Page 326 - Failure Analysis Case Studies II

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Table 1. The effect of various water constituents and characteristics on Type I pitting
Chemical species Effect
Sulphate (SO:-) Assists pit initiation and growth, but its effect depends on the concentration of other chemical
species.
Chloride (Cl-) Essential for pitting attack. Assists the breakdown of protective surface films and results in the
formation of wide, shallow pits
Nitrate (NO;) Inhibits pitting
PH Increases in pH generally decreasing the probability of pitting.
Dissolved oxygen (0,) Increased 0, content increases the probability of pitting.
Carbon dioxide (CO,) Increased CO, content increases the probability of pitting due to a decrease in pH.

and these are summarised in Table 1. An empirical screening process has also been developed to
assess the risk of Type I pitting in various waters [2] (Fig. 5). This process has been used extensively
with reasonable success.
A number of characteristics of Type I pitting discussed above were evident in the failed copper
tubes from the shopping centre. The presence of tubercules of corrosion product and the greenish
scale on the internal surface of the tubes were clearly evident (Fig. 1). The friable underlying layer
of shiny, dark cuprite was also observed (Fig. 2). The wide, saucer-shaped pits and their approxi-
mately linear distribution were also evident and are shown in Fig. 3. It is also evident that pits at
various stages of development were observed.

5. CONCLUSIONS
It was concluded that the failure of the copper tubes was due to Type I pitting attack. It is not
clear at this stage what the exact cause of pitting failure was, particularly given the fact that pitting
only became evident after 12 years’ service. It is highly unlikely that it may be due to the presence
of foreign matter deposits introduced during installation of the system. The introduction of foreign
matter in the water is, however, a possibility, particularly if the water is not filtered. A change in
water quality or content (e.g. resulting from mixing of the water with borehole or well waters) could
also be responsible for pitting.
Once initiated, pitting attack can in some cases be halted through the application of appropriate
treatments of the water and the metal. The extent of pitting observed in the present case, however,

+Yes
-No Is the ratio of aggressive C02 to total C02 above 0.05? 1
+ Yes
0 +No Is the pH in the range 6.8 - 7.51
z
+ yes
4 NO Is the ratio of sodium to nitrate greater than I?