Characterization of zinc and zinc-alloy coatings

B.1 General

This annex describes briefly some of the zinc and zinc-alloy coatings which are commercially available.

Additional information regarding zinc-coated steel can be obtained from the applicable ISO standards.

B.2 Hot-dipped zinc-coated (ISO 3575)

This product is commonly referred to as hot-dipped galvanized steel. The coating process involves immersing cold-rolled steel sheet or strip in a bath of molten zinc. The molten zinc bath might contain traces of aluminium which suppress the formation of zinc-iron alloys during the coating process. This results in a virtually pure zinc coating on the surface of the steel strip. Unless special steps are taken, the zinc coating will exhibit a “full spangle” pattern of zinc crystals, like the familiar pattern seen on sheet metal air ducts. For improved paint appearance, special processing can create a “minimized spangle” or a “spangle-free” zinc coating. Zinc coating mass per unit area can be as high as Z700 (700 g/m2), but a coating mass per unit area of Z350 (350 g/m2) or Z275 (275 g/m2) is more common. Hot-dipped zinc coatings can also be produced with a different coating mass per unit area on each side of the strip, or with a coating on only one side of the strip.

B.3 Zinc-iron alloy coated (ISO 3575)

This type of coating is often referred to as “galvannealed”. The coating is produced by heating or wiping a hotdipped zinc-coated surface in a manner that causes the zinc coating to alloy with iron from the steel surface.

The resulting zinc-iron alloy coating has a dull grey appearance. Standard coatings with given mass per unit area are available up to ZF180 (180 g/m2). The coating designation specifies the total mass of the coating on both sides of the sheet.

B.4 Electrolytically zinc-coated (ISO 5002)

This type of coating is often referred to as “electrogalvanized”. To produce this type of coating, pure zinc is electroplated onto a steel surface in a continuous process. Common coating designations range from ZE10/10 (1,0 μm each side) to ZE75/75 (7,5 μm each side). Variations of this coating include zinc-nickel and zinc-iron alloy electroplated coatings, as well as differentially coated (where the coating mass per unit area is different on each side of the strip) and coated on one side only.

B.5 Zinc-5 % aluminum alloy coated (ISO 14788)

This type of coating is often referred to as “galfan”. Coating application is similar to that used in hot-dip zinc coating, except that the molten zinc bath contains a mass fraction of approximately 5 % aluminium. Standard coating mass per unit area ranges up to 700 g/m2. The coating mass represents the total amount of coating on both sides of the steel sheet or strip.

B.6 Aluminum 55 %-zinc alloy (EN 10215)

This type of coating is often referred to as “galvalume”. Coating application is similar to that used in hot-dip zinc coating, except that the molten zinc bath contains a mass fraction of approximately 55 % aluminium.