A single phase microstructure with a close packed crystal structure is most useful in resisting corrosion. Presence of a passive film would be an added advantage. In chromium cast irons, an adherent and impervious layer of chromium oxide is formed on the surface. Due to this, chrome irons are resistant to the action of different corroding media.14,15 Copper appears to act in the same way as chromium in the formation of a protective layer. This is substantiated by the EDX analysis which shows an enrichment of copper at the surface. Similar findings were reported by Szpunar16


on the


vealed by electron diffraction analysis. Conclusions


effect of copper on corrosion resistance of Ni-Mn-Cu cast iron in chloride solution. According to Szpunar, the layer nearest to the metal surface contains Fe3


O4 .Fe2 O3 .CuO as re-


bides in the matrix which are acting as cathode and the matrix as anode in the chloride media.


• The increase in corrosion resistance property by copper addition is perhaps due to the formation of an adherent and impervious protective layer of copper oxide on to the surface of Cr-Mn irons.


REFERENCES


1. Levi, L.I. et.al., “Wear-Resistant Chromium- Manganese Cast Iron,” Russian Casting Production, pp. 409-412 (August 1967).


2. Basak, A., Penning, J., & Dilewijns, J., “Effect of Manganese on Wear Resistance and Impact Strength of 12% Chromium White Cast Iron,” International Cast Metals Journal, vol. 6, no. 3, pp.12-17 (Sept. 1981).


• Addition of copper increases the corrosion resis- tance of Cr-Mn white cast irons in chloride medium.


• The best corrosion resistance property is obtained with a predominantly austenitic matrix.


• The corrosion rate increases when the austenitic matrix is destabilized by an ausaging treatment. This is due to the formation of fine secondary car-


3. Basak, A., Penning, J., & Dilewijns, J., Metals Technology, vol. 9, no. 9, p.381 (Sept. 1982).


4. Chakrabarty, I. & Basak, A., “Aus-Aging” and Corrosive Wear Behavior of Cr-Mn-Cu White Cast Irons,” AFS Transactions, vol. 98, p. 707 (1990). 5. ASM Metals Handbook; 9th


Edition, vol.1, Properties


and Selection: Irons and Steels, p.75 and p.597 (1978). 6. Cox, G.J., “Developments in Alloy Cast Irons,” The


Figure 10. SEM micrographs of the corroded surface in 5% NaCl solution: a) alloy 330 ausaged, b) alloy 333 ausaged and c) Ni- hard alloy IV ausaged.


International Journal of Metalcasting/Winter 11 55


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