Surface spangles

Surface Finishes. Galvanized coatings are commonly characterized by surface spangles. In cross section, an Fe2Al (Zn) inhibition layer develops first, preventing any iron-zinc intermetallic phase formation. The overlay layer is made up of dendrites of pure zinc (t ) phase and appears as a poly-crystaUine structure. The three surface finishes commonly produced are ... 

The iron is then immersed in molten zinc, withdrawn, and allowed to cool, whereupon the zinc crystallizes in the characteristic spangled design. To provide protection from rusting, the quantity of zinc retained on the surface of the iron should be not less than 2 oz/ft2. 

Reflected light Light which is reflected at the surface of a specimen, or from an internal fracture (e.g. sun spangles in amber). 

The most important change in materials over the years has been the transition from uncoated mild steel to zinc pre-coated steel and other corrosion-resistant metals. Hot-dip galvanized steels were used in the early stages in the motor vehicle industry. Hot-dipped galvanized steels were corrosion resistant but had a spangled surface that resulted in poor appearance after painting. Hot-dipped galvanized steel is still used on most body parts and interior surface of body outer panels. 

The crystals, or grains, of a metal are made up of unit cells repeated in a three-dimensional array. However, the crystalline nature of metals is not readily obvious because the metal surface usually conforms to the shape in which it has been cast or formed. In some instances, the crystal structure can be observed naturally. Figure 2.3 is a photograph of the surface of a hot-dip galvanized post with characteristic spangle... 

New steel bright, smooth spangled surface. Dark patches on some thicker members. 

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