WCN Autumn 2010

The presence of mischmetal was justified as being required to promote fluidity, wettability and absence of intergranular corrosion. Cerium and lanthanum are expensive rare earth metals, increasing the cost of coating. From a financial point of view, their presence is undesirable, but are they required to give the technical improvements they are credited with? Regarding high temperature perfor- mance, no differences were seen with regards to the wires coated with (3.4mm 1083) and without (1.85 1005) mischmetal additions. A study done at Bekaert (8) has shown that 35-50% of mischmetal in the hot dip alloy bath is lost as dross. The mischmetal present in the coating can be up to 10x less than that in the molten bath ( Figure 21 ). The greater the mischmetal addi- tion, the greater the amount of dross produced. When more than 0.02% mischmetal is present, processing is seriously hindered. With regards to corrosion, relatively little effect is seen on performance in the humidity chamber ( Figure 22 ). Electrochemical potential tests in both 5% aqueous NaCl ( Figure 23 ) and 0.05 M K2SO4 ( Figure 24 ) show no significant effect of mischmetal additions on corrosion rate. Conclusions Information from trials concerning the high temperature behaviour of hot-dip zinc and zinc aluminium coatings has been presented. For 3.4mm high carbon (0.83% C) ACSR zinc and zinc-5% aluminium coated core wires, no problems other than a small loss in salt spray resistance have been found at temperatures at 190°C for up to 185 days. However, a low carbon zinc coated wire was found to show increased alloy layer growth and wrap test problems after 120 days at 190°C.

Measured values 240/gm 2 Measured values 150/gm 2

Actual % Ce+La in Coating

Actual % Ce+La in Melt

Figure 21 ▲ ▲ : Mischmetal contents in melt and coating (8) Figure 22 ▼ ▼ : Effect of mischmetal additions on humidity test (8)

Accelerated Humidity Test – Bezinal Samples 150g/m 2

100% WR

Minimum hours to failure

% Ce+La in Coating

Figure 23 ▼ ▼ : Effect of mischmetal additions on corrosion rate in electrochemical potential test in 5% aqueous NaCl (8)

Corrosion Rate as a fuction of % Ce, % La in % Al in 0.05 M K 2 SO 4

% Ce % La % Al

Linear (% Al) Linear (% Ce) Linear (% La)

Concentration Al ( %)

Concentration Ce & La (both in %)

Corrosion Rate (µm loss per year)

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WCN Issue N° 43 www.iwma.org