PINHOLE CORROSION IN METAL PIPEWORK
Corrosion is a well-known problem that occurs in (metallic) water pipework due to the presence of oxygen. However, basic errors at the setup stages can sometimes lead to an increase in the incidence of corrosion.
- WHAT IS METAL CORROSION?
In metal corrosion, a refined metal reacts with oxygen that is dissolved in water, to form reactants such as oxides, hydroxides, sulphides. If the oxygen is readily replenished (through leaks in the system requiring regular top ups) then the material will continue to corrode until it fails.
There are different types of corrosion which include:
- Pitting Corrosion This is a form of localised corrosion that results in pin-holing of the pipe and failure may occur within months of the installation. There are different types of pitting corrosion which occur in a range of conditions and these will vary with temperature, pH, pressure, and velocity.
- Erosion-Induced Corrosion This type of corrosion usually occurs in larger buildings with recirculating hot water systems such as hotels etc. rather than domestic houses. It tends to occur at sites within the system where the flow pattern changes i.e. bends, reducers and pumps etc. Causes include the turbulent flow of water which releases air bubbles and removes the protective patina layer formed on the pipework and lead to corrosion. Another possible cause is due to significant amount of sediment and undissolved solids being transported through the system, which erode away the metal.
- Microbiologically-Induced Corrosion (MIC) This type of corrosion is caused by bacterial microbes in pipework growing due to the presence of certain conditions: metals, nutrients, water and oxygen. Microbial growth leads to pitting corrosion unless these conditions are limited which damages the pipes.
- SYSTEM SETUP
Pre-commissioning and commissioning are critical to bring the system into the correct operational condition. This involves, flushing out the pipework, disinfection and carrying out biological tests and measuring flow rates. Pressure tests (typically 1.5 times normal working pressure) are also conducted in order to test for leaks because the presence of a leak can allow the oxygen in the system to be replenished, promoting further corrosion, and require the system water level to be topped up. If any water treatment is required, this is also carried out at this stage.
- ISSUES AT SETUP LEADING TO CORROSION
There are various different issues that can arise at the setup stages which can increase the likelihood of corrosion occurring. These can include: limited time, lack of documentation and inexperienced operatives.
The errors that can be caused are:
- Safety pressure devices not being set up correctly leading to high pressure in the pipework which can damage the oxide protective layer (patina).
- If the Flow Velocity of water is not checked at commissioning and it exceeds the critical flow velocity, this causes turbulent flow which can lead to damage of the patina layer.
Examples:
