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Sandelin curve and effect of silicon on coating thicness
Sandelin Curve

The amount of silicon added during the steel-making process can create differences in the appearance of galvanized products. The Sandelin Curve, as seen in the image to the left, compares zinc coating thickness to the mass percentage of silicon in the steel. The recommended silicon composition is either less than 0.04% or between 0.15% and 0.25%. Any steels not within these ranges are considered reactive steels and can be expected to form thicker than average zinc coatings.

Reactive steels tend to produce thicker galvanized coatings with a matte gray appearance instead of the typical shiny appearance. This difference in appearance is a result of the rapid zinc-iron inter-metallic growth that consumes all of the pure zinc layer (the growth of the inter-metallic layer is out of the galvanizers control).

Rough Coating Phosphorus
Rough Coating

In addition to silicon, the presence of phosphorus influences the reaction between molten zinc and steel. The image to the right shows steel with phosphorus levels over 0.04% which produce dull coating areas and ridges of thicker coating where there is increased inter-metallic growth. The end result is a rough surface with a ridged appearance.

Read more in the AGA publication - Hot-Dip Galvanizing Coating Appearance

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