溶融亜鉛めっき鋼板の電極寿命に及ぼすめっき皮膜中のＡｌの影響

Abstract

The purpose of this study is to investigate the effect of Aluminum in Zn coating on electrode life of galvanized steel sheet. Three hot-dip galvanized and one electro-galvanized steel sheets were prepared for this study. Aluminum content in coatings was changed from 0.26 to 0.87 mass% among three hot-dip galvanized steel sheets.The approach used here included EDX, AES analysis of the coating layers, electrode life tests and EPMA analysis of electrodes after 900 welded.The obtained results are as follows:(1) Electrode lives of HDG materials were changed between less and more than 0.3-0.4 mass% Al content in Zn coating. Materials with low-Al coating content showed over three times longer electrode lives than materials with high-Al coating content.(2) Although thickness of Al oxide layers was in proportion to Al content in Zn coatings, the obvious correlation between electrode life and thickness of Al oxide layers was not observed.(3) In case of low-Al coating content, it was observed that Fe-Zn alloy grew from the steel-coating interface to Zn coating. It was considered that, in the initial stage of welding, the content of Fe in Zn coating increased immediately.(4) In case of high-Al coating content, Fe-Al Alloy was observed at coating-steel interface instead of Fe-Zn alloy. It was known that Fe-Al alloy suppresses Fe-Zn alloying reaction. Zn coatings was not alloyed with Fe on initial stage of welding.(5) From these results, it was concluded that Aluminum in coatings affected electrode life by changing the melting point of coating layers between electrode and steel. Melting point of low-Al content coating layer rose because of the diffusion of Fe into Zn coating. This phenomenon decreased electrode wearing and electrode life was long. In contrast, melting point of high-Al content coating layers remained low. This phenomenon caused electrode alloying easily and also increase electrode wearing. As the result, electrode life became shorter.