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Metal coated steels

Protection against rust with zinc coating

Zinc coating (Z) produced on both sides by hot-dip galvanizing extends the service life of the end product by protecting the steel from corrosion.

The continuous hot-dip galvanizing process offers a wide protection range from Z100 to Z600 in addition to a tight bond between the coating and the steel. These properties make zinc coatings well suitable to forming and demanding corrosive atmospheres.

The composition of zinc coating consists almost entirely of zinc (>99%) and is lead free, resulting in finely crystallized zinc spangle that meets high requirements for visual appearance. The corrosion resistance provided by the zinc coating is in direct proportion to the coating's thickness. The Z600 coating (42 µm on both sides) can achieve a service life of up to 80 years. Therefore, in certain applications, batch galvanizing of finished components can be substituted by using Z450 or Z600 coated sheet steel thereby simplifying the overall process chain.

Because of the sacrificial nature of the zinc, the coating provides corrosion protection for areas of exposed steel surfaces, such as cutting edges and areas where coating has damaged (scratches, impacts, etc.). Full corrosion protection is also achieved in areas that have been heavily formed because the peeling tendency of low-friction and tightly bonded coating is low.

SSAB offers zinc coatings with different coating thicknesses, surface qualities and surface treatments to meet the demands of various applications.

Coating thickness

Coating designation Minimum total coating mass, both surfaces (g/m2) * Guidance value for coating thickness per surface, typically (μm)
Z100 100 7
Z140 140 10
Z180 180 13
Z200 200 14
Z225 225 16
Z275 275 20
Z350 350 25
Z450 450 32
Z600 600 42

* in triple spot test

In addition to these Zinc coating thicknesses defined according to EN10346:2015, the offering contains different asymmetric coatings, coatings with equal coating minimum mass per surface, and other OEM specifications that are available upon request.

Processing of zinc coated steel

Forming

In general, zinc (Z) coating endures severe deformations thanks to its ductility and good frictional behavior. Consequently usually the same forming processes as for uncoated steels can be applied for zinc-based coated steels without any substantial modifications to the process conditions. Small differences in surface behavior may require some changes, for instance in lubrication, tooling geometry and holding forces. The metal coating has an advantageous characteristic of serving as lubricant, which works greatly with low and moderate surface pressures taking place 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 selections made regarding the geometry of component, steel grade, metal coating type and thickness, surface quality and protection, and a tool used in the forming. 

Welding

Metal 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 are equal to those of non-coated steels. 

Resistance welding methods, like spot welding, are most common and give excellent results with metal coated steels. Advantageous anticorrosion properties of the zinc-based coating mainly remain in the area of the properly made spot weld. Spot welding of metal coated material requires slightly higher current and electrode force than for uncoated steels due to the lower contact resistance of the coating. Similarly the increase in coating thickness increases slightly the required welding current. Therefore, in order improve weldability and achieve longer service life of welding electrodes, it is recommended to avoid unnecessarily thick coatings in welding applications. Galvannealed (ZF) coating is recommended for resistance welding applications with numerous welds.

Laser welding is also ideal for metal coated materials 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 in order to limit the heated zone in the metal coated sheet. Similar to scratched areas of the coating, also in the narrow weld the sacrificial effect of the zinc-based coating provides cathodic corrosion protection. However, the weld area is recommended to be painted or protected by other appropriate coating after the fusion welding process.

Ventilation of the workplace must be arranged properly because welding of zinc-based coated steels produces welding fumes containing zinc oxide. 

Joining

All zinc-based coatings are well suited for adhesive bonding provided that the surface is matched with the adhesive (epoxy, acrylic or polyurethane adhesive) used. One advantage of adhesive bonding is that it preserves the anticorrosion properties of the coating because the coating basically remains intact in the joint area. In order to ensure good adhesion, any oils or impurities should be carefully removed from the surfaces to be adhesively bonded. Thin organic coating (TOC) is recommended in many cases providing best surface for adhesive bonding. The compatibility of the surface with the adhesive used should always be considered case by case. 

Painting 

In addition of providing the desired color for the final product, painting also further improves the corrosion protection of the final product. The skin-passed surface quality B is recommended when high surface quality for the painted surface is required. 

Zinc coating provides a good substrate for painting when the surface is prepared correctly and right paints are used. In order to ensure good adhesion, any oils or impurities should be carefully removed from the surfaces to be painted. The product can be pretreated by zinc-phosphating or alternative pretreatment suitable for zinc-based coated steel in receiving plant to improve the coating adhesion.