Abstract: The relationship between the structure defect and weldability of Sn coating, electrodeposited from the acidic bright bath containing Ce³⁺ ion as the additive, has been investigated by means of flu-orometry, X-ray photoelectron spectroscopy (XPS) and positron annihilation lifetime spectroscopy (PALS), It was shown that the co-deposition of Ce and Sn did not occur during the electrodeposition process.The.effect of Ce³⁺ in the bath was to inhibit the hydrolysis and oxidation of Sn²⁺ resulting in the stabilization of the electrolyte as shown in Table 2 and Fig.1, which in turn decreased the oxygen content in the Sn coating surface(Fig.2).This is one of the reasons for that Ce³⁺ ion in bath is favorable to improving the weldability of the coating.But the experimental results of PALSshowed that only those Sn coating obtained from the bath containing～3.5 g/Lof Ce³⁺ possessed a lower structrue defect as indicating by the positron average lifetime τ, which was responsible for the higher weldability as shown in Fig.3.

Key words: Electrodeposition, Sn coating.Structure defect, Ce³⁺ additive, Weldability