1. Coating Structure and Composition
Typical alloyed galvanized layers contain about 90–95% zinc, 5–10% iron and trace aluminium (to control the galvanizing bath). The structure from substrate to surface is: steel substrate → dense columnar Fe-Zn alloy layer → surface oxide layer (ZnO, FeO, etc.). The alloyed layer has no zinc flowers, the surface is silver-grey or dark grey with low gloss, and because the alloy phase is evenly distributed, the surface flatness is better than unalloyed galvanized sheet.
2. Corrosion Resistance
The electrode potential of zinc in the Zn-Fe alloy layer is lower than that of iron, so sacrificial anode protection is still provided. Due to the higher iron content the corrosion potential is slightly higher than a pure zinc layer - the initial corrosion rate is slightly faster - but the long-term corrosion products (basic zinc carbonate and iron hydroxide) form a dense protective film and corrosion resistance stabilises. Reference salt-spray data (neutral salt spray; state the standard and acceptance criteria per the test): a 50 μm alloyed coating reaches about 500–1000 h to white rust and over 2000 h to red rust - better than bare cold-rolled sheet and comparable to or slightly better than pure zinc depending on the alloying degree.
3. Formability and Processing
Formability. The alloyed coating is harder than pure zinc, but the strong coating-substrate bonding prevents peeling during stamping - suitable for deep drawing.
Welding. The higher melting point of the Zn-Fe alloy reduces zinc evaporation in the heat-affected zone and electrode burnout - suitable for resistance welding (spot, seam) and laser welding.
Paint adhesion. The surface oxide layer bonds more strongly with paint; salt-spray performance after painting is better than pure zinc - used where high coating quality is required, e.g. automotive exterior panels. Phosphating can be performed directly to improve the pre-treatment.
4. High-Temperature and Surface Behaviour
The alloying layer is stable below about 200 °C (reference value), higher than a pure zinc layer - suitable for parts requiring subsequent heat treatment. The surface has no zinc flowers, so the appearance is less decorative and more functional.
5. Applications
Automotive. Over 70% of consumption (market reference) - body floors, door inner panels, chassis parts; major global automakers use alloyed galvanized sheet to improve weld-zone corrosion resistance and paint adhesion.
Appliances and steel structures. Parts requiring welding - refrigerator back panels, air-conditioner brackets; corrosion-resistant structures such as bridges and containers.
Special fields. Marine engineering (corrosion resistance plus weldability), agricultural machinery parts (soil-corrosion resistance).
