TB 036 - pinhole-corrosion-in-pipework

TECHNICAL BULLETINPINHOLE CORROSION IN METAL PIPEWORKCorrosion 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.1. WHAT IS METAL CORROSION?In metal corrosion, a refined metal reacts with oxygen that is dissolved inwater, to form reactants such as oxides, hydroxides, sulphides. If the oxygenis 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 Figure 1 Pinhole Corrosion in PipeworkThis 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, velocity, carbon film and phosphates i  Erosion-Induced CorrosionThis type of corrosion usually occurs in larger buildings with recirculating hot water systems such as hotels etc. rather than domestichouses. It tends to occur at sites within the system where the flow pattern changes i.e. bends, reducers and pumps etc. Causesinclude the turbulent flow of water which releases air bubbles and removes the protective patina layer formed on the pipeworkand lead to corrosion. Another possible cause is due to significant amount of sediment and undissolved solids being transportedthrough 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 thepipes.2. SYSTEM SETUPPre-commissioning and commissioning are critical to bring the system into the correct operational condition. This involves, flushingout the pipework, disinfection and carrying out biological tests and measuring flow rates. Pressure tests (typically 1.5 times normalworking pressure) are also conducted in order to test for leaks because the presence of a leak can allow the oxygen in the systemto be replenished, promoting further corrosion, and require the system water level to be topped up. If any water treatment isrequired, this is also carried out at this stage. Guidance on commissioning water systems can be found in BG2/2010ii and guidanceon flushing and disinfection water service in a domestic environment can be found in PD 855468:2015. TB/036 1 of 3

PINHOLE CORROSION IN METAL PIPEWORKTB/0363. 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 - The critical flow velocity is <3m/s for cold water and <2m/s for hot water pipesiii - In base-exchanged softened water, critical flow velocity is even lower at <2m/s cold water and <1.2m/s for hot water. This is possibly due to the non-scaling nature of over-softened water which in theory has a higher risk of corrosion in copper alloys iv . The Langelier index can be used as an indicator for this risk by taking into consideration factors such as pH and temperaturev. A guide on how to calculate this can be found here.  Excess use of soldering flux is also a problem and the flux can run downstream and lead to flux induced corrosion within a few months. This can be reduced by using the correct flushing procedure.4. SOLUTIONIn order to reduce the risk of corrosion due to improper setup, certain steps can be taken to ensure commissioning occurs properly.These include: Preparing a pre-commissioning and commissioning plan involving all relevant parties who are involved with the work. This will help identify issues in early stages and ensure activities are completed correctly. Following BG29/2012 which highlights effective pre-commission cleaning of Pipework Systems vi and keeping records of measurements and test results for handover documentation and future reference. Flushing the system correctly according to BS EN 806-4 after installation to remove deposits and excess flux from the system. Flushing and disinfecting system according to PD 855468:2015. Setting the flow velocity below critical velocity which is <2m/s for cold water and <1.2m/s for hot watervii if possible. Otherwise, consider implementing regular pipework inspections.It is important to get pipework commissioned by a competent contractor with experience in pipework commissioning and watersoftening in order to limit the risk of corrosion. TB/036 2 of 3

PINHOLE CORROSION IN METAL PIPEWORKTB/036Note: this document is based on knowledge available at the time of publication and is meant for general purposes, not for reliance on in relation to specific technicalor legal issues, in which case you should always seek independent advice. No responsibility of any kind for any injury, death, loss, damage or delay however caused,resulting from the use of the advice and recommendations contained herein, is accepted by the authors or others involved in its publication (including the Building& Engineering Services Association). 21/08/2017Image taken from: http://www.flcorrosioncontrol.com/photo-gallery/i http://www.fwr.org/copper.pdfii https://www.bsria.co.uk/information-membership/bookshop/publication/commissioning-water-systems/iiihttp://resource.npl.co.uk/docs/science_technology/materials/life_management_of_materials/publications/online_guides/pdf/avoidance_of_corrosion_in_plumbing.pdfiv https://www.bsria.co.uk/information-membership/bookshop/publication/water-treatment-for-closed-heating-and-cooling-systems/v http://www.mae.gov.nl.ca/waterres/quality/drinkingwater/pdf/calculation_langelier_index.pdfvi https://www.bsria.co.uk/information-membership/bookshop/publication/pre-commission-cleaning-of-pipework-systems/viihttp://resource.npl.co.uk/docs/science_technology/materials/life_management_of_materials/publications/online_guides/pdf/avoidance_of_corrosion_in_plumbing.pdf TB/036 3 of 3