If Copper is a Noble Metal then Why Are My Pipes Corroding?
Copper possess a resistance to corrosion that is unmatched by many metals, thus giving copper pipes a lengthy service life. However, this metal has been widely mischaracterized as being corrosion-proof and permanent due to the fact that it is classified as a noble metal.
For decades, copper piping has been a popular material for piping in a vast number of households. It is estimated that over 80% of households in the U.S. use copper piping to supply hot and cold water feeds and central heating pipework. The popularity of copper piping is due in part to its inherent high levels of corrosion resistance. However, contrary to popular belief and despite its corrosion resistance, copper piping does not last indefinitely. Copper pipes, like other metals, are susceptible to corrosion damage.
In this article, we shall explore the various factors that contribute to the corrosion of copper piping.
Corrosion of Copper
In chemical terms, copper is defined as a noble metal, which are metals that are highly resistant to corrosion and oxidation. Copper’s resistance to corrosion is attributed mainly to a thin oxide film that forms on the surface of the copper when exposed to air and moisture. This oxide layer, called patina, firmly adheres to the outer surface of the copper and protects the underlying copper layers from further corrosion.
The patina layer, most times, is able to provide sufficient protection to the copper pipe for a lengthy service life. However, changes to water chemistry, water quality and other factors have caused water to become increasingly aggressive towards copper piping. This increased aggressiveness can sometimes exceed the capacity of the corrosion resistivity of the copper piping and shorten its intended lifespan, resulting in pipe damage and leaks.
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Factors Contributing to Corrosion in Copper Pipes
The source of the raw water and the treatment methods applied by municipal treatment plants to produce potable water for distribution determines the chemical properties of the water. This ultimately has a direct effect on the taste, odor and corrosiveness of the final product. It is well known that the chemical properties of the water flowing through copper pipes influence the formation and propagation of corrosion in the piping system. (For more on this topic, see The Role of Corrosion in the Flint Water Crisis.) Some of the factors that affect the chemical properties and corrosiveness of water are:
Surface Water Source – Surface water (typically from lakes and rivers) is usually soft and low in alkalinity (mildly acidic), having a total dissolved solids (TDS) content of fewer than 100 milligrams per liter. With a pH of less than 7.0, untreated surface water can be particularly damaging to copper pipes. Such a low pH value promotes corrosion that can lead to the release of dissolved copper ions into the water giving it a bluish/green discoloration. This type of corrosion is known as cuprosolvency.
Groundwater Source – Groundwater (water collected from wells and aquifers), in contrast to surface water, is generally hard and possesses a relatively high alkalinity with a total dissolved solids content between 200 to 300+ milligrams per liter. In addition, groundwater can also contain high levels of dissolved carbon dioxide which, if left untreated, can also be aggressive towards copper plumbing. Untreated groundwater with a high total dissolved solids content and a pH between 7.0 and 8.2 can lead to pitting corrosion. This is more likely to occur in cold water, and this effect is appropriately referred to as cold water pitting.
Water Treatment Methods – Raw surface water and groundwater are typically treated in a number of ways. For instance, chloramine, which is being used as a substitute for chlorine in the U.S. to comply with Environmental Protection Agency (EPA) standards, is used to disinfect bacteria from potable water. However, chloramine can be corrosive towards copper piping and can lead to pitting corrosion and pinhole leaks.
Also, the use of alum (aluminum sulfate) to coagulate dissolved solids has also been identified as a contributing factor to the formation of pitting corrosion and pinhole leaks in copper pipes.
Microbiological corrosion refers to the internal corrosion caused by bacteria present in the water and piping walls. (This topic is covered in detail in the article Testing For Microbiologically Influenced Corrosion in Pipelines.) The metabolic activity of bacteria in the water results in the formation of hydrogen sulfide gases. When the sulfide gas makes its way to the surface, it reacts with the air to form sulfuric acid, which is highly corrosive and aggressive enough to cause corrosion in most metal and even concrete piping.
High Water Velocity
When the water pressure is not properly regulated, or if the copper pipe diameter is inadequate for the available flow, then this can result in excessively high flow rates. This is characterized by increased water pressure and high fluid velocities that can erode the protective oxide layer of the copper tubing, leaving bare unprotected copper exposed to potentially corrosive fluid. In extreme cases, the metal itself may be removed due to the high velocities. This is known as erosion corrosion and can lead to the formation of grooves and rounded holes (usually in a directional pattern) on the inside walls of the copper piping. As one would expect, erosion corrosion will have the most devastating effects in areas where the water is forced to change direction such as elbows and tees.
Sand, Sediment and Grit
The presence of sand, sediment, grit and other particles makes the water abrasive in nature. When abrasive water flows through the system, the result is, once again, erosion corrosion. High-velocity water in combination with debris in the piping system is a recipe for elevated erosion rates and can drastically reduce the serviceable life of copper piping.
Another, less common type of corrosion from particles occurs when the interior of hot water tanks corrode and release steel particles in the copper hot water system. This can give rise to galvanic corrosion due to the contact of two dissimilar metals (i.e., the copper pipe and the steel from the tank) and can also result in localized pinhole leaks. (Further reading: Why Do Two Dissimilar Metals Cause Corrosion?)
Improper Pipe Installation
Improper installation due to an excessive application of soldering flux has been identified as a cause of corrosion in copper piping. Flux is used to clean oxidation from the surfaces being joined in the soldering process. If excess flux is used, heat applied during the soldering process can cause the flux to flow to various areas within the copper pipe. If the pipe is not properly flushed with hot water, then the flux remains attached to the inner walls of the pipe. Flux is naturally corrosive to copper piping due to the presence of chemicals such as zinc chloride and ammonium chloride that actively remove the copper’s oxide layer. Leaving excess flux in copper piping can eventually cause localized corrosion where the flux was deposited.
Copper is classified as noble, due to its reluctance to corrode, however it has been wrongly characterized as corrosion-proof and permanent. Copper became the popular industry standard for piping in the 1960s and was initially expected to last 25 to 40 years. Therefore, the prevalence of copper pipe failures in the 21st century should come as no surprise.
The evaluation of water chemistry and poor workmanship is not easily identified by visual inspection; therefore homeowners should be aware of the early signs that indicate issues with their copper plumbing, such as:
- Water discoloration and stains in porcelain fixtures
- Rattling/hammering pipes
- Noticeable chlorine smell from the tap water
- Wet spots indicating early signs of leaking
Early detection of possible copper pipe corrosion can help reduce repair costs and inconvenience in the long run.