Hot-Dipped Galvanizing Appearance

A. Is that a mouse grey or pearl grey? Steel Chemistry will have the final word on what galvanized structural members will look like. Galvanizing is a zinc-iron alloy that can represent a wide range of "colors" based on ranges of steel chemistry (silicon, phos, carbon...). Of course galvanized sheet can provide more consistent coatings that can range in different spangles and appearances (galvannealed). A galvannealed coating would tend to be a flat grey color that is consistent. I guess it depends on your steel supplier and your part/ steel that you are using as to whether I could show you something that is consistent in color. Galvanized steel will change in appearance over time as the material "weathers." If you can be more specific of the part or application I might be able to speculate if you can achieve this "gray even color."

A. Hot dip Galvanizing is an iron alloy layer comprised of different combinations of zinc and iron. The eventual appearance of the product will depend on steel chemistry of the product being galvanized, time of product in the kettle, and the retention of heat in the product after withdrawal. Silicon, Phosphorous, Carbon and Manganese in steel have the most profound affect that can lead to heavy and potentially brittle coatings if in excess (ASTM ASTM A385). This coating will most likely be dull and frosty. When these levels are exceeded the iron in the zinc-iron alloy coating comes to the surface. When a product is brighter the pure zinc layer comes to the surface leaving a brighter coating. The pearl grey and mouse grey reference was a little sarcastic, since it is very difficult to project the result of a product without knowing steel chemistry or the product. Different galvanizers use different additives to the zinc bath such as nickel, bizmuth, aluminum, and tin to change the appearance of a coating and to aid in the types of products that they run in there area. The American Galvanizers Association can help you with some technical question as well as provide literature to explain this a little better.

A. Hi Francisco. I am not a galvanizing expert, but I don't think the mottled, crystalline look of galvanizing is a lot different than that of hot-dipped tin or a frosted windowpane. As the molten metal or the condensed water cools, crystals start growing simultaneously from a number of spots … and eventually they run into each other. The lightness or darkness is probably due to the orientation of the crystals (exactly perpendicular to the surface or the reflected light or somewhat askew).

People may have tried to control the mottled look via different amounts of silicon, galvanizing temperature, immersion time, withdrawal speed, or whatever to control the number of seed points but personally I know nothing about that. When soldering a wire in place if you accidentally let it wiggle at just the wrong time you get an unreliable "cold solder" joint with a mottled or grainy gray look.

Regards,

A. I'd recommend that you go to galvanizeit.org (the website of the American Galvanizers Association) and put 'spangle' in the search box.

A. Some things "just are" ! How does tree bark get its look?

Galvanizing is zinc alloyed to steel. That alloying varies in nature as steel varies a lot from one piece to another - the chemistry is different in minor ways, but enough to affect the formation of that alloy formed which is galvanizing. Si is especially influential, but P also has a part to play. Examining test certificates of steel shows the varying chemistry, even within one grade of steel.
That variation changes the way the crystals of zinc and alloy form, and that changes their appearance.
Colour changes can come from two main sources. The composition of the alloy (can be up to 10%Fe, 90% Zn), and the level of oxide formed, or deposited on the surfaces.