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Zinc
2.5-4.5 usd/kg
Zinc is used mainly as protection for steel (galvanised or electroplated), in brass (copper alloy) and as an industrial additive. A very important metal, it is the forth most consumed, behind iron, aluminium and copper.
Zinc forms a protective oxide (ZnO) layer on its surface when exposed to the atmosphere, which helps protect it from corrosion. While this surface is not usually left bare on products or in automotive applications – zinc is coated or plated to produce a durable and aesthetic surface, such as in kitchen appliances and car body panels – unpainted zinc and zinc-coated steel is suitable for a wide range of industrial and exterior applications.
Zinc coatings are applied to steel by galvanising (HDG), electroplating, painting and spraying (metallising). Hot dip galvanising is by far the most common. Parts are submerged in a bath of molten zinc to produce a uniform coating of zinc and zinc-iron alloy. All types and sizes of parts can be coated in this way. Electroplating is economical, but only for thin layers, and so tends not to be suitable for parts that require a durable and weather-resistant zinc finish.
Steel car bodies are galvanised before being painted. The zinc provides a durable barrier and oxide layer. It provides cathodic protection, which means the zinc will preferentially corrode to protect the underlying steel even if there are small gaps, imperfections, or stone chips.
On its own, or as an alloy, zinc has very good die casting properties, which are utilised in kitchen appliances, tableware, toys and automotive parts. It has excellent fluidity when molten, which means it reproduces surface details and thin wall sections dow to 0.15 mm very well. Complex castings are possible in a single step, which can help reduce fabrications to a single part.
There are two main types of zinc die casting alloy: Zamak and ZA. Zamak alloys contain c. 4% aluminium and provide excellent die casting properties and reasonable strength. The ZA alloys contain higher amounts of aluminium and offer superior strength, hardness and creep resistance. The numbers 8, 12 and 27 represent the approximate weight % of aluminium in each: ZA-8, ZA-12 and ZA-27. In some cases, they offer an economical alternative to steel fabrications, and castings in iron, bronze and aluminium.
These alloys can be electroplated, powder coated, painted and chromate conversion coated. For decorative parts, such as handles and bathroom fitting, electroless nickel is often used. It smooths over holes and imperfections to produce a superior finish. The finished part is plated or coated to look like gold, silver, copper or brass, for example.
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Sustainability concerns
Non-renewable ingredients
Raw material generates polluting by-products
Rolled zinc cladding and roofing is prized for its longevity, low maintenance and workability. It is relatively easily formed and this helps with shaping parts for roofs, walls and drainage systems. Uncoated, it develops a lovely blue-grey patina over time. Through a series of reactions with oxygen, water and CO2, a layer of zinc carbonate (ZnCO3) forms on the surface. This durable layer, which is insoluble in water, is responsible for the high corrosion resistance of zinc. It progresses over months and years until it becomes stable.
The patination process that helps to protect zinc causes some of the metal to be separated. This will be washed off with the rain and into the drainage and sewage system. While zinc is essential to life, it can be harmful to aquatic systems depending on the chemical form and concentration. This is a concern in locations with a high density of roofing, urban areas for example, and after heavy rain when the water can overflow and bypass treatment.
Zinc cladding and roofing relatively expensive, especially compared to coated steel sheet. However, while zinc cladding is expected to last 75-100 years on roofs and 200-300 years on walls, sheet steel may only be guaranteed for 40.
Zinc on its own is too soft and brittle to be useful. Therefore it is alloyed with small amounts of copper, titanium and other alloys to produce the required strength and workability. It is formed by casting and rolling into sheets and strip.
The surface is suitable for patination, phosphate coating and painting. Patinated zinc does not have any coating and the surface colour depends on the alloy contents. Over time, any scratches of differences in production will even out to a uniform colour. Phosphating produces a durable matte finish. Over time, the natural patination process will take over and infiltrate the phosphate layer, creating longterm protection. If colour is required, then a fluoropolymer coating, such as polyvinylidene fluoride (PVDF) is used. Alternatively, a transparent top layer, such as polyurethane resin (PUR) may be applied to patinated or phosphated zinc to lock the colour in. This top layer may be tinted to add an element of colour. Over time, the natural patination of the zinc will take over.
