Premature darkening problem and its prevention in galvanized sheet surface

Studies were undertaken in detail to identify the root cause of the darkening problem in unpassivated as well as laboratory simulated passivated hot dip galvanized steel sheet having regular spangle with the help of corrosion studies,i.e., salt spray, humidity chamber, atmospheric exposure and electrochemical tests. The samples were characterized by using Scanning electron microscopy/energy dispersive X-rays analysis (SEM/EDX), Auger electron spectroscopy (AES) and also measuring the volume percentage of bright and dull spangles on galvanizes sheet by a new technique developed in our laboratory. Based on the laboratory results, a special passivating solution was developed and implemented in our continuous galvanizing line (CGL), in order to improve the darkening behaviour of galvanized sheets. The sample from CGL with special passivating solution was tested and results were further corroborated by potentiodynamic polarization and SEM/EDX studies.