Multilayer composite pipes are used for underfloor heatings and drinking water installations for more than 30 years. There are great numbers of pipe systems on the market and suppliers available. And their importance – respectively for heat pumps – is increasing.

Multilayer composite pipes are cost efficient extruded  by meter or as pipe rods from polymers like polyethylene PE (polyethene), polypropylene PP (polypropene) or polybutylene PB (polybutene) and traded. The sufficient stiffness of crosslinked PE (PE-X) makes it possible to form comple pipe geometries. Composite pipes are easy to mount. Thermal processes like soldering or welding are not needed. Individual sections are only compressed using plastic or metal fittings being traded by well-known manufacturers. They may also be fitted to conventional metal pipes, using connectors made of gunmetal or brass. Therefore, complete installations can be mounted by less trained co-workers, that should only be instructed by a manual of the provider.

Driven by sale it is claimed again and again, that “installations made of composite pipes are damage-free because they do not corrode”. This statement is not valid, because polymers may fail, too, but not even by corrosion. The expression ”corrosion damage” is limited to metal parts by the definition of standards. DIN EN ISO 8044 does not allow to use the term “corrosion damage” for multi-layer composite pipes. It defines corrosion as "a physicochemical interaction between a metal and its environment that results in changes in the properties of the metal, and which may lead to significant impairments of the function of the metal, the environment, or the technical system, of which these form a part. This interaction is often of an electrochemical nature."

Polymers are hydrocarbons. Their chemical bond is linear directed (covalent). Their valence electrons are not mobile as it is in metals, but located between the atomic nuclei. Pure polymers are almost non-conductive and electrically very high resistant. Therefore, the interaction between polymers and their environment is usually chemical and not yet electrochemical. For this reason, the damage of polymers is called ageing or degradation.

The terminology of DIN EN ISO 8044 is scientifically justified. But the difference is for users more academic. Polymers and metals are used in drinking water, heating or air conditioning systems; the terms of use are almost identical; chemical reactions can cause significant damage. the destructive agent is water; it can contain compounds or substances that are added for water treatment and can change the properties of polymers and influence their function, the nature of their environment or the respective technical system. The current article even proves that damages similar to corrosion occur in water containing systems in practice.

Experts must therefore comment on damages that - comparable to corrosion - occur under aqueous attack on thermoplastic pipes made of PE, PP or PB. VDI 3822 "Damage Analysis" takes this trend into account. Tis Technical Guideline has been revised. The section “Defects in thermoplastic products” has been added. Sheet No. 2.1.7, for example, explains "Damages in thermoplastic products due to chemical stress"; Sheet No. 2.1.10 deals with "Significant instrumental analysis methods for damage analysis of plastic products".

VDI 3822 overviews some damage patterns observed and test methods used on thermoplastics. It doesn´t explain, how to deal with an individual damage case. The current publication is intended to close this gap. It offers a possible procedure for future investigations by analyzing the cause of damage on multi-layer composite pipes, that are observed at the drinking water installation of a one-family house. The damage patterns and the process of disintegration are illustrated by many figures; the investigation procedure and the methods used to analyze the damage are explained, too.