted to increase the mechanical properties [8][9][10] and corrosion resistance [11][12][13] of casting alloys. In the present study, the thixotropic behavior of the semi-solid AZ91D magnesium alloy was investigated with the changes in the rest time and the up time.…”