Coated steel is bought on corrosion life, and the coating system, not the base steel, decides that life. Two suppliers can quote the same grade and thickness yet deliver coatings that last very differently, because coating weight, coating composition and adhesion are what actually protect the steel. This guide covers the main coated products, the specification axes and the incoming checks.
The coated product families
Hot dip galvanised, GI
Hot dip galvanised sheet, grades such as DX51D, SGCC and SGH440, carries a zinc coating that protects the steel by barrier action and by sacrificial action at cut edges and scratches. It is the general purpose coated product for construction, appliances, automotive and general fabrication. The key specification is the coating weight, expressed in grams per square metre total or per side, and it should be chosen for the expected corrosivity of the service environment rather than on price alone.
Galvannealed, GA
Galvannealed sheet is galvanised and then annealed so that the coating becomes a zinc iron alloy. The surface is matt and, crucially, is suitable for painting without further treatment, which is why galvannealed automotive outer panels are common. The coating is harder and less ductile than pure zinc, so forming behaviour differs and should be confirmed on a trial.
Zinc aluminium and zinc aluminium magnesium coatings
Zinc aluminium coated sheet trades some sacrificial protection for much better barrier and elevated temperature performance, and is common in roofing, cladding and automotive exhaust. Zinc aluminium magnesium coatings, such as ZAM, add magnesium, which substantially improves cut edge protection in chloride bearing environments, making them attractive for coastal and high humidity service.
Prepainted and colour coated, PPGI
Prepainted coil applies an organic coating system over the metallic coating, and the system, not the topcoat colour alone, determines performance. A typical system is a primer plus a topcoat, and the resin family matters: polyester for general use, silicon modified polyester for better heat resistance, PVDF for the best colour and gloss retention in outdoor architectural service, and polyurethane where flexibility plus durability is needed. For architectural use, specify the coating system and the total dry film thickness, not just the colour.
Low carbon and recycled content grades
Coated sheet increasingly carries a sustainability specification as well, such as a high recycled content produced through an electric arc route. Where a project has a carbon or recycled content requirement, specify it in the same way as any other property, with a defined measurement basis, because recycling routes differ in their energy intensity and their scrap ratio.
Specification axes that decide coating life
Coating weight and coating weight per side
Specify the coating weight per side, not only the total, because unequal coatings affect both corrosion life and forming. Ask for the test method and the sampling plan.
Coating adhesion and forming
Specify the adhesion requirement by a defined test, and confirm it after the forming operation your part will see, since a coating that passes on a flat sample may flake at a severe bend or a drawn feature.
Surface quality and flatness
For exposed applications, specify the surface quality class and the acceptable level of defects, and specify flatness, because coated sheet is often used where appearance is part of the function.
Substrate properties
The base steel grade still sets the mechanical properties, so yield strength, elongation and the grade designation remain part of the specification. For structural and automotive use, specify the substrate as well as the coating.
Incoming verification checklist
- Measure coating weight per side and compare with the order, using the agreed method.
- Check coating adhesion by a bend or impact test, ideally after a forming trial.
- Confirm the substrate grade and mechanical properties from the mill certificate.
- Inspect surface quality and flatness against the required class.
- For prepainted coil, confirm the coating system and, for architectural use, the dry film thickness.
- Confirm thickness and width tolerance, which are often where a low quotation has been trimmed.
Common sourcing mistakes
- Comparing coated sheet on price per tonne without comparing coating weight per side.
- Specifying a prepainted product by colour and gloss without specifying the resin system and film thickness.
- Assuming a galvannealed coating will form like pure zinc, then meeting cracking at the forming stage.
- Ignoring the chloride environment, then finding a standard zinc coating fails at cut edges.
- Overlooking thickness and width tolerance, so the delivered material is thinner than the design assumed.
Sourcing coated steel from Hunan and China
China produces the full range of coated steel products, including galvanised, galvannealed, zinc aluminium, zinc aluminium magnesium and prepainted coil, supported by integrated steel lines and a large appliance, construction and automotive industry. Hunan hosts advanced steel suppliers within its advanced materials base. LiiFoo maintains verified supplier profiles in the advanced steel materials cluster covering coated steel products.
For grade matched quotations, coating specification data and sample coils, see the cluster above or contact our sourcing team.
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