Water does corrode copper pipes, but the process is not the simple rusting seen in iron or steel plumbing. Corrosion is the irreversible degradation of the metal through an electrochemical or chemical reaction with the water and its environment. Copper pipes offer excellent long-term durability, but their longevity depends entirely on the quality of the water flowing through them. When corrosive conditions exist, the metal begins to dissolve, potentially leading to leaks and the release of copper into the drinking water supply.
The Basic Mechanism of Copper Oxidation
The chemical reaction that initiates copper corrosion requires both water and dissolved oxygen. When copper is exposed to oxygenated water, it undergoes an oxidation reaction, forming a thin layer of cuprous oxide (Cu2O) on the pipe’s inner surface. This reddish-brown or black layer is commonly known as the patina.
In most plumbing systems, this stable patina layer acts as a passive barrier, separating the underlying copper metal from the water. This protective film significantly slows down further corrosion, allowing copper pipes to last for many decades. If the water chemistry disrupts or prevents the formation of this stable layer, the corrosion process will accelerate.
Key Water Quality Factors Driving Corrosion
Water quality is the most important determinant of whether a copper pipe develops corrosion issues. Water that is too acidic (pH below 6.5) is cuprosolvent, meaning it actively dissolves the copper oxide patina and the metal itself. Highly alkaline water (pH above 8.5) can also lead to localized corrosion. Maintaining a near-neutral pH range, typically between 6.5 and 8.5, is optimal for patina stability.
Dissolved oxygen fuels the initial oxidation reaction and sustains the corrosion process. High levels of oxygen accelerate the breakdown of the protective layer, increasing the rate at which the copper is attacked. Elevated water temperatures also increase the speed of chemical reactions, making hot water lines more susceptible to corrosion than cold water lines.
Certain dissolved solids and ions can destabilize the protective patina. High concentrations of chlorides and sulfates are particularly aggressive toward copper and can accelerate localized corrosion. Soft water that lacks sufficient mineral content, such as calcium and alkalinity, may also be corrosive because it cannot form the thin mineral scale that reinforces the cuprous oxide layer.
Excessive flow velocity causes a physical form of damage called erosion-corrosion. When water moves too quickly, often exceeding 5 to 8 feet per second, the force of the moving liquid physically scours the inner pipe wall. This mechanical wear strips away the protective patina, exposing the raw copper metal and resulting in rapid material loss, especially near bends and fittings.
Identifying Different Forms of Copper Damage
Corrosion in copper pipes typically manifests in two primary forms: uniform corrosion and localized pitting corrosion. Uniform corrosion involves a relatively even thinning of the pipe wall across a large surface area. This slow, generalized attack is less common but results in a gradual reduction of the pipe’s structural integrity.
The most frequent and destructive form of copper failure is localized pitting corrosion, which creates tiny, focused holes, often referred to as pinholes. This occurs when the protective patina breaks down in a small area, allowing the water to attack the copper aggressively at that single point. The resulting pits are narrow but deep, leading to pipe perforation and a leak long before the overall pipe wall thins.
Erosion-corrosion is another distinct form of localized attack, physically identifiable by grooves or horseshoe-shaped depressions inside the pipe. This damage is typically found near elbows, tees, or pumps where the water flow is turbulent and fast enough to mechanically remove the protective layer. Identifying the pattern of damage helps diagnose whether the underlying cause is chemical (pitting) or physical (erosion).
Practical Methods for Corrosion Prevention
Mitigating copper corrosion focuses on managing the corrosive factors within the water supply. Water treatment facilities often employ alkalinity and pH control to keep the water within the optimal range of 6.5 to 8.5, minimizing copper dissolution. Some systems also use corrosion inhibitors, such as orthophosphates, which are added to the water to form a thin film that reinforces the natural patina.
For homeowners, ensuring that water flow rates are not excessive is a practical preventive step. High flow velocities that cause erosion-corrosion can be avoided by verifying the plumbing system is properly sized or by checking for flow restrictions. Preventing water stagnation by ensuring regular water use also helps maintain consistent water quality inside the pipes.
Another important measure involves avoiding contact between copper pipes and dissimilar metals, which can cause galvanic corrosion. Where copper connects to a more active metal, such as galvanized steel, a non-conductive dielectric union should be installed to electrically separate the two metals. This prevents the electrochemical reaction that accelerates corrosion in the copper pipe. Proper electrical grounding practices are also important, as stray electrical currents running through the plumbing system can accelerate corrosion.