Pitting Corrosion of Copper in High-pH and Low-Alkalinity Waters

Figure 1. Pit cap.

Localized corrosion, or “pitting” corrosion, is a major cause of failure in copper household plumbing. It’s important to investigate copper corrosion because copper is popular in household plumbing—it’s easy to install, abundant in nature, and low in cost. However, after only a few months following the installation of copper plumbing, pitting can lead to pipe failure in the form of pinhole leaks. The cost of plumbing repair and the associated expense of repairing water-damaged items can be excessive. Additionally, pinhole leaks may go undetected in walls or basements for months, providing an ideal environment for the growth of mildew or mold. As a result, many homeowners elect to replace their entire plumbing system to avoid the hassle of continually repairing damage caused by copper pitting. Examining copper pitting corrosion in high-pH and low-alkalinity waters will help water utilities and engineers reduce the occurrence of copper pitting and the problems it causes. Figures 1 and 2 show signs of a pitting corrosion attack.

Figure 2. Copper pitting corrosion.

Copper pitting corrosion remains poorly understood despite a number of reports released in recent years. There have been cases of copper pitting problems in waters having high pH, low alkalinity, and significant levels of sulfate and chloride. In some of those cases, aluminum, silica, total inorganic carbon, and other materials have been implicated as the cause of pitting. Also, the role of orthophosphate as a corrosion inhibitor and reducer of the pitting tendency of water has shown some promise. EPA continues to research pitting corrosion, as in the following study.

Study Objective

The objective of this study was to systematically investigate the effect of water chemistry (pH, sulfate, and alkalinity or inorganic carbon, and orthophosphate) on the nature of copper corrosion, both localized and uniform.

Methods and Materials

Figure 3. Experimental pilot-scale copper recirculation system.

Research was conducted using recirculating water pipe rigs (shown in Figure 3). A detailed, solids surface‑analysis approach was used to investigate the nature of copper corrosion in the copper plumbing. Various techniques were used, including X-ray diffraction and scanning electron microscopy (SEM) at the completion of the exposure period. Water quality was monitored and adjusted regularly to attain goals.

Results to Date

• Examining Pitting Corrosion at the Pilot Scale — The research demonstrated that some forms of localized or pitting corrosion can be studied at the pilot scale in a reasonable time frame (i.e., several months), at least in relative study periods needed to examine uniform corrosion and metal solubility.
• Experimental Systems and Water Quality — The findings suggest that experimental systems can be set up to systematically examine the effect of all types of water quality variables on pitting and pinhole leaks.
• Key Water Chemistry Results — Pitting corrosion occurred only in low dissolved inorganic carbon (DIC) (5 and 10 mg C/L) and high-pH (9) water in the presence of chloride (20 mg/L) and was not observed at pH 7 or 8. Sulfate was not necessary to develop pitting corrosion; however, it did affect the composition of the corrosion by-products associated with pitting corrosion. Increasing the DIC from 10 to 50 mg C/L or adding 3 mg PO₄/L prevented pitting corrosion at pH 9. A conceptual model was established that attributed pit initiation to the deposition of copper chloride or sulfate compounds on the pipe at the anode.

Implications

The results of the examination of pitting corrosion at the pilot scale provide some supporting evidence to:

• Suggest that water utilities can construct relatively simple pilot studies to investigate the tendency for their water to induce pitting
• Predict the effect of water treatment on pitting corrosion tendencies
• Examine water quality changes on continuously flowing copper pipe loops operated in a single flow-through design at the appropriate flow rate

Water quality results show that pitting can be initiated without silica, aluminum, organic carbon, or other water quality variables—variables that have been suggested to be important in pitting or frequently associated with copper plumbing that has failed from pitting corrosion.

Technical Contact:

Darren Lytle (513) 569-7432,

See Also:

The Effect of Water Chemistry on the Solubility and Properties of Freshly Precipitated Copper Solids