Page 325 - Failure Analysis Case Studies II

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3.1. Chemical anaZysis
An analysis of the chemical composition of the tubes was carried out using a wet chemical analysis
method. From the high phosphorus content it was evident that the tubes were made from phosphorus
de-oxidised copper.

4. TYPE 1 PITTING

Pitting corrosion is the most common failure mechanism for copper tubes in water distribution
systems. Essentially two different types of pitting attack have been identified, and these are referred
to in the literature as Type I and Type I1 pitting*. The former is known as cold water pitting and
occurs more frequently than the latter.
Type I pitting is usually encountered in cold water systems carrying borehole or well waters free
from organic matter [I]. It occurs sporadically and can result in tube wall penetration within a few
months. In some cases, however, penetration occurs only after 15 years or more. The internal
surfaces of tubes undergoing Type I pitting are usually covered with a greenish scale of a copper
compound called malachite. Beneath this scale, the tube surface is covered with a smooth, shiny
layer of dark cuprite which is very friable and easily spalled off. Pits are usually associated with the
presence of tubercules which form over pin hole defects in the cuprite layer.
The characteristics of Type 1 pitting attack are such that many pits at all stages of development
can usually be found [I]. Larger pits are generally linearly arranged along the bottom half of
horizontal water lines. When pits are very close together, tubercules can extend over a number of
pits to form one long tubercule. Although pitting has been observed in annealed, half-hard and
hard-drawn tube, susceptibility is generally greatest in the annealed condition. The pits formed are
usually saucer-shaped and relatively wide.
A number of causes of Type I pitting have been identified [I]. Firstly, the incidence of pitting has
been associated with the presence of carbonaceous films on the internal surface of the tube. These
films are residues of the lubricant used for the drawing operation and which are carbonized during
annealing. The quantity and distribution of these films on the internal surface appears to affect the
severity of pitting. The problems arising from the presence of these carbonaceous films can be
overcome in practice by scouring the tubes with a water-sand or a water-air blast.
Secondly, pitting has been associated with the presence of foreign matter deposits on the bottom
half of horizontal tubes [l]. This is in agreement with observations on the preferential location of
pits discussed above. The foreign matter deposits can be introduced into the water lines in a
number of ways. Metal chips and filings and dirt can be allowed to contaminate the system during
installation. If these are not properly removed before service, they may deposit along sections of
the water lines where the water velocity is low. Foreign matter deposits may also be introduced into
the system in the water or may be due to corrosion products formed during surface corrosion of the
tubes during service. The concentration of these deposits, and hence their deleterious effects, can be
reduced by the installation of filters in the water line.
Thirdly, another factor said to cause pitting attack is the presence of soldering pastes on the
insides of the tubes. This generally results from bad workmanship and can be avoided by ensuring
that adequate quality standards are maintained during installation. The soldering pastes may act as
deposits in the same way as foreign matter. Alternatively, during soldering or brazing these pastes
may be converted to oxides which form as a thin film on the copper surface. These oxides are
gcncrally cathodic to copper and can therefore give rise to pitting corrosion.
The effect of water quality on the incidence of Type I pitting is the subject of some controversy
and no consensus has been reached in this regard. Some general observations have been made,
however, on the effects of various constituents and characteristics of water on the extent of pitting,

*Some researchers have also reported the existence of Type 111 and Type IV pitting, but these appear to be variations of
Type I pitting [I].