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Alloy Coatings
Published in Frank Porter, Zinc Handbook, 1991
The first alloy to be used significantly was zinc-iron in the form of galvannealed coatings. Tests on zinc-iron alloys have shown corrosion resistance up to 30% greater than obtained with the same amount of pure zinc, notably in acid industrial atmospheres. Such zinc-iron coatings form an integral part of the hot-dip-galvanized coating and the whole of the sherardized coating is of such alloys.
Characterisation of industrially produced galvannealed coating by open circuit potential (OCP) measurement technique
Published in Transactions of the IMF, 2018
Pure zinc coated steel provides an inferior performance against spot weldability as the melting point of pure zinc is low (418°C). This leads to there existing a high chance of a sticking problem during spot welding operation and, therefore, a shorter electrode useful life. The galvannealed (zinc–iron alloy) coating was developed to resolve this chronic problem. The galvannealed steel is used in automobiles and high-end white goods applications. This coating is designed not only to resolve the spot weldability problem but also to improve paintability as the coating texture enhances paint adhesion.1–4 An annealing operation is carried out after galvanisation to obtain zinc–iron alloy phases. During the annealing treatment, four iron–zinc intermetallic phases appear in the galvannealed coating, including, Γ, Γ1, δ and ξ5–9 which are stacked in layers on the steel substrate. Different conventional characterisation tools such as scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS),10 X-ray diffraction (XRD),11–12 glow discharge optical emission spectroscopy, (GDOES)13 transmission electron microscopy (TEM),14–16 Auger electron spectroscopy (AES),17 galvanostatic scanning,18–19 colour etching20–21 have all been used to identify all the phases present in the coating. Furthermore, a model has been developed to predict the iron content22–23 in galvannealed coatings. All these techniques provide only the presence of different phases rather than the information regarding their stability in chloride environment. Separate electrochemical and Mösbauer tests have been employed to identify the stability of different electrodeposited zinc–iron alloys and it was confirmed that the Fe0.25 Zn0.7524 phase is more resistant to chloride attack than the others.