VOICES | STEWART JONES Hard choice In mixed metal HVAC applications, its important to understand when carbon steel is more appropriate than stainless steel, says Tata Steels Stewart Jones W hen specifying or installing building service pipework, particularly within heating, ventilation and air conditioning (HVAC) systems, it is essential that equal consideration is given to the selection of the pipework material. Over recent years, there has been a change within the building services market, with specifiers and installers moving away from carbon steel pipes to those manufactured using stainless steel possibly because of the latters reputation for improved corrosion resistance. Stainless steel has a high chromium content, creating an invisible protective layer on the metal which, in turn, prevents staining and corrosion. That said, carbon steel is less aesthetically pleasing and, while its higher carbon content makes it more malleable and durable, it does not have the same reputation for corrosion resistance as its stainless alternative. However, it is vital to note that stainless steels aesthetic appearance and corrosion performance does not automatically make it more suitable for use within commercial building service systems especially as the widely held belief that the material does not corrode is not necessarily correct. There are some applications in which stainless steel can be more prone to corrosion than alternatives such as carbon steel. For example, when brought into contact with dissimilar metals particularly within a damp environment (such as that encountered during storage, transportation, installation, post-commissioning and in-service) stainless steel can become contaminated. This affects the stainless-steel pipes and can degrade other metal materials in the pipework system, including more expensive metal components a process called galvanic corrosion. In turn, this could impact on a systems overall service life and result in costly repairs. Stainless steel has a high tendency to react when close to metals Corrosion risks There is also the potential for other corrosion mechanisms to impact on a project, especially if there is a conflicting understanding of preventative practices. For example, installers may try to prevent high system oxygen levels in order to control corrosion risks. However, stainless steels protective layer a result of its high chromium content requires a constant supply of oxygen to maintain corrosion-resistance on the metals surface. Unfortunately, while an oxygen-rich or open system would be beneficial to stainless steel, it could have a negative effect if there are mixed metals for example, non-stainless fittings or valves within the same pipework system. This could potentially cause accelerated rates of corrosion. Also, stainless steel has a high tendency to react when close to other metals. This can lead to further complications and practical issues on site, even prior to installation. For example, to avoid contamination, stainless steel requires specialist storage, completely separated from other metal materials, which can be problematic if space is restricted. Careful handling and dedicated tooling for fabrication and welding may be needed, all of which can result in expensive time delays and extra labour costs. Making informed choices In order to support the industry and improve awareness of such issues, Tata Steel has been working with the Building and Industrial Services Pipework Academy (Bispa) a collaboration between industry and academic centres of excellence, such as the School of Architecture, Building and Civil Engineering, Loughborough University, and the Corrosion Group at Cranfield University. Specifiers and installers should educate themselves on the characteristics of different metals to make informed product choices and help ensure the long-term satisfactory performance of their building services pipework. For example, for HVAC pipework within a building, traditional hot-finished carbon steel may be more appropriate. Not only does it typically have thicker walls, making it more robust and better able to accommodate initial corrosion, it also has no heat affected zone (HAZ) an area of weakness around the weld seam. With its increased strength and durability, as well as its positioning within the galvanic series, carbon steel may be considered more complementary to other dissimilar metals in turn reducing the risk of serious or accelerated corrosion occurring. It is also less prone to contamination during storage, fabrication and installation, and is far more malleable and ductile when compared with stainless steel. Budget will always be a concern, and a hot-finished carbon steel tube, with its comparatively lower price and ease of handling on site, may often be the soundest choice. While carbon steel as a standalone metal is perhaps not as resistant to corrosion as its stainless-steel alternative, it is more complementary and therefore more suitable for installation alongside other materials commonly seen within building services systems. STEWART JONES is customer technical advisory engineer, conveyance tubes at Tata Steel www.cibsejournal.com October 2019 69 CIBSE Oct19 pp69 Stainless steel.indd 69 20/09/2019 16:21