the ausaged alloy 333 are shown in Figure 8. Chromium is found to be distributed in primary as well as secondary car- bides and copper in the matrix. But copper distribution in the matrix is not homogeneous. As the solid solubility of copper is poor, except in austenite, it will precipitate out from the transformed austenite phase.13
Manganese and silicon are
present both in carbides and the matrix. B) Corrosion Behavior:
The corrosion rate measurements using the cathodic polar- ization method were carried out on alloys 330, 333 and Ni- hard alloy for comparison and a typical overvoltage versus current density plot for as-cast alloy 333 is shown in Figure 9. The important electrochemical parameters obtained are shown in Table 2.
The analysis of the polarization data indicates that amongst the alloys studied, the alloy 333 in as-cast condition has a minimum corrosion current and the same is comparable to that of nickel-chromium iron (type IV). However, in ausaged condition the same alloy shows an increased rate of corro- sion. The copper free chromium-manganese ausaged alloy 330 with almost the same microstructure as the ausaged al- loy 333, indicates a considerable increase in corrosion rate. These observations indeed prove the beneficial effect of cop- per on increasing the corrosion resistance. The alloy 333 in ausaged condition with a large number of precipitates in the matrix, exhibits a decreased corrosion resistance than the as- cast predominantly austenitic alloy of the same composition. This is due to local cell formation in a chloride solution with the secondary carbides as cathode and the matrix as anode. This is evident in the corroded SEM micrographs (Fig. 10).
Figure 9. Cathodic polarization curve for as-cast alloy 333 in 5% NaCl solution.
Table 2. Electrochemical Parameters of Cathodic Polarization
54
International Journal of Metalcasting/Winter 11
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