Dr. Edwards continued his prior work examining the roles of aluminum and chlorine as potential promoters of copper pitting.
His tests also looked at how flow (versus stagnant water) and different pH levels would impact copper pitting tendencies.
He used off-the-shelf copper pipe (soft copper, hard copper and copper couplings) and "synthetic" water. His man-made water contained approximately the same dissolved salts (hardness, alkalinity, sulfate, etc.) as does water at WSSC. The water meets all federal EPA regulations and was actually "cleaner" than federal standards as it did not contain any natural organic matter (NOM). Some NOM are natural corrosion inhibitors.
The combination of high pH, aluminum and chlorine levels, and no NOM gave every indication of initiating pitting of copper in the lab for the first time. (Note: this does not mean a pinhole leak formed in the pipe, although the extent of corrosion observed in just 20-28 days testing was impressive by any measure.)
The impact of higher pH contradicts the conventional scientific wisdom that higher pH is generally less corrosive to copper. For example, higher pHs are used to reduce copper leaching to water in order to meet federal lead/copper rule regulations. This is an industry standard practice reinforced by the EPA lead/copper rule recommendations. The observation that higher pH may also increase the likelihood of pitting when aluminum and chlorine are present is especially troubling, given that so many utilities around the country have recently increased pH to control lead and copper leaching to water. It is important to note that previous research indicates that higher pH by itself (without the presence of aluminum and chlorine) does not cause pitting of copper tube.
The adverse impacts of aluminum on copper pitting corrosion were also unexpected. Earlier published work actually supported the idea that such films would be beneficial in preventing aspects of pipe corrosion. In some instances, in fact, utilities were considering purposefully forming such aluminum films on pipes to aid in corrosion control. This work shows that these films, at least at higher pHs and in the presence of free chlorine, can actually increase the likelihood of pitting corrosion.
Of specific interest to WSSC is his work that raises questions of the effectiveness of orthophosphate to slow or stop copper pitting corrosion at higher pH levels.
The impacts of high chlorine, aluminum and pH, and removal of NOM suggest that standard industry best practices and EPA regulations (NOM removal, lead/copper rule, etc.) may be combining to cause a major problem for homeowners across the country with copper pipes.
Recent results about the effect of pH are in direct conflict with the longstanding scientific observation that higher pH leads to fewer copper corrosion problems.
No one – not the copper industry, water utilities or EPA – could have anticipated this reaction. The adverse impacts of NOM removal on copper pitting were predicted in a peer reviewed journal article in 1994. The apparent impact of Cl2 and Al was discovered only during this project in work directly funded by WSSC and the Copper Development Association (CDA). WSSC funded the first phase of this research and CDA funded the current phase.
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