Page 322 - Failure Analysis Case Studies II

Failure Analysis Case Studies II
D.R.H. Jones (Editor)
0 2001 Elsevier Science Ltd. All rights reserved 307

TYPE I PITTING OF COPPER TUBES FROM A WATER
DISTRIBUTION SYSTEM

PAUL0 J. L. FERNANDES
Advanced Engineering and Testing Services, MATTEK, CSIR, Private Bag X28. Auckland Park, 2006,
South Africa
(Received 9 Augusf 1997)

Abstract-Samples of copper tubes from a cold water distribution system which had failed due to pitting
whilst in service were subjected to a detailed failure investigation. Analysis of the tubes showed that failure
was a result of Type I pitting attack. While the exact cause of pitting was unknown, it was hypothesised that
it could have been due to changes in the water quality and/or content. The tubes were found to be made from
phosphorus de-oxidised copper and no anomalies were evident in either the chemical composition or the
microstructure which could have caused the pitting observed. It was recommended that the tubes be replaced
and that due attention be given to ensure that the new tubes are free of internal carbonaceous deposits or
other foreign matter. 0 1998 Elsevier Science Ltd. All rights reserved.
Keywords Corrosion, pitting corrosion.

1. INTRODUCTION
Copper tubes are used extensively in water distribution systems due to their corrosion resistance
and ease of installation. In Europe and North America they account for more than 80% of all tubes
installed in water service [l], amounting to over 100 million metres of tubing. In spite of these large
quantities, tube failures are relatively rare. Of the failures that do occur, pitting corrosion accounts
for approximately 60%.
This study presents an investigation of the failure of copper tubes from a cold water distribution
system carrying potable water in a shopping centre. The tubes, which were built into the brick walls,
sprang leaks in several premises in the shopping centre after approximately 12 years’ seMce, causing
severe staining of the walls. Examination of the tubes revealed the presence of pin holes perforating
the tube walls.

2.. EXPERIMENTAL PROCEDURE
2.1. Visual examination
Several tubes sections were received for analysis. These were sectioned to reveal the internal
surfaces, which were found to be covered with a greenish-white scale (Fig. 1). Furthermore, localized
deposits of green corrosion product in the form of tubercules were also evident (see arrow in Fig.
I). Some tubercules were carefully removed by light scrubbing to reveal the underlying metal. A
shiny, black layer of an unidentified compound was found to exist beneath the greenish-white scale.
Beneath this black layer, in turn, pits penetrating into the tube wall were found. An example of the
various layers and the underlying corrosion pit is shown in Fig. 2. Some of the pits observed were
relatively large and deep, as shown in Fig. 3.

2.2. Chemical analysis of internal scale and corrosion products
Samples of the tubes were examined in a scanning electron microscope (SEM) equipped with an
energy dispersive spectroscopy of X-rays (EDS) facility. The results of the EDS analysis of the
greenish-white scale found on the internal surfaces of the tubes are shown in Fig. 4. The large copper
Reprinted from Engineering Failure AnaZysis 5 (l), 35-40 (1 998)