Pitting on copper of a water pipe


1. History

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.

Technical specifications

Operating temperature approx. 10 °C,
Service time approx. 30 years
Pipe diameter 20 mm
Wall thicknesse 1 mm
Connection technology brazing
Water hardness for about 2 years 13° dH (Calcium 80 mg/l), before 18° dH [dH = German Hardness]

2. Investigations

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).

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. 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.

In the region of the pustules, the pipe wall is eroded. This area has a length of approx. 30 mm as it can be seen in Fig. 2 and Fig. 3.

Fig. 2:
Base material exposed,
linear failure in area of pustules (detail from Fig. 1, lower shell)
Fig. 2: Base material exposed, linear failure in area of pustules (detail from Fig. 1, lower shell)

Corrosion in the area free of deposits (detail from Fig. 2)
Fig. 3: Corrosion in the area free of deposits (detail from Fig. 2)

The attack went furrow-like in the depth and has apparently under-run the surface (Fig. 4).


linear pit
(0.8 * 3 mm) as a dark area
Fig. 4: linear pit (0.8 * 3 mm) as a dark area (detail from Fig. 3)

In the following, the pustule should be considered closer. The scanning electron micrograph (SEM) shows cauliflower-like structures as they arise when the mass grows continuously (Fig. 5).


Cauliflower-like growths as a pustule surface
(recording with the SEM using backscattered electrons)
Fig. 5: Cauliflower-like growths as a pustule surface (recording with the SEM using backscattered electrons)

X-ray analysis (energy-dispersive) of the pustules revealed the presence predominantly of oxygen and copper, along with calcium, aluminium, silicon and phosphorus (the latter in each case below 1%), see Fig. 6.

Analysis of a pustule: oxygen and copper,
besides some iron, calcium, aluminium, silicon and phosphorus (less than 1% each)
Fig. 6: Analysis of a pustule; oxygen and copper, besides some iron, calcium, aluminium, silicon and phosphorus (less than 1% each)

In an ultrasonic bath the sample was cleaned with dilute citric acid. The leak is now visible. Its diameter is about 0.8 mm. The hole is situated in a roughened dark area, which is linearly distinguished from the copper-coloured residual wall. Its width is about 4.5 mm. Remains of pustules are still clinging here. In the middle of this zone, a longitudinal furrow is visible (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. 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

Within this strip, the sheet was thinned until finally the leaking was achieved (Fig. 8).

:
Sheet was thinned
(detail from
Fig. 7)
Fig. 8: Sheet was thinned (detail from Fig. 7)

3. Discussion

The 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::

2Cu + H2O + CO2 + O2 = Cu (OH)2 + CuCO3

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. [1]). 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. Summary

The 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

[1] Konrad J., and A. Kundig: Copper Applications in Plumbing


German version:
Lochfraß an Kupferrohren einer Wasserleitung

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