How do I avoid corrosion of copper pipes?

Concerned Plumber, Bridlington

Enough of the theory for now – here are some practical tips for avoiding, or at least reducing the risk of, corrosion in drinking water systems (known as domestic water systems).

  • Do not allow water to stagnate in the system for a long period after installation. Regular renewal with fresh aerated water is required to form protective patinas or corrosion product layers on copper and other metals in the system.
  • Do not drain down the system after pressure testing. It will be impossible to remove all the water and localised attack will quickly develop at the three phase boundary: metal/water/air.
  • Reduce as much as possible dead-legs in the system. These can lead to stagnant water conditions in use, which favours the development of pitting corrosion.
  • Only use copper pipe to EN 1057 with the BS kitemark. Copper pipe not complying to this standard may still have carbon film residues on the inside from the extrusion process, which can lead to type I pitting in hard water areas.
  • Do not used galvanised pipe in hot water systems and never use downstream of copper.
  • Design a system so that water flows in copper pipework are in the range 0.5-1m/s in hot water and 0.5-2m/s in cold water. Too low water flows allow debris to settle out and may lead to under-deposit corrosion. Flow rates above these values may induce erosion corrosion on copper.
  • For larger systems, if there is a risk of deposits being brought into the system, i.e. from surface upland waters, fit a water filter to the incoming mains.
  • Deburr the cut ends of copper pipe and do not have large changes in section at joints. This will reduce the risk of turbulent flows immediately downstream of the joint and hence the likelihood of erosion corrosion.
  • Do not use excessive soldering fluxes and wipe any excess from the outside of the pipe after soldering. Flux residues left after soldering may lead to pinholing of pipework several months later.
  • Lag pipework, especially hot water pipes, not only to save energy but also to keep water temperatures in the pipes outside the range 25-50°C for any length of time. At these temperatures, micro-organisms can flourish which increases the risk of legionella and also later of microbial induced pitting corrosion.

For systems with an open header tank, always make sure that the lid is securely fitted. This will prevent insects and even birds ending up in the tank stimulating microbial growth.

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