temperature. To select the critical temperature for ausaging, the as-cast alloy 333 was tempered isochronally for a fixed time of one hour at temperatures of 600,700,800 and 900C (1112, 1292, 1472 and 1652F respectively) followed by air cooling. The effect of aging time was then studied at the optimum tem- perature selected from the isochronal treatment results.


The matrix and the carbide morphology in the as-cast and heat treated conditions were examined by optical micros- copy. The bulk hardness and the matrix hardness were mea- sured in HRC and HV(0.1) scales respectively. The distribu- tion of elements Si, Cr, Mn and Cu in the microstructure of ausaged sample were analyzed by electron probe micro- analysis (EPMA) in a Jeol-Superprobe 733 analyzer.


The corrosion rate was measured using the cathodic polar- ization method with as-cast and destabilized alloys with and without copper. For comparison purposes, the corrosion rate of a standard nickel-chromium alloy (Ni-hard type IV) was also tested. The experimental set-up is shown schematically in Fig. 1. The electrolytic cell (C) consists of a 5% NaCl solution in a glass beaker. The working electrode (W) i.e., the alloy of which the corrosion rate is to be measured and the counter electrode (A) were introduced into the cell. The counter elec- trode was a flat platinum foil of 4cm.x 4cm. in size. The Lug- gin capillary probe (S) that sensed the potential of the work- ing electrode, contained saturated KCl in agar agar, and was connected to the reference electrode and the counter electrode through a ‘Wenking’ Potentiostat type 70TS1.


measured by a nomogram12


The specimen used for the polarization study were cylindrical in shape (1cm. diameter x 5cm. length) and were coated with cold setting resin except for one of its cross-sectional sides. This bare surface was polished and cleaned with alcohol and dried. The specimen and the counter electrode were placed into the electrochemical cell and were pre-exposed to the test solution for a sufficient length of time to attain a steady state potential and the same was noted. The cathodic polarization was then initiated from the steady state potential. The current density and the corresponding potential were measured. The corrosion current (Icorr


) and Tafel constants (βc and βa) were of overvoltage (η) vs.’iapplied


’ using


. The nomogram is shown in Fig. 2. The following steps were adopted for determination of the system parameters:


different sets of βc and βa values. The icorr cm2


• The overvoltage (η) vs. current density curves were plotted using the same overvoltage scale as used in the nomogram.


• The nomogram was placed onto the graph so that the two axes coincided with those of the nomogram.


• The best fit curve was selected on the nomogram with which the experimental curve matched by slid- ing the nomogram along the x-axis on both sides of the origin. The βc and βa values corresponding to the selected curve were noted. The current value of the experimental curve which matched with the current value of 1mA/cm2 value.


of the nomogram is the required icorr


is fixed at 1mA/


Figure 3. Bulk hardness vs. isochronal treatment temperature plot for as-cast alloy 333.


Figure 4. Optical micrographs of alloys; a) as-cast alloy 330 (without copper) and b) as-cast alloy 333 (with copper). International Journal of Metalcasting/Winter 11 51


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