Current cases of damage
The current damage phenomena can be described by the following observations:
- Damages in old, hard soldered installations made before 1996, where damages occur sporadically (temporal wave-like movement of damage maxima) or first damages occur after a long service time (> 20 years);
- Damages in installations made of half-hard copper pipes (cold and warm water).
Until the middle of the 1990s the pattern of damage was basically determined by copper pitting corrosion in installations where hard copper pipes (R 290) had been handled by hard soldering or soft annealing (for bending). The primary cause of damage could be determined as the partially explicit pre-existing defect of the pipes as a consequence of the significant impact of the heat on the surface.
The structure alteration associated with that and the sensitisation of the basis material manifest in the metallographic microsection in the area of the heat affected zone (surface area affected by hard soldering) as a corrosion attack advancing along the grain boundaries (Figure 1a). During the further growth of an active pitting site it may develop into a spherical shaped shallow pit which is a typical sign of pitting corrosion type 1 (Figure 1b).
Figure a, source: Denzig, 2001
Figure b
Figure 1: Corrosion attack in the heat affected zone, after four weeks of service (picture a); spherical shaped shallow pit (picture b)
The damage susceptibility can therefore be referred to the manufacturing while the damage likelihood in the further process is dependent on the start-up, the service conditions and the corrosion chemical properties of the drinking water. The wave-like occurrence of damage frequency and the occurrence of damages in very old installations can possibly be referred to the water composition, but reliable research results on this topic are not available yet.
Since 2003 to 2005 an increasing number of damages is reported from drinking water installations where copper pipes of the temper half-hard (R 250) had been installed and from supply areas which had not been known as remarkable in terms of copper pitting corrosion so far. These damages have been determined only in pipes, not in fittings yet.
Novel damages in half-hard copper pipes
Figure 2: Seven perforations of the wall of the pipe on a length of 30 cm; warm drinking water
Some specialties with the novel damages have been observed, especially the intensity (high number of deep pits and perforations, Figure 2), the position – horizontally and vertically installed pipes are affected – and the morphology of the pitting sites. Additionally the damages occur equally in cold and warm drinking water.
The cappings of the pitting sites (tubercles) equal those typically detected in cases of type 1 or type 2 pitting – optically and by the composition of the corrosion products. What has not been described for drinking water is the structure of the pitting sites.
Figure 3a: Corrosion attacks in half-hard copper pipes; initial corrosion with fine branching
Under initial corrosion conditions the attacks show fine branching (Figure 3a) and/or tunnel-like structures which can spread into a significant undermining attack (Figure 3b).
Figure 3b: Tunnel-like structures with significant undermining attack
Sufficient scientific explanations or model conceptions which could explain the causes of these damages do not exist at the moment. But they are necessary for the assessment of the corrosion risk. Especially the question of the initiation of pitting corrosion as the primary process of pit growth is mostly unanswered.
Key aspects of research – phase 1
The aims of the research project are the clarification of the cause of the damages and the development of measures to avoid future damages.
In the first phase of the research project the partially only rudimental pieces of information will be compiled and a systematic survey of damages will be conducted. For this an enquiry of the damages and the remarkable supply areas will be carried out by a nationwide survey of the water supply companies affiliated with DVGW. Parallel to that an assessment system for the investigation of damaged copper pipes will be developed and after typification and classification of the damages the results will be evaluated by a multi-attribute analysis.
For statistical validation of the data IWW is – additionally to the pipes already available for investigation – looking for further damaged pipes to put the investigations and the enquiry of information on a wider basis.
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Pitting corrosion of copper – current damages in drinking water systems in Germany
Electrochemical testing of different corrosion inhibitors and their effect on copper pitting corrosion in drinking water
Influence of drinking water treatment processes on the pitting corrosion behaviour of copper pipes in domestic installations, CEOCOR Brüssel 2010
Influence of drinking water treatment processes on the pitting corrosion of copper pipes in domestic installations_EUROCORR 2010, Moskau
