Wire

Galvanising

Galvanising is a process that coats the surface of the steel with a layer of zinc, to lessen the rate of corrosion. In clean air, zinc corrodes 15–20 times more slowly than steel. It also has the useful property of corroding preferentially to steel in localized areas, so at cut ends or scratches where the steel is exposed, the zinc corrodes to cover the exposed steel.

The length of life of any wire product depends on the effectiveness of the zinc coating, and the atmosphere in which the wire is used. At the time of publication, most fencing wire available in Britain is galvanised to British Standard 443: 1982. This standard has been withdrawn and is superseded by BS EN 10244: 2001. The details below relate to BS 443: 1982.

The effeciveness of the zinc coating is determined by:

• The weight or thickness of the zinc coat: British Standard BS 443: 1982 requires a minimum coat weight of 260 grammes per square metre on a wire of 2.50mm. This is known as heavy galvanising. Products described as commercially galvanised have a coat weight of about 90 grammes per square metre.
• The adherence of the zinc to the steel. This depends on the quality of the steel and the process used.
• The concentricity of the coating. Corrosion starts at the point where the coating is thinnest.

Any deficiencies in the zinc coating are not visible on purchase, and only become obvious when the wire rusts and breaks after a relatively short time. It is important only to use wire that is manufactured to BS 443 or EN10244:2001, and bears the label of a reputable manufacturer.

The atmosphere in which the wire is to be used should be taken into consideration. Zinc corrodes more quickly in air polluted with sulphur dioxide, and also on coasts, where the air is heavily salt laden. Table 4b shows the expected wire life for three weights of galvanising in different situations.

Note that the life of heavy galvanising is estimated to be about 20% of the total life of the wire, so wire can still last for many years even after the galvanising has deteriorated and the wire is rusty.

Another important factor in the reduction of corrosion is in the storage and handling of the wire.

• Wire stored in damp conditions, on concrete floors, or in contact with fertilizers, chemicals or other corrosive materials may be damaged.
• Sharp kinks due to careless unwinding, or rough handling with fencing pliers or other equipment with serrated jaws will damage the zinc coating and increase corrosion. In addition, kinking weakens the wire, possibly causing it to snap.
• Loose fastening with staples on intermediate posts will allow the wire to rattle in the wind, which will damage the galvanising.
• Frequent wetting and drying, such as occurs at ground level, will increase corrosion.

Table 4b Typical lives of galvanised coatings in different environments

Coated wires

A zinc and aluminium alloy is used as a coating to make a wire which has three times the life of wire with heavy galvanising. An example is Triple Life, coated with Galfan, manufactured by Estate Wire Ltd. Triple Life stock netting, high tensile stock netting, chain link and barbed wire are available.

Plain wire

Table 4c lists the types of plain wire available. Wire is described by its type, either spring steel, high tensile or mild steel (see below), and by its diameter or SWG number (Standard Wire Gauge).

The main difference between the different types of wire is in their tensile strength. Tensile strength is the maximum load that a material can support when being stretched, divided by the original cross sectional area of the material. It is measured in Newtons per mm2. The tensile strength depends on the constituents of the wire and the method by which it is ‘drawn’ during manufacture. Tensile strength can vary for wires of the same diameter.

Mild steel wire, when pulled, stretches in two distinct stages. The first is the elastic stage, when if the load is removed, the wire springs back to its original length. If pulled further, into the second or plastic stage, the wire elongates but does not spring back when the load is removed. The wire elongates considerably in the plastic stage before it breaks. The change from the elastic to the plastic stage is called the ‘yield point’.

In contrast, high tensile and spring steel wires have no yield point, but stretch elastically under high loads.

Because high tensile wire has greater elasticity than mild steel wire, it is able to retain tension under impact and varying atmospheric conditions. In cold weather, wire contracts and the tension increases. In hot weather the wire expands, decreasing the tension. The change in length is similar for all types of wire, but the resultant increase or decrease in tension depends on the wire elasticity. If the contraction of mild steel wire in cold temperatures takes it beyond the yield point, the wire will lose all springiness as temperatures rise.

High tensile fencing, using high tensile wire with appropriate straining and intermediate posts, should be specified where possible for new fences, as it is very much more efficient than mild steel wire fencing.

Table 4c Plain wires

Barbed wire

Prior to the invention of barbed wire in the 1860s, post and rail fencing was the only type of fencing available to most farmers. Following the mechanisation of the manufacture of barbed wire in 1874, this cheap and quick method of fencing spread rapidly in America and later in Europe. It had a major impact on the American West, allowing vast tracts of land to be fenced for cattle.

Barbed wire remains in widespread use for fencing, although it is not an ideal fencing material.

• Barbed wire is unpleasant to handle, even when new. Removing and disposing of old barbed wire is such an unpleasant job that often the old wire is left in place and more wire or another fence is put up next to it.
• As well as the distress caused to wild and farm animals that get injured by barbed wire, considerable damage is caused to animal skins, reducing their value.
• Barbed wire does not deter animals from jumping fences, but merely injures them when they do. Electric scare wires can be positioned to deter animals from jumping, or fences should be heightened.
• Barbed wire does not deter animals from leaning on or putting their heads through fences in order to forage on the other side. Electric fences provide a more effective and less damaging way of discouraging animals from leaning or rubbing on fences. A strong rubbing post is useful in paddocks which have no trees or other objects on which animals can rub themselves, and reduces damage to the fence.

Advice from experienced fencers is to avoid the use of barbed wire wherever possible. Always consider carefully the purpose of the fence, and don’t merely put up a top line of barbed wire because it is the tradition to do so.

Types of barbed wire

• 2 ply mild steel. This should be galvanised to BS 443 or EN 10244: 2001. Each strand of the 2 ply wire is 2.5mm wire, with 2.00mm wire 4 point barbs, spaced at 85mm (3.5in) intervals. Normally supplied in 200m reels.
  
• 2 ply high tensile steel. This should be galvanised to BS 443 or EN 10244: 2001. Line wires are 1.60mm diameter, with barbed wires of 1.45mm diameter. A heavier high tensile barbed wire is also available from Sentinel Fencing, with 2.00mm wires and 1.60mm barbs. Normally supplied in 200m reels. High tensile barbed wire has a reverse twist in the ply, in order that it can be unreeled without getting tangled.
  
• Concertina barbed wires and tapes. Various types are available for security fencing.