New York Paper - Effect of Impurities on Zinc-aluminum alloys (with Discussion)

Among the zinc base alloys used for casting in metal moulds, pnrticularly die casting, those alloys containing aluminum usually together with copper, are probably the most widely used. The reason lies in their desirable mechanical properties and the fluidity and absence of hot shortncss which facilitate ca,sting. Objection to the use of these alloys may be raised on account of the rather frequent instances of failure of a type variously described as disintegration, exfoliation and intererystalline corrosion. In all cases which have come to our kno~vlcdge where the conditionsof service were known, failure has occurred upon cxposurc to moisture and heat varying from a hot moist atmosphere to lligh pressure steam. RIoreover, the alloys described in all such roses contain impurities or alloying metals which the present authors have found to have enormous accelerating effects on intererystalline oxid:~t'ion in zinc-aluminurn alloys. Thus Williams1 experimented with an alloy containing a large percentage of tin exposed in Cuba. Baucr2 reports failure of a boiler plug containing over 1 per cent. of impurities, doubtless lead for the must part, and Rawdon's3 examinations were madc on an alloy containing 0.1 per cent. lead also exyosctl to thc Tropics. No thorough investigation of the nature and cause of this type of failure has been made heretofore, so far as the authors are aware, although Rawdon correctly classified it as internal oxidation. We have, therefore, carried out this investigation in order to det'ermine the exact nature of this phenonienon and the effect of other metals present as impurities or intentional additions in accelerating or retarding it. In the appendix we include a considerable amount of data which may be valuable in selecting an alloy combining good physical properties with high resistance to oxidation. At the time this work was undertaken the best available equilibrium *Investigator, Metal Section, Research Division, The New Jersey Zinc Company. 'H. W. Williams—Jour. Am. Inst. of AIet,als, Vol. 11, p. 221 (1917). 2o. Bauer—ZeiL, fiir i'vletall Kunde, Vol. 12, .No. 7, pp. 129-31 (1920). 3H. S. Rawdon—Trans. Am. Elcctrochcmic.al Society (1921).