Stainless steel has long been a preferred material of pipe makers, and its unique anti-corrosive properties make it especially ideal for use with compressed air systems. Air Energy’s reputation for excellence in quality and service has made us one of the leading suppliers of stainless-steel pipe Australia-wide.
But despite its strength and durability, stainless steel is not totally immune from corrosive damage. In this blog, we take a look at some of the different kinds of corrosion that can affect stainless steel, what typically causes them, and what you can do to prevent them.
A more accurate name for this material would be “highly stain-resistant steel,” but we realise it’s unlikely to catch on. Stainless steel was invented in 1913, when an English metallurgist discovered that he could increase the stain corrosion-resisting properties of low carbon steel by adding at least 12% chromium. The chromium combines with oxygen to create an invisible layer of chrome oxide on the surface of the steel that is called “passive film.” When this layer is scratched, new oxide molecules quickly form to cover the damaged area, effectively making stainless steel a self-repairing metal.
If the damage is severe enough or the atmospheric conditions are poor, the passive film might not be able to regenerate, and corrosion can occur. Of the various kinds of corrosion that can affect stainless steel, pitting is of greatest concern, as it can compromise the structure of the steel. Pitting in stainless steel is caused by the presence of chlorine ions and exacerbated by continuous wetting. This typically occurs on stainless steel pipes that are directly exposed to the sea and is rarely observed inland or that are repeatedly exposed to chlorine based cleaners,
Tea-staining is a brownish discolouration on the surface of stainless steel. This is a cosmetic issue that does not damage the integrity of the steel and generally does not impact its lifetime. Like pitting, it is caused by corrosive particles in the atmosphere, and it is especially common when the steel is in close proximity to salt water and in areas of high humidity.
The best way of avoiding corrosion is to use the material that is best suited to the given application and setting. For instance, high-quality stainless-steel pipes such as Europress 316L have the best corrosion resistance for most industry applications. The main difference between 316 and 316L is the amount of carbon that is in the material -316 has .08 Max carbon content while 316L has a .03 Max carbon content. The primary reason for using an L grade of stainless steel is that this will reduce the tendency of the material to crack after welding.
Eurotubi Pressfittings AISI 316 are suitable for use in environments with low salinity, while AISI 316L pipes work well for areas of moderate salinity. The lower carbon content of 316L means the chromium molecules have less opportunity to bond with the carbon and form chromium carbides, leaving more chromium to create the protective passive layer.
While high-quality stainless steel such as 316 or 316L works for low to medium salinity environments, it is not suitable for transportation of saltwater or sea water. For these applications, Cupronickel press fittings are the best choice, as nickel is highly resistant to the kind of crevice corrosion that is frequently seen in high-salinity environments.
Natural salt water is the most common culprit for steel corrosion, but you should also be aware of the other chemicals that come into contact with your pipes. For instance, one customer of ours had an issue with pitting caused by a chlorine-based wash-down disinfectant drying on hot stainless-steel pipes that were heated by the hot water they were conveying.
304 is a cheaper grade and is corrosion resistant but not as effective as 316 grades, it is ideal for fuel & oil piping. 304 is widely used for kitchen equipment, but 316 grades are recommended for harsher industrial environments.
At Air Energy, we have built a reputation for consistently providing our customers with excellent products at competitive prices. We are committed to a policy of continual improvement, and we are always on the lookout for the latest industry innovations so we can provide you with cutting-edge technology from around the world. To speak to one of our friendly team members today, head to the contact section of our website and get in touch by phone or email.