Here, Stewart Jones, Customer Technical Advisory Engineer, Conveyance Tubes, at Tata Steel, looks at why carbon steel can provide a more robust solution than stainless steel, particularly within mixed metal building services applications – and why it is important for specifiers, fabricators and installers to fully consider a metal’s individual characteristics in order to make an informed product decision.

When 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 – as well as the other associated HVAC products – for they all play a vital role in ensuring a system’s long-term performance.

Over recent years, it is evident that 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 due to the latter’s 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. In comparison, carbon steel is less aesthetically pleasing and, whilst its higher carbon content makes it both more malleable and durable, it does not have the same reputation for corrosion resistance as its stainless alternative.

However, it is vitally important to note that stainless steel’s aesthetic appearance and corrosion performance does not automatically make it more suitable for use within commercial building service systems; especially as, despite the widely-held belief that the material does not corrode, this is not necessarily correct.

There are in fact some applications in which stainless steel can be more prone to corrosion than other alternative materials, such as carbon steel. For example, people are often unaware that, 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. Not only does this affect the stainless-steel pipes themselves, but it can also result in increased levels of degradation to the other metal materials in the pipework system, including more expensive metal components – a process called galvanic corrosion. In turn, this could impact on a system’s overall service life and result in the requirement of costly repairs and maintenance.

There is also the potential for other corrosion mechanisms to impact on a project, especially in the event of there being a conflicting understanding of preventative practices. For example, within traditional building services systems, installers will often try to prevent high system oxygen levels in order to control corrosion risks. However, stainless steel’s protective layer, a result of its high chromium content, requires a constant supply of oxygen in order for it to maintain its corrosion resistance on the metal’s surface. Unfortunately, while having 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, potentially causing accelerated rates of corrosion.

Even in an open system, stainless is still at risk, as in very tight crevices, or within certain applications where debris may act as a barrier, it can be increasingly difficult for oxygen to reach the stainless-steel surface; thereby reducing its protection and making it vulnerable to attack.

What’s more, due to stainless steel’s high tendency to react when in close proximity 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 would require specialist storage, completely separated from other metal materials. This could prove especially difficult on urban developments, where sites are often highly restricted in terms of available storage space. There would also be the need for careful handling and dedicated tooling for fabrication and welding, all of which can result in expensive time delays and additional labour costs.

It is evident, therefore, that stainless steel is not always as robust as people may believe, with the potential for severe consequences should it be specified in an application to which it is un-suited. In order to support the industry and improve awareness of such issues, including the complexity of galvanic and mixed metal corrosion, 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.

It is clearly vital that specifiers and installers educate themselves on the individual characteristics of different metals in order 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 still be a more appropriate choice.

Not only does it typically have thicker walls, making it more robust and better able to accommodate any initial corrosion until the system stabilises, but it also has no Heat Affected Zone (HAZ) – an area of weakness around the weld seam – as is frequently seen within imported cold-formed carbon steel alternatives. 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 to stainless steel.

Another concern on building service projects is of course the monetary budget, with specifiers and installers generally under pressure to deliver the optimum solution for the best price. In these situations, and particularly on larger projects where a greater volume of steel is required, hot finished carbon steel tubes, with its comparatively lower price and ease of handling on site, may often be the soundest choice.

As demonstrated, while carbon steel as a stand-alone 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. With its robustness, reduced risk of contamination or accelerated system corrosion, in addition to being more malleable, durable and affordable; its use within building services applications should never be overlooked.