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COPPER PITTING AND PINHOLE LEAK RESEARCH STUDY
Citation:
Lytle*, D A. AND J. Payne. COPPER PITTING AND PINHOLE LEAK RESEARCH STUDY. Presented at WWSWRD Peer Review, Cincinnati, OH, September 27 - 29, 2004.
Impact/Purpose:
To inform the public.
Description:
Localized copper corrosion or pitting is a significant problem at many water utilities across the United States. Copper pinhole leak problems resulting from extensive pitting are widely under reported. Given the sensitive nature of the problem, extent of damage possible, costs of repair, threat of legal action, and the fact that rarely is a water quality regulation exceeded, utilities are not anxious to make a pitting problem public. Homeowners may not even notice the problem due to hidden plumbing or failure to regularly examine the plumbing. Further, pitting is complicated, difficult to predict and difficult to remediate. The objective of this study is to describe the approach taken to study a community's pitting problem and discuss the findings of the study. The research study approach focused on three parts: analysis of failed pipes from the distribution system, customer survey on the nature of the pitting problem (location, hot versus cold water, etc..), and an examination of water chemistry. The findings together along with comparisons to similar systems (water quality and water treatment) were used to develop a causative pitting model. Copper pipe sections were removed from homes at the time of replacement and transported to the U.S. EPA research center (Cincinnati, OH) by U.S.EPA investigators. Prior to removal, home plumbing details including the relative position of the removed section(s) were carefully documented on-site when possible, and the survey questionnaire was answered. The external and internal surface of the pipe section was photographed using a digital camera and close-up images of area of interest such as pit caps were collected using a stereomicroscope with a 10x to 63x range of magnification. Pits were dissected using a modified dental pick. Solids removed (e.g. pit cap, solids within the pit, etc) were collected for solid analysis, and stereoscopic imaging was performed in the dissected region. Regions of uniform corrosion were also photographed and solids were removed for solid analysis. Solids were analyzed using a variety of approaches including: :X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) (Concord, MA ) and acid digestion followed by inductively coupled plasma - mass spectrometry (ICP-MS). The pitting problem in the case study community was limited to cold water plumbing and, in general, leaky pinholes were most evident along the first 20 to 30 feet of horizontal plumbing after entering the homes. Analysis of a pit cross-sectional structure showed a blue-green colored pit cap over a thin red-orange colored porous thin membrane. The membrane covered a deep penetrating pit that was full of nearly perfect ideally grown cubic crystals of cuprite (CuO2). The pit extended completely through the copper pipe wall in regions of the pipe where pin hole leaks were identified. The pit caps consisted of two copper-sulfate minerals: brochantite [Cu4(OH)6(SO4)] and ponsjakite [Cu4(OH)6(SO4)*H2O]. The minerals were banded with the more hydrated and blue form in the outside (in contact with the water). The pits themselves were as large as 5 mm in diameter and wider at the top giving the appearance of a semicircle. The relatively homogenous portion of the scale surrounding and sometime on top of the pit was comprised of silica and a relatively smaller amount of aluminum and magnesium. Interestingly the same material was found at the bottom of the hot water heater of one home. Isolated particles of the same composition were observed at various sizes across the surface of the pipe. Removal of the particles revealed various stages of pit development. The observations led to the conclusion that the deposition an growing of the solids in an irregular manner led to the development of corrosion cells and initialization pitting. The particle deposition tendencies were linked to the relatively high pH and low alkalinity of the source water. Higher sulfate levels in the source water may have been additional conducive to pitting propagation.