Combined effects of pb and sn additions on the corrosion of Zn-12 Pct Al-1 Pct Cu-0.02 Pct Mg sand cast alloy in wet ste
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IN an e a r l i e r p r o g r a m of work~ the individual effects of Pb and Sn on the corrosion of the sand cast Zn-12 pct A1 alloy were determined at various levels of Cu and Mg. It was shown that the alloys had a g r e a t e r tolerance for Pb than would be anticipated from the proposed specification limits and anomalous effects were observed for Sn in that corrosion resistance was better at Sn contents both lower and higher than the generally accepted maximum of 0.003 pet. To complete the picture, therefore, the work was extended to cover a limited r a n g e of combined Pb and Sn contents. This has further practical significance because both Pb and Sn would be expected to be present as impurities in Zn s c r a p and it was therefore desirable to determine whether a possible interaction between the two impurities would necessitate lower impurity limits. The optimum alloy with 1 pet Cu and 0.02 pet Mg as d e t e r mined in the e a r l i e r work was used for the present study.
aluminum-10 pct Mg alloy. Two ~160 lb (~80 kg) m e l t s containing, respectively, 0.005 and 0.015 pct Pb were made up from the b a s i c alloy and in each the Sn was v a r i e d from 0.01 to 0.00125 pct (nominal) by successive dilution. The alloys labelled "< 0.00125 pct Sn" were obtained by diluting the remaining 0.00125 pct Sn material with the remaining b a s i c alloy at each Pb content. For each P b / S n combination, 8 cast-to-size tensile test bars were cast in two 4 - b a r moulds (ISO Standard 2379). Corrosion susceptibility was assessed by measuring the effect of 10days in saturated water vapour at 95°C on expansion (increase in length of a 6-in. (150 ram) test bar), loss in tensile strength and corrosion depth. The details of these estimates are given in the e a r l i e r report.1
CHEMICAL ANALYSIS
The melting and casting techniques were the same as used in the e a r l i e r work.1 The b a s i c Zn-12 pct A1 alloy was made up using 99.995 pct electrolytic Zn, super-purity A1, high-purity A1-Cu m a s t e r alloy and
The results of chemical analysis are given in Table I. The actual Pb results a g r e e reasonably well with the nominals although the Pb content of the base alloy (~0.002 pct) s e e m s high. The Sn analysis results for the alloys with the lowest intentional Sn contents (
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