Copper does corrode in water, although it is highly durable and often selected for plumbing due to its general resistance to degradation. Corrosion is the deterioration of a material resulting from a reaction with its surrounding environment, such as flowing water in copper pipes. While this process is usually slow under normal conditions, specific water chemistries can significantly accelerate the rate of material loss. Copper’s longevity relies on a naturally forming protective layer that, if compromised, can lead to premature pipe failure.
The Copper-Water Reaction: Patina Formation and Failure
Copper’s stability in water relies on an initial electrochemical reaction with dissolved oxygen and water molecules. This reaction quickly forms a thin, dense layer of cuprous oxide (\(\text{Cu}_2\text{O}\)) on the pipe’s interior surface. This surface film, known as a patina, acts as a barrier, separating the underlying metal from the water and dramatically slowing further corrosion.
The protective patina is only stable within a narrow range of water conditions; if the water chemistry shifts, the film can dissolve or become unstable. When the patina fails or is stripped away, the fresh copper surface is continuously exposed to the water. This instability leads to a continuous corrosion cycle, known as cuprosolvency, which releases copper ions into the water supply. A sign of this instability is often the appearance of “blue water” due to the suspension of copper corrosion by-products.
Distinct Forms of Copper Damage
Corrosion damage in copper plumbing manifests in several distinct physical patterns, each presenting a different risk to the system’s integrity. The most concerning type for homeowners is pitting corrosion, a highly localized attack on the pipe wall. Pitting focuses corrosive action on a small area, eventually eating a tiny pinhole through the pipe and leading to sudden leaks.
Uniform corrosion, by contrast, is a general thinning of the pipe wall that occurs evenly across the interior surface. While less likely to cause sudden failure, uniform corrosion slowly reduces the system’s lifespan. Erosion corrosion is a mechanical-chemical process accelerated by high water velocity or turbulence, typically near bends or fittings. The force of the water flow physically removes the protective patina, exposing the bare metal to rapid chemical attack and causing a characteristic grooved pattern of material loss.
Water Quality Variables That Drive Corrosion
pH and Chemical Composition
The rate of corrosion is largely controlled by the chemical and physical characteristics of the water itself. Water acidity, measured by \(\text{pH}\), is a significant factor; water with a \(\text{pH}\) below \(6.5\) is often highly corrosive and can dissolve the protective cuprous oxide layer. Conversely, excessively alkaline water (above \(\text{pH}\) \(8.5\)) can lead to scale buildup that creates localized corrosive conditions underneath the deposits. High concentrations of dissolved oxygen are necessary for the initial patina to form, but excessive amounts can drive the corrosion reaction once the protective layer is compromised.
Flow and Temperature Factors
Certain dissolved solids, particularly chlorides and sulfates, are corrosive because they can destabilize or penetrate the passive film. Water temperature also plays a role, with hot water systems often experiencing accelerated corrosion rates compared to cold water lines. The water’s buffering capacity, related to its dissolved inorganic carbon content, also influences corrosion by affecting the stability of the protective film. High flow rates, particularly in commercial buildings, can increase the risk of erosion corrosion if the velocity exceeds \(5\) feet per second.
Addressing Corrosion: Indicators and Preventative Measures
Indicators of Corrosion
The first visible signs of copper corrosion often appear outside the plumbing system as blue-green stains on porcelain sinks, tubs, and fixtures. These stains are caused by copper ions that have leached from the pipes and precipitated onto the surfaces. Homeowners might also notice a metallic or bitter taste in their drinking water, indicating elevated copper levels. The most severe sign is the development of chronic pinhole leaks, which result from localized pitting corrosion. These leaks commonly occur in clusters and signal a breakdown in the pipe’s protective mechanisms.
Preventative Measures
To mitigate corrosion, water treatment is often employed, such as adjusting the water \(\text{pH}\) to an optimal range (typically between \(6.5\) and \(8.5\)). Water utilities may also add corrosion inhibitors, such as orthophosphate, which helps stabilize and reinforce the protective patina. Ensuring the plumbing system is properly grounded can help prevent stray electrical currents that may exacerbate corrosion. For homes with extremely aggressive water chemistry, the solution may involve installing a whole-house water treatment system or replacing sections of pipe with more resistant materials.