Protection against external chemical influences
Bitumen paints, zinc, zinc-aluminium
In the 1960s the health-related ban on the use of tar became effective and resulted in the use of bitumen paints. In aggressive soils, external coating with “bitumen paint” was no longer sufficient and at the beginning of the 1970s it was replaced by the “zinc plus bituminous finishing layer” system. Initially it consisted of a 130 g/m² layer of metallic zinc with a coating of bitumen paint at least 70 µm thick. Later, this zinc layer was increased to 200 g/m². In around the year 2000, this was supplemented by a zinc-aluminium layer of 400 g/m², with a finishing layer of epoxy paint.
The protective effect is based on the position of iron and zinc in the electrochemical series of metals. This is so in most cases, in particular if the pH value of the soil electrolytes is above 6.5. In boggy and marshy soils with their acidic water, the zinc ions remain in solution and the protective mechanism is inhibited. Thick coatings have been developed for these soils which act as barriers and separate the iron from soil electrolytes with a very high electrical resistance. The Table shows the coatings which have been standardised for cast iron pipe systems in Europe.
Cement mortar
A fibre-reinforced cement mortar coating to EN 15542 has a hybrid status. It is 5 mm thick and is applied to the galvanised pipe with an organic adhesive primer. With a polymer-modified mortar, the adhesive primer can be omitted. Both versions can be used in all soils. A cement mortar coating is extremely mechanically robust and has proved its worth above all with the trenchless technique, where sharp obstacles often lie undetected in the ground where the pipe is to be laid. But also, with installation in alpine areas to which it is almost impossible to transport bedding sand, the excavation material from the trench with coarse, sharp-edged stones and boulders can be used again as backfill.
