Corrosion of metals and metal components in heating and water supply installations will occur due to the presence of free oxygen in the water. Oxygen will almost always be present in any system as it can enter through a number of points, such as open header tanks, valves, threaded joints and pumps, and also through gas permeable materials.

In closed loop heating systems which are not continually supplied with hot water, minimisation of oxygen ingress through the pipe wall will significantly reduce the occurrence of corrosion. For this reason, Polybutene-1 barrier pipe has been developed.

Barrier pipe consists of 3 or 5 concentric co-extruded layers of material which are combined into one integral pipe wall. In a 3-layer structure, Polybutene-1 constitutes the inner layer and ethylene vinyl alcohol copolymer (EVOH) the outer layer. The central layer is an adhesive material which is compatible with both the inner and outer functional materials to form an integral structure.

EVOH is the oxygen barrier material that has extremely low oxygen permeability characteristics. To improve the oxygen barrier performance even further, a 5-layer pipe structure has been developed.

In this case, the inner and outer layers are Polybutene-1 and the centre layer is EVOH. On either side of the EVOH are 'adhesive tie' layers

The advantages of the 5-layer structure are that the EVOH layer is protected from both physical and atmospheric effects, and hence provides a superior oxygen barrier performance per unit weight of EVOH. 

Small amounts of oxygen will enter all heating water circuits whatever the type of pipe used, and therefore to protect the system against the effects of corrosion, inhibitors are added to the circulating water.