Application Example 8: Cross-Sectional Examination of a Galvanized Steel

Abstract

AbstractGalvanizing is a simple process where a steel sheet is immersed in a bath of molten zinc and then withdrawn from the bath. In this manner, the surface of steel is coated with a layer of Zn, ~20 μm thick, which protects the steel substrate from corrosion. However, there is a problem here; zinc reacts with steel substrate to form a coarse and brittle Γ phase, which is very detrimental for subsequent fabrication of galvanized steel sheets into complex shapes of industrial importance, such as car bodies. It is well known that the formation of Γ phase is prevented effectively by addition of a small amount of aluminum, normally less than 1%, in the melt. In the presence of aluminum, steel reacts preferentially with aluminum to form a thin, continuous layer of Fe2Al5 at the melt/steel interface. Direct contact between zinc melt and steel is now avoided and the growth of Γ phase is prevented. In both car and steel industries, formation of Fe2Al5 layer at the interface has been a matter of great concern for many years, since product performance depends critically, among other things, on the thickness, uniformity, and continuity of Fe2Al5 layer. Thus, researchers in steel and car industries have been working hard to examine the Zn/steel interface by transmission electron microscopy. They tried to prepare cross-sectional specimens, suitably thin for transmission electron microscopy, using focused-ion-beam (FIB) milling, but without much success.KeywordsSteel SheetSteel SubstrateContinuous LayerGalvanize SteelIndustrial ImportanceThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.