Zinc has unique properties and these are exploited in its uses. The predominant use for zinc (50-60%) is in corrosion protection, primarily galvanising, that is the coating of steel with metallic zinc in order to protect it against corrosion. Zinc is a very reactive metal (less reactive than magnesium or aluminium but more reactive than iron) and it may seem strange that it should be able to protect steel from corrosion when one might imagine that zinc itself is more vulnerable and needs all the protection that it can obtain! However, when oxygen in the air reacts with the surface of zinc, a very dense and impermeable coating of zinc oxide is formed and it is this physical barrier that protects the zinc surface from further attack. An article coated in zinc, for example a motorway crash barrier, a roadside lighting standard or a galvanised dustbin, quickly loses its shiny silver new appearance and becomes dull grey; this is because of the formation of this zinc oxide coating which is a natural ageing process. The physical barrier of the zinc coating and its surface oxide then protects the steel. The interface between the zinc and steel is not, in fact, just one metal directly onto another. The galvanising process creates a reaction between the zinc and steel whereby an intermediate zinc-iron compound is formed between the steel and zinc, and this provides a further barrier to corrosion and an unbreakable bond between the zinc and the steel. The zinc does not peel away from the steel in the way that paint would.
This is not, however, the complete story as far as the protection of the steel is concerned. If the surface of the galvanised steel becomes scratched or broken, one might expect the exposed steel area, which has now lost its physical protective barrier, to corrode. Zinc, however, being more reactive than iron, provides a further defence, because the zinc and iron form an electrolytic cell, like a battery, whereby the natural reactivity of the iron and its desire to turn to rust is suppressed in favour of the more reactive zinc. Zinc will therefore oxidise a little and the zinc oxide product will then heal the scratch in the zinc coating. In any case, because the area of zinc coating is so large in comparison to the area of the break in the coating, the actual amount of zinc loss in protecting the steel is very small. Zinc is therefore the perfect protector. For protecting steel car bodies, the long guarantees of 12 years or more now provided against corrosion, speak for themselves, when one compares with car owners’ experiences only 20 years ago, at which time car bodies used to start rusting away in 2 years or less. However well a steel item is painted the paint cannot match zinc for protection because it only provides a physical barrier and cannot provide a secondary defence if the coating is broken. In fact, once the paint surface is broken, corrosion advances rapidly underneath the remaining paint.
There are two main classifications of galvanising, general galvanising and continuous galvanising. In general galvanising the steel article, after cleaning, is immersed in a bath of molten zinc and is then removed and drained; the zinc coating soon becomes solid and the article can then be handled normally. General galvanising can be used for large articles such as steel girders and structures as well as for small articles such as nuts, bolts and washers. The zinc used for general galvanising contains a small amount of lead, which settles to the bottom of the galvanising bath, and it helps to protect the bath against zinc attack as well as providing a liquid surface on which any free iron-zinc compounds will collect and can be more easily removed.
Continuous galvanising is used for coating coils of steel strip or wire and is a continuous process. Thus steel strip or wire is unravelled from a coil and passes, via cleaning steps, through a bath of molten zinc and is subsequently re-coiled. This process is obviously much faster than general galvanising but is only suitable for regular coils of steel strip and wire. The zinc used for continuous galvanising contains up to 1% aluminium and this has the effect of modifying the zinc-iron compounds formed between the zinc and steel, making them less brittle, so that the strip can be subsequently bent and formed into shapes, for example automobile under-body parts.
Zinc can also be applied in a continuous galvanising process by electrolysis, although this process is less used than hot-dip continuous galvanising. Molten zinc can also be sprayed on to steel to provide protection and this method has been used to protect large structures such as bridges. Arguably the protection is not as good as if the individual components had been galvanised on initial manufacture, since the preparation and application cannot be as well controlled on a construction site.
A further application of zinc in corrosion protection is the use of zinc anodes (lumps of zinc), which are attached to (un-galvanised) steel structures. Ships’ hulls, steel pipelines and oil rigs at sea are all examples of structures often protected in this way and which, in general, cannot themselves be galvanised. When zinc is used in this way it is called sacrificial protection because it is intended that the zinc should corrode and be “sacrificed” in preference to the steel structure. The zinc anode, being more reactive than the steel, creates an electric cell between itself and the steel, rendering the steel un-reactive to the conditions. Whilst the zinc, in making itself vulnerable to attack in this way, will require replacement at intervals of time, actual corrosion rates can be kept quite low.