Pitting Corrosion of Copper in High-pH and Low-Alkalinity Waters
Copper is popular in household plumbing—it’s easy to install, abundant in nature, and low in cost. Localized corrosion, or “pitting” corrosion, is a major cause of failure in copper household plumbing. 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. This provides an ideal environment for the growth of mildew or mold. 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.
Copper pitting corrosion remains poorly understood despite a number of reports released in recent years. There have been cases of pitting in waters having:
- high pH,
- low alkalinity, and
- significant levels of sulfate and chloride.
These materials may also cause pitting:
- total inorganic carbon.
Orthophosphate has shown some promise as a corrosion inhibitor and reducer of pitting tendency of water.
EPA continues to research pitting. EPA researchers are systematically investigating the effect of water chemistry, such as:
- sulfate, and
- alkalinity or
- inorganic carbon and
- both localized and uniform.
Uniform corrosion is identified by the presence of a relatively uniform layer of copper corrosion by-products across the inner surface of a pipe wall. It is typically associated with elevated copper levels at our taps.
Methods and Materials
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 corrosion in the copper plumbing. Various techniques were used, including X-ray diffraction and scanning electron microscopy (SEM). Water quality was monitored and adjusted regularly.
Results and Impacts 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. Researchers 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). It 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 recognized the start of pitting to the deposition of copper chloride or sulfate compounds on the pipe at the anode.
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:
- organic carbon, or
- other water quality variables—variables that have been suggested to be important in pitting or frequently associated with copper plumbing that have failed from pitting corrosion.