The hot-dip galvanizing of steel constructions is generally executed in large kettles made of steel with a length of up to 20 m. Intermetallic phases ofiron and zinc form on the kettle wall. This means that the kettle wall thickness slowly decreases over time. Certain (especially thermal) effects can cause a faster attack on the steel of the kettle wall. This may significantly shorten the lifetime of the zinc kettle (generally 8-12 years). In the worst case it could lead to a wall throughput, which results in high costs and a threat of life and health. For many decades lead was used as a common alloying additive in hot-dip galvanizing at concentrations of 0.3 - 1.2 wt-% Pb. The addition of lead has the following effects: 1. Reduction of the surface tension of the liquid zinc for the improvement of wetting of the dipped steel constructions (good results in hot-dip galvanizing) 2. Protection of the kettle bottom by a liquid lead sump 3. Simplified removal of dross from the kettle bottom. However, lead and its chemical combinations are EU-classified as hazardous for health, toxic for reproduction and hazardous to the aquatic environment. Lead enriches on the surface of zinc coatings on steel constructions with up to 5 wt-% Pb. During weathering of the zinc coatings lead combinations can be released into the environment. A research project at IKS Dresden investigates the possibility of substitution of lead in zinc melts by bismuth. Bismuth and its chemical combinations are not hazardous for health or for the environment. Bismuth causes a much stronger decrease of the surface tension of the liquid zinc bath than lead. Compared to lead, a much lower concentration of bismuth is necessary in zinc melts. But in literature bismuth or rather bismuth in combination with tin is suspected to cause a faster attack on the intermetallic phases ofiron and zinc on the kettle wall. Within the research project several comprehensive long-term dipping tests were performed in a small zinc kettle at IKS Dresden (30 kg Zn) with different amounts of bismuth in zinc baths over a period of 6 - 10 weeks, respectively. The results clearly show that bismuth in zinc melts at concentrations of 0.15 to 4.5 wt-% Bi does not cause a faster attack on the intermetallic phases of iron and zinc on the kettle wall or on the steel of the kettle wall itself. Compared to lead-containig zinc melts bismuth even leads to an additional stabilisation of the intermetallic phases on the kettle wall. Hot-dip galvanized steel sheets from lead-free zinc baths with bismuth concentrations of 0.15 and 0.25 wt-% Bi showed a very good appearance, a high adhesion and a very high mechanical resistance of the zinc coating at conventional film thicknesses. These findings have been confirmed in practice in recent years. Three industrial kettles with lead-free and bismuth-containig zinc melts (approx. 0.1 wt-% Bi) were operated for a period of 8 - 10 years without showing any extensive attack on the kettle wall.
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