Copper Tube Corrosion Resistance
No other plumbing material has the service performance record of copper water tube and fittings.
Copper tube has been used for hot and cold water systems since the 1930s. And it is not uncommon for these early installations to still be performing satisfactorily 60 years later! There are thousands upon thousands of installations completed in the fifties, sixties and seventies that have provided decades of trouble-free service and continue to function satisfactorily.
Copper’s corrosion resistance is related to its ability to form a uniform, adherent, protective oxide film in contact with most waters. However, there are instances where the protective film may not form, or it may be damaged or disrupted, and corrosion may occur. These instances are exceptionally rare when one considers the many millions of feet of copper tube that are in service in Canada, as well as North America, Europe, and other regions.
Cuprosolvency may occur in soft waters, with low hardness and low alkalinity, and a pH of 7 or lower. It may cause a blue/ green water colour and staining of plumbing fittings or laundry. The general dissolution of copper tube associated with cuprosolvency is a very slow process which thins the tube but does not usually result in failure of the wall of the tube.
Cold Water Pitting is associated with well or other ground waters containing free carbon dioxide in conjunction with dissolved oxygen. Such waters are generally referred to as being aggressive. Pits develop from the inside of the tube, and typically they have a blue/green tubercle or hollow mound of corrosion products over the pit. Cold water pitting can be mitigated by treatment of the water to eliminate its aggressiveness. A variety of water treatment methods are available.
Flux Corrosion is another form of pitting which is attributable to the use of aggressive soldering fluxes and poor workmanship. If too much flux is used or insufficient heat is applied during soldering, a waxy petrolatum residue may remain in the tube. The corrosive residues may eventually result in pitting. It is important to emphasize again that flux corrosion does not occur often when one considers the millions of solder joints that are made annually.
Erosion Corrosion is caused by excessive localized water velocity and/or turbulence. Affected areas are typically free from the protective oxide film and corrosion products, and may be bright and shiny with horseshoe-shaped pits present. Failure to deburr the inside edge of a tube after cutting is one of the most common causes of turbulence in a system. Another cause is too many abrupt changes in direction.
Hot Water Recirculating Systems require special mention. Excessive velocity in such systems is a common cause of erosion corrosion and failure. Installations which use small sizes of tube or too large pumps result in higher than recommended flow rates. Other factors such as system design, installation workmanship, operating temperature and water chemistry must also be taken into consideration.
Galvanic, or Dissimilar Metal, Corrosion of copper and copper alloys is exceptionally rare. Incidents often attributed to galvanic corrosion are usually erroneous. In the galvanic series of metals, copper is one of the most noble metals. This means that copper is the most corrosion resistant. In other words, when copper is in contact with iron, steel or aluminum in water distribution systems, for example, the copper does not corrode; the other metal will eventually fail if the conditions for galvanic corrosion are present. This situation can be prevented by using a dielectric fitting between the copper and the less noble metal. It should be added that electrolysis should not be confused with galvanic corrosion.
Underground Copper lines are renowned for their excellent performance in a wide variety of soil conditions. Copper does not corrode in most clays, chalks, loams, sands, and gravels.
There are a few aggressive soil conditions that may result in corrosion when moisture is present. Cinder fill containing sulphur is one example. In such conditions, the tube should be insulated from the cinders by a layer of sand mixed with lime, or a layer of limestone, or by wrapping with moisture-prooftape.
Concrete is often thought to cause corrosion of copper, but this is a misconception. Copper is unaffected by Portland cements which provide an alkaline environment. However, non-alkaline cements containing sulphurous ash or other inorganic acids should be avoided, as should foamed concretes which employ ammonia-containing foaming agents.
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