Hot-dip galvanizing submerges steel into a vat of molten zinc to form a zinc-iron alloy coating that is highly durable and resistant to corrosion. It is often the most cost-effective method of adding corrosion resistance to steel since it can be done immediately after cold rolling.
Hot-dip zinc galvanizing goes by many designations: GI (for VDA standards), Z (for EN 10346:2015 standard), G (for the ASTM standard), as well as HDG and HDGI.
The tight bond between the zinc coating and the steel makes hot-dip galvanizing well suited to demanding forming applications and aggressively corrosive environments. With its tight bonding and low-friction, GI coatings have a low peeling tendency, providing full corrosion resistance to even heavily formed steel components. Zinc’s sacrificial behavior provides a level of corrosion resistance to areas where the coating has been damaged from scratches or impacts.
Corrosion resistance is directly proportional to the hot-dip zinc coating’s thickness. Minimum total coating mass for both surfaces of the steel range from 100 g/m2 (Z100) to 600 g/m2 (Z600). Z600 coating with 42 µm on both sides can achieve a service life of up to 80 years. In certain applications, using Z450 or Z600 zinc coated sheet steel can replace the batch galvanizing of finished components, reducing costs for the overall process chain.
Z coatings are lead-free and >99% pure zinc, producing a finely crystallized zinc spangle that meets high requirements for visual appearance. SSAB offers zinc coatings with different coating thicknesses, surface qualities and surface treatments to meet the demands of various applications.
*In triple spot test
In addition to the above zinc coating thicknesses as defined by EN10346:2015, we offer asymmetric coatings, coating with equal coating minimum mass per surface, and GI coatings to other OEM specifications upon request.
SSAB offers zinc coatings with different coating thicknesses, surface qualities, and surface treatments to meet the demands of various applications.
In general, zinc coatings can endure severe deformations thanks to their ductility and low frictional behavior. So usually the same forming processes used for uncoated steels can be applied to zinc coated steels without any substantial process modifications. Small differences in surface behavior may require some changes, for instance in lubrication, tooling geometry and holding forces. The GI coating’s low-friction surface acts can act as a lubricant when low- to moderate-surface pressures are used in forming. The bending performance of zinc coated sheets with coating masses up to 275g/m2 can be regarded as equal to that of corresponding uncoated sheets.
The successful forming of metal coated steels depends on the selections you make regarding the geometry of component, steel grade, metal coating type and thickness, surface quality and protection, and the tool used in the forming.
In addition of providing the desired color for the final product, painting also further improves the corrosion resistance of the final product. The skin-passed surface quality B is recommended you require a high surface quality for the painted area.
Zinc coating provides a good substrate for painting when the surface is prepared correctly and right paints are used. To ensure good adhesion, all oils or impurities should be fully removed from the surfaces to be painted. To improve coating adhesion, the GI coated steel can be pretreated by zinc-phosphating or a suitable alternative pretreatment in the receiving plant.
Zinc coated steels can be welded by various welding techniques – including different methods of resistance welding, laser welding, and arc welding. When welding recommendations are followed, the mechanical properties of welded joints on GI coated steels are equal to those of non-coated steels.
Resistance welding methods, like spot welding, are the most common and give excellent results with zinc coated steels. The advantageous anticorrosion properties of the zinc-based coating mainly remain in the area of the properly made spot weld. Spot welding of Z coated steel requires slightly higher current and electrode force than for uncoated steels due to the lower contact resistance of the coating. Similarly, increasing the zinc coating thickness will require a slight increase in welding current. In order improve weldability and achieve longer service life of welding electrodes, it is recommended to avoid unnecessarily thick GI coatings in welding applications. Galvannealed (ZF) coating is recommended for resistance welding applications with numerous welds.
Laser welding is also ideal for zinc coated steels thanks to the narrow (only few mm) weld and low heat input. When any fusion welding methods are used, the heat input must be as minimal as possible to limit the heated zone in the GI coated sheet. The narrow weld benefits from the sacrificial effect of the zinc coating providing cathodic corrosion resistance. However, we recommend the weld area is painted or protected by other appropriate coating after the fusion welding process.
Be certain there is adequate ventilation in the welding area to avoid exposure to zinc oxide welding fumes.
GI coatings are well suited for adhesive bonding – provided that the surface is matched with the appropriate epoxy, acrylic or polyurethane adhesive. The compatibility of the zinc coated surface with the adhesive should always be considered case-by-case. Adhesive bonding preserves the anticorrosion properties of the GI coating because the coating remains basically intact in the joint area. All oils or impurities should be carefully removed from the surfaces to be adhesively bonded.
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