Study On The Mechanism Of Sandelin Effect And Its Solutions

The role of silicon in steels to common hot dip galvanized coating, known as Sandelin effect, brings a detrimental influence on the quality of the coating. This has been an oppressive and difficult problem to galvanizing industry. The presence of silicon at certain levels (in the vicinity of 0.1wt% and above 0.3wt%) in steels,known as reactive steels in galvanizing industry, can result in the rapid growth of the Fe-Zn alloy layer, producing a coating of excessive thickness, having gray appearance and poor adherenc. Up to now, there is still lack of uniform and convincing knowledge about Sandelin effect. The present technologies such as'Technigalva'have only effect on the over-growing coating of the steel with less than 0.25%Si. For solving the proplem which has disturbed galvanizers for a long time, the influence of silicon in steels on galvanized coating was sysmetically studied and new solutions of restraining the quick growth of the coatings of high-silicon (especially over 0.25%Si) steels were discussed.In this thesis, five types of silcon-containing steels(0.04%, 0.09%, 0.14%, 0.28% and 0.36%Si) were especially smelted and the growth dynamics and microstructure of hot dip galvanized coatings of those steels under different circumstances at 450℃were studied systmetically by a series of galvanizing experiments and by various of testing methods such as OM, SEM, and EPMA; the effect of silicon on the galvanized coating was discussed; a new and relatively complete interpretation model about Sandelin effect was establised; the Sebisty phenomenon was interpreted. Based on the results above, three solutions to Sandelin effect were brought forward, which were the iron-electroplated method, the nickel-electroplated method and the Zn-Sn-Ni alloy bath method. By systmetically studing the growth dynamics and microstructure of the coatings obtained by the three solutions, their mechanism of inhibiting the abnormal coating of silicon-containing steels were proposed.The study on the hot dip galvanized coatings of the steels in pure zinc bath reveals that the growth of the coatings displays a typical Sandelin effect and the longer the immersion time is, the more distinct the Sandelin effect is. The galvanized coatings of hypo-Sandelin steel and Sebisty steel are continuous and compact. The coating of Sandelin steel consists of stable region and reactive region. The microstructure in stable region is similar to the one of the coating of hypo-Sandelin steel while the microstructure in reactive region is composed of thin and continuousδ...