WCN Autumn 2010

In the case of Galfan, small mischmetal additions between 0.03 and 0.1% (mainly of lanthanum and cerium) are also specified (3, 4) . However, the value of these additions is in question and is discussed later in this article. Zinc-5% aluminium coatings are well known for their significantly better corrosion performance in comparison to standard zinc (about 3x better salt spray resistance) (5, 6) . In addition, the microstructure of such coatings is quite different to that of hot dip zinc, especially with regards to the nature of the alloy layer and the absence of the brittle Fe-Zn binary compounds mentioned above. Thus, one would also expect differences in response of the coating to temperature. This paper compares the response of Zn and Zn-Al coated wires in terms of microstructure, wrapping properties and corrosion to different exposure times at various elevated temperatures with and without mischmetal additions. In addition to coating, the influence of steel type is also examined. Finally, some observation the influence of mischmetal additions to Zn-5%Al coatings are made. In this paper, the designation Zn5Al refers to a coating composition close to that of the Zn-5% Al eutectic, but may be found to vary slightly from close to the eutectic composition to slightly above in terms of aluminium content. Experimental Exposure of Zn and ZnAl Coated Wires to Elevated Temperatures Actual 1/0 AWG 6X1 Raven ACSR cables containing both Zn5Al (with Figure 6 ▼ ▼ : Microstructures of 1.85mm core 1005 wires from start to 185 days at 190°C. Start microstructures are from 150g/m 2 . Note only alloy layer is present in the case of the Zn coating exposed for 185 days (free zinc has peeled off)

Figure 2 ▲ ▲ : Iron-zinc compounds (with associated hardnesses) at the coating/steel interface

Figure 3 ▲ ▲ : Peel-off of free zinc layer due to the formation of voids (the Kirkendall effect) resulting from zinc-iron diffusion at elevated temperatures (1)

Figure 4 ▲ ▲ : Arrangement of ACSR segments in oven

voids (the Kirkendall effect) between the unalloyed top layer of zinc and alloy layer, making the free (unalloyed) zinc layer susceptible to peeling ( Figure 3 ). Different values have been given for the maximum safe use temperature of galvanised coatings. Steel manufacturers advised 245°C in the early 70s (2) , but more recently the American Galvanizer’s Association stated that galvanised coatings have an extended time upper service temperature of around 200°C (1) . Zn-Al coatings, such as Bezinal and Galfan, offer an alternative to standard zinc galvanising coatings. Wire Zn-Al coatings are usually based near the zinc-aluminium eutectic composition, near 5% aluminium. Figure 5 ▼ ▼ : Microstructures of 3.4mm ACSR 1083 core wires from start to 185 days at 190°C (note: variation in total coating thickness is normal for hot-dipped coatings)

The average operating temperature for such a cable is typically around 80°C. However, due to increasing trade in the power market, the need for conductor capacity has increased while moratoriums have limited the construction of new lines for environ- mental and economic reasons. This has now resulted in the need for power lines to be capable of higher capacities. With this comes the need for the lines to be able to withstand higher temperatures. When this is combined with peak periods of usage, the cable operating temperature could rise as high as 250°C. This can be of concern for traditional zinc hot-dip coatings due to their temperature sensitivity. Galvanised coatings are sensitive to temperature due to iron-zinc diffusion phenomena across the steel/coating interface which lead to the creation of a number of iron-zinc intermetallic compounds (some of which can be brittle) at the coating-steel interface referred to as the alloy layer ( Figure 2 ). This alloy layer is initially formed during the galvanising temperature, but its growth stops as it is cooled. However, with sufficient subsequent reheating it is possible for the alloy layer to grow again. Because the diffusion of iron and zinc atoms do not occur at the same speed, it is possible to create

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