《Table 4 Chemical compositions of selected points in as-cast specimens obtained by emission spectrum

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《Mg-Al-Pb及Mg-Al-Pb-Zn合金在3.5% NaCl溶液中的腐蚀行为（英文）》

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Fig.7 shows the SEM（secondary electron）corrosion morphologies of A and B alloys in different heat treatment conditions immersed in 3.5%NaCl aqueous solution for 48h.The corrosion morphology of B-T4specimen，which is not shown in Fig.7，is similar to that of A-T4 specimen.The chemical compositions of the selected points in the as-cast specimens of A and B alloys obtained by emission spectrum analysis（ESA）are listed in Table 4.The corrosion area is enlarged and spread to the whole surface of the specimen for 48himmersion.For T4treated specimens，many pits are still observed in the low magnification morphologies（Fig.7 （a）） and these pits are mainly situated along and/or adjacent to the grain boundaries（Fig.7 （b）） .This result indicates that the corrosion develops downward，and still along the grain boundaries in the form of pitting corrosion，after it spreads to the whole surface of the alloy.For as-cast specimens，the second phases are still embedded in theα-Mg phase after 48h immersion，which can be observed in Fig.7（c）and（d）.The corrosion pits are still surrounding the second phases，which is more obvious in B as-cast specimen.Besides，many pits are observed in the corroded surfaces.This result implies that the corrosion develops downward，and they are always surrounding the second phases，after it spreads to the whole surface of the alloy for a long time immersion.