Carbon steel and Stainless Steel are two very popular grades of pipe used in the Mechanical Industry. Each type of material comes with its own particular set of benefits and disadvantages to the contractor. In this article, we look at the main considerations to take note of when selecting which system to use and the different fabrication factors that which you should be aware of.
Carbon Steel
When specifying any product, cost, longevity and sustainability are key issues. Carbon steel over the past decade has been increasingly used as a greener alternative to mild steel. Carbon steel is up to 50% lighter than traditional mild steel pipe. This means that not only is installation easier but as carbon steel uses fewer materials it produces less CO2 during manufacture. It’s usage also plays a significant part in the recycling of mild steel. Traditionally mild steel screwed pipework has been the fail-safe for applications like closed-circuit heating or chilled water.
Carbon steel with press-fitted connections is now widely used thanks to its corrosion resistance in closed heating and chilled water systems. Boasting enhanced eco credentials plus money-saving benefits, as long as it is installed correctly, carbon steel makes for a convincing alternative. One of the key benefits of carbon steel is its anti-corrosive nature in closed-circuit systems. However correct storage, handling, installation and commissioning is vital to illuminate any damage caused by corrosion.
However serious problems can develop when the manufacturers guidance is not followed.
There are 4 key points to take note of with Carbon steel
1) Keep it clean: Prior to fitting the piping must be protected from dirt and damage. Carbon-steel products should not be stored alongside stainless-steel items otherwise contact corrosion can occur.
2) Moisture Free: It is absolutely critical that the pipe is protected against moisture and the influence of weather. For chilled-water applications a vapour barrier must always be maintained in areas where condensation is likely to occur.
3) Oxygen free: Crucially, the probability of corrosion is increased if oxygen is present in the circuit; concentrations in excess of 0.1 g/m3 indicate a greater probability of corrosion. This can occurs through compression glands, screw connections or automatic air-valves, however there is little risk of corrosive damage from oxygen when filling and supplementing with water since the amount of oxygen is very low.
4) Correct commissioning: This may seem an obvious point however many contractors have fallen down because they haven’t followed the correct commissioning procedures. Installers Should not be tempted to test sections of pipe with water as they go along, draining them down in between. Residual oxygen and water in the pipework can begin the corrosion process, which once started is almost impossible to stop. The system can be purged with Nitrogen if left empty.
Stainless Steel
Stainless steel is a unique form of steel which does not corrode. It gets this property by employing chromium as a special hardening agent. With the use of chromium, stainless steel has the basic properties of steel with the significantly increased corrosion resistance. Like carbon steel, stainless steel also reacts to oxygen in the air to form an oxide layer. However rather than form an iron oxide layer it forms a chromium oxide layer. Iron oxide is a volatile molecule which continues to aid in the oxidation which is the problem we get on carbon steel. However chromium oxide is inert – it forms a protective layer that prevents further oxidation. For this reason it can be used in open systems and also applications requiring high levels of hygiene, such as the food and brewing industries.
The durability and flexibility of Stainless steel is key when deciding on a system. It can offer a much longer lifespan than carbon steel – up to 50 years due to its excellent anti-corrosion properties.
316 grade Stainless steel can also be used for potable water and vented systems so can be used as a replacement for copper, this is worth considering in project situations where theft of copper can be a problem, particularly on larger dimensions.
Unlike mild steel fabrications, stainless steel does not need to be inspected for paint deterioration, it does not require repainting, saving time and money on plant shut downs, removal of insulation etc. Stainless steel can also be used in in highly corrosive environments like coastal, off shore or underground locations making it a very versatile option on any project.
The downside to stainless steel is that it is higher cost than carbon steel, installation costs are similar due to the quick press-fit systems available today.
In areas where there is a raised likelihood of corrosion, then to minimise risks, stainless steel is an excellent alternative. It is also a cost effective alternative to copper on larger installations offering security and higher impacts.
