A spangle is the visible aesthetic feature of crystallites on the surface of a galvanized steel sheet. The spangle appears as either a snowflake or a six-pointed star pattern. This is produced on the steel sheet when certain alloying elements are either added to the liquid zinc or available as impurities.
The size and orientation of the zinc grain affects the corrosion and mechanical properties of the zinc alloy. This can be altered by alloying elements such as:
Spangles give the zinc coating a decorative appearance determined by orientations of the zinc crystals and the distribution of the alloying elements in the coating.
The spangle is formed when liquid zinc adhering to a steel surface is cooled to temperatures below the melting point of zinc. The zinc atoms, which are randomly arranged in the liquid form, start to position themselves in an orderly pattern at random locations within the molten zinc coating. This process of transforming from disorderly atoms in the liquid state to an orderly pattern is solidification or crystallization.
These small solidifying regions in the molten zinc are referred to as grains. Grain growth occurs when individual atoms from the molten zinc continue attaching themselves to the solidifying grain in an orderly pattern. The individual atoms of the growing solid grain arrange themselves into the often-visible hexagonal symmetry of the final spangle. When the coating solidifies completely, the individual spangles formed represent the respective zinc grains.
Dendritic growth is a different solidification process that also gives rise to spangles in a galvanized steel sheet. Spangles produced in this process have a snowflake appearance.
Factors that affect spangle size are:
Spangle sizes on galvanized steel sheets are classified as regular, minimized or zero spangle.
Surfaces with spangles have lower corrosive and mechanical resistance properties as compared to the uniform zinc-iron alloy surface. Sheets with spangles are suitable for many ordinary applications. However, spangles are undesirable when painting or when the use of an outer layer of synthetic plastic is required for finishing or decorative purposes
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