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For many years, copper tubing has been used to distribute water to buildings around the world. The main reason for this is that the metal retains its stability, its resistance to the environment and forms insoluble corrosion that insulates it after it comes into contact with natural water. This means that it can take up to 280 years for the tubing to wear completely through. Despite this reliability, however, contact with some cold hard waters may actually cause pitting in the copper, which can cause it to fail within 2 years. Hard, pale green nodules of copper sulfate and copper hydroxide salts usually cover the pitting that forms in copper tubing. It is only once these nodules are removed that the pitting is revealed; the hemispherical holes are often filled with coarse crystals of red cuprous oxide and green cuprous chloride. This kind of pitting is referred to as Type 1 pitting and is said to be caused by water that is cold, less than 30 degrees Celsius, hard (or moderately hard), organically pure and possess a specific chemistry. Even though there are many waters that are considered to possess the capacity of Type 1 pitting, no problems will arise unless a pit is initiated in the wall of the tubing. When the copper is first exposed to hard water, salt deposits on the wall and the metal itself will react with the water, producing a thin protective layer that is comprised of mixed corrosion and hardness scale. For pitting to occur, this layer must be disrupted; unfortunately, there are three common ways in which this disruption can occur. • Carbon Films – Tubing is created from large billets of copper that are gradually worked and drawn until they reach the desired size. The organic oils and greases that are used to lubricate the metal break down during heat treatment, which coats it in a film of carbon. If the carbon is left inside the tubing it can disrupt the formation of the protective scale discussed above. • Stagnation – If water is left standing inside the tubing for an extended period of time, its chemical characteristics can change as the mixed scale and corrosion are deposited. Particles of scale that have not adhered to the walls and have not been washed away can settle on the bottom and produce coarse, porous deposits. Air pockets that develop will disrupt the formation of protective scales on the sides and top. • Flux – This is used in plumbing systems to keep the ‘mating’ surfaces clean during soldering. These fluxes often consist of corrosive chemicals (such as ammonium chloride and zinc chloride). If too much flux is applied to the joint the excess will melt down and pool in the copper tubing. Whilst this can protect the metal from corrosion, the edges are often subjected to pitting. It is believed that Type1 pitting can be avoided in many applications through good work practices. This means that you should only use copper tubing that has been manufactured to the correct standards (BS EN 1057), that a system should either be put immediately into service or drained and dried and that flux should be used sparingly during any soldering tasks. This will help you to avoid the three common causes of pitting – carbon films, stagnation and flux – without compromising the project.
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