Zn55Al Alloy Coatings

The purpose of Zn-55Al alloy coatings is to combine the excellent long-term atmospheric corrosion resistance of aluminum with the sacrificial characteristics of galvanized in a single coating (Ref 23, 24). Long-term studies showed that the 55% Al alloy was the optimum composition of the aluminum-zinc system. Steel sheet coated with this alloy was first produced commercially in 1972 in the United States under the trademark Galvalume. Coating thicknesses are usually in the range of 20 to 25 pm (0.8 to 1 mil).

About 80 vol% of the complex microstructure of the Zn-55Al alloy coating (Fig. 15) is composed of cored, aluminum-rich dendrites, representing the first solid to form during cooling. As predicted by the phase diagram (Ref 22) (Fig. 13), the final liquid to freeze in the interdendritic regions is enriched in zinc. A thin (1 to 2 /m) iron-aluminum-zinc intermetallic layer is evident at the steel surface.

Fig. 15 Zn-55Al coating microstructure. Scanning electron microscope cross section

About 1.5% Si is added to the Zn-55Al bath for the purpose of minimizing alloy-layer growth during dipping. It is present in the form of scattered, needlelike particles, mostly in the interdendritic region. Some of the silicon is also concentrated in a thin layer at the overlay/alloy-layer interface, where it functions as a barrier to alloy-layer growth during the coating process.

Atmospheric corrosion resistance of the Zn-55Al alloy coating is generally at least two to four times that of an equal thickness of galvanized coating, as evident in Fig. 8 and Table 8. Most of the corrosion of the alloy coating takes place in the zinc-rich interdendritic regions. This enables the coating to exhibit the sacrificial characteristics of a galvanized coating. As the zinc-rich interdendritic regions of the coating corrode, zinc corrosion products are trapped in the interdendritic interstices and act as a barrier that slows further corrosion. As a result, the corrosion rate of the alloy coating decreases with time, as shown in Fig. 8.

Although the effective distance of the galvanic action provided by a Zn-55Al coating is roughly half that of galvanized, it is generally adequate to protect the cut edges of steel sheet with a thickness of one millimeter or less in most environments. Moreover, because of the greater durability of the 55% aluminum-zinc coating, it provides galvanic protection for a longer duration.

As shown in Fig. 8, the corrosion rate of the 55% Al alloy is greater than that of an aluminum coating. However, unlike an aluminum coating, the 55% Al alloy is able to prevent rust staining at cut edges and scratches, as well as resist crevice corrosion in marine environments.

As shown in Table 6, the spot welding tip life of the 55% Al coated sheet is less than that of galvanized. This reduction is attributed to the adverse effect of aluminum.

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