ABSTRACT

Results of a failure analysis of a copper pipe sample which was removed from a domestic hot water system are presented. The failed piping was in service for 12 years. The pipe failed due to extensive wall thinning which led to through-wall perforation. The corrosion products/deposits on the affected ID surfaces were rich in iron. No sulfur was detected on the examined ID surfaces; however, the damage was attributed to the presence of sulphate reducing bacteria (SRB). The morphology of the corroded surface was examined employing scanning electron microscopy/X-ray energy dispersive analysis, stereomicroscopy and optical metallographic techniques.

INTRODUCTION

Corrosion of Copper In Domestic Water Systems

There are several types of copper corrosion which occur in potable water systems. These include general corrosion, pitting corrosion, erosion/corrosion, and microbiologically induced corrosion (MIC). Localized (pitting) corrosion of copper is typically classified into three categories Type I (Cold-water pitting), Type II (Hot-water pitting) and Type III (Soft Water pitting). General and pitting corrosion of copper is often associated with specific water qualities (1, 2). In general, uniform corrosion of copper occurs when exposed to soft waters of low pH. As a result of general corrosion the affected surface may be covered with a loose powdery scale or have a tarnished copper appearance. Type I pitting produces deep and narrow pits and is typically the result of exposure to hard, cold, well waters with pH in the range from 7 to 7.8 and high levels of sulfate to chloride and bicarbonate ratios. Soft waters with elevated pH (above 8.0) are associated with type III pitting (3). Typical of this attack are wide and shallow pits. Type II pitting has been observed in hot waters with relatively low pH (below 7.2) and high sulfate to bicarbonate ratios.

MIC Of Copper

In the past copper was believed to be immune to MIC corrosion. In recent years many cases of MIC of copper and copper based alloys have been reported (4-6). Microbiologically induced pitting attack of copper pipes in domestic water systems was reported to affect both cold and hot water lines. The attack is often observed in soft waters. Other factors known to contribute to MIC of copper piping include long-term or periodic water stagnation, high suspended solids content, low or nonexisting chlorine levels, and high organic carbon content (7). MIC is often identified by the presence of biofilms on the affected surfaces. In the case of sulphate-reducing bacteria (SRB) frequently intense localized attack is present. These bacteria are anaerobic and occur in most natural waters including fresh, brackish, and sea waters. The mechanism of biocorrosion of copper by SRB is not well developed. One of the theories suggests that SRBs promote corrosion by cathodic depolarization due to removal of hydrogen from cathodic sites. SRBs use hydrogen to reduce inorganic sulphates to sulphides. Also they produce H2S, which directly attacks the metal surface. As a result a black copper sulphide (CuS) -rich scale forms on the metal surface. This scale is porous and easily removable from the metal surface (4). Hydrogen sulfide may cause rapid degradation of the parent metal.

COPPER PIPE FROM DOMESTIC HOT WATER SYSTEM - CASE STUDY A copper pipe sample with extensive wall thinning was examined. The sample was a portion of a horizontal hot water piping run and was removed due to multiple leaks. The failed horizontal section was installed above the ceiling of a retirement home. The pipe was in service for 12 years. The potable water had the following characteristics: pH = 7.8; hardness = 90 ppm as CaCO3; total alkalinity = 70 ppm as Ca

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