Pitting on copper of a water pipe
The copper pipe for cold water had been laid under plaster. A wet spot in the wall suggested the formation of a leak. After removal of the pipe, a fine hole was visible.
|Operating temperature||approx. 10 °C,|
|Service time||approx. 30 years||Pipe diameter||20 mm|
|Wall thicknesse||1 mm|
|Water hardness||for about 2 years 13° dH (Calcium 80 mg/l), before 18° dH [dH = German Hardness]|
By means of a handsaw, the tube was divided longitudinally. Now we consider the inner surface of the two pipe shells. In the upper shell, a blue-green coating becomes visible, which appears relatively dense. In the lower shell the coating is more of whitish colour. However, the covering is missing linear; instead blue-green pustules have formed (Fig. 1).
Fig. 1: Pipe opened; the lower shell contains the leak, dense blue-green coating in the upper half; in the lower shell the material is linearly uncovered, but formation of blue-green pustules.
|Fig. 2: Base material exposed, linear failure in area of pustules (detail from Fig. 1, lower shell)|
|Fig. 3: Corrosion in the area free of deposits (detail from Fig. 2)|
|Fig. 4: linear pit (0.8 * 3 mm) as a dark area (detail from Fig. 3)|
|Fig. 5: Cauliflower-like growths as a pustule surface (recording with the SEM using backscattered electrons)|
|Fig. 6: Analysis of a pustule; oxygen and copper, besides some iron, calcium, aluminium, silicon and phosphorus (less than 1% each)|
|Fig. 7: Hole (0.8 mm) in a roughened linear region (4.5&xnbsp;mm wide) embedded in an otherwise bright-coppery tube wall, longitudinal furrow in the middle of the zone, remains of pustules|
|Fig. 8: Sheet was thinned (detail from Fig. 7)|
3. DiscussionThe formation of a leak was preceded by the growths of a blue-green layer. In these crusts the main chemical partner of copper is oxygen. It is the "patina" as it known from copper roofs, which protects the surface from further corrosion. The patina is formed by a mixture of copper hydroxide and copper (II) carbonate::
In contrast to the formation of the natural roof patina, the attack continued in the present case.
Concerning the corroded zone it is noticeable that not only the edges are linear, there is also a pronounced centre furrow. The darkening of this zone can only be explained by oxidation. This area was less noble than the rest of the wall resulting in dissolution i.e. an electrochemical element was formed.
Due to its linearity, the furrow formation can only have occurred during the production of the pipe.
In manufacturing copper pipes, the extrusion process is used first. The mass is heated until it is pliable. Then it is pressed over a mandrel by a mould (die). After cooling, the pipe is drawn to final gauge. In the tube, a mandrel is introduced whose diameter is slightly larger than that of the die opening. The mandrel settles in front of the matrix (“floating plug mandrel” cf. ). In the present case, the mandrel has caused a shallow furrow on the inner wall of the pipe, a so called die mark.
The oxidation of the furrow surface requires the presence of high temperatures. Accordingly, the furrow was introduced during the extrusion process. Uncleanliness when mounting the pipes (introduction of solder flux) ore the lack of a water filter could be excluded. The reduction in water hardness had also had a secondary effect.
4. SummaryThe leaking of a copper pipe is based on the formation of a furrow during manufacturing. The walls of this “die mark” were oxidized. This area offered a weak region for corrosion due to formation of an electrochemical element. The oxidation indicates that the furrow has already arisen during hot forming of the pipe (extrusion).
Martin Moeser, 23.11.2017
German version: Lochfraß an Kupferrohren einer Wasserleitung