“Influence of Si, Sb and Sr Additions on the Microstructure ...

_ _WChapter 2: Literature Review
This discrimination is explained by the presence <strong>of</strong> zinc in the solid solution. It is also
reported that in Mg-Al alloys, presence <strong>of</strong> Zn influences the Eco" <strong>of</strong> both matrix <strong>and</strong>
precipitates <strong>and</strong> has a beneficial effect on the corrosion resistance <strong>of</strong> the alloys [I97].
(ii) [3-Mg17Al1;
The [3-Mg17Al1g phase is cathodic with respect to the matrix <strong>and</strong> exhibits a
passive behavior over a wider pH range than either <strong>of</strong> its components Al <strong>and</strong> Mg. B­
Mg17Al1; is found to be inert in chloride solutions in comparison with the surrounding
Mg matrix <strong>and</strong> could act as a corrosion barrier [200]. It is reported that the resistance
<strong>of</strong> [3-Mg17Al1;; to corrosion is due to the presence <strong>of</strong> a thin passive film on its surface.
However, literature also suggests that B phase serves dual role in corrosion;
the B phase can acts either as a barrier or as a galvanic cathode [37, 201-203]. if it is
present as a small fraction, it serves mainly as a galvanic cathode, <strong>and</strong> accelerates the
overall corrosion <strong>of</strong> 0. matrix. However, if its fraction is high, then it may act mainly
as an anodic barrier [35].
If the (1 grains are very fine, the B phase fraction is not too low, <strong>and</strong> the B
phase is nearly continuous like a net over the a matrix, then the [3 phase particles don
not easily fall out by undermining. Instead, in this case the or matrix is much more
easily corroded <strong>and</strong> even undem1ined because many more or grains are completely
separated by the B phase net. Therefore, it can be expected that [3 fraction increase
with time during corrosion, <strong>and</strong> finally the fraction becomes high enough to make the
flphase become nearly continuous. The corrosion rate should be low after this steady
state surface condition is reached. Hence, if or grains are fine, the gaps between [3
precipitates are narrow <strong>and</strong> the B phase is nearly continuous, the corrosion is greatly
retarded [20l, 202]. Figure 2.22 provides a schematic illustration <strong>of</strong> change <strong>of</strong> surface
composition <strong>of</strong> die cast during corrosion. This kind <strong>of</strong> behavior normally observed
with die cast AZ9l alloy.
On the other h<strong>and</strong>, if