Spangle Formation on Hot-Dip Galvanized Steel Sheet and its Effects on Corrosion-Resistant Properties

Samples of bright and dull zinc coatings produced on a continuous galvanizing line from a zinc bath containing small concentrations of aluminum and lead was examined under a scanning electron microscope. Large grain structures termed “spangles” are produced on the galvanized coatings when lead is added to the zinc bath. In the spangle, different regions can be distinguished (viz., mirror, feathery, dimpled, orthogonal dendritic, and ridged types of surface topography). The goal of the present investigation was to identify the behavior of the spangle type and size on corrosion-resistant properties of post-treated galvanized samples. Detailed microstructure analysis of the galvanized sheet was made with the help of scanning electron microscopy/energy dispersive x-ray analysis (SEM/EDX). The salt spray test and the sulfur dioxide (SO₂) test were carried out to study the corrosion resistance of the galvanized samples having different spangle appearances. These tests were further corroborated by electrochemical tests such as potential-time profile and galvanostatic polarization.