Stainless claddings can have a very low corrosion rate and remain functional over the life of the structure. But each stainless steel grade is a little different, so specify and use them appropriately.
WHEN CARBON STEEL (such as mild steel) is exposed to a corrosive environment, such as a humid atmosphere, it forms an iron oxide on the surface, commonly called rust. This iron oxide film is not protective.
Many types of stainless steel
Adding at least 11–13% chromium to carbon steel changes the nature of the surface oxide – a continuous chromium-rich oxide film forms that protects the metal from corrosion. These are stainless steels.
The formability and weldability of stainless steel can be improved by adding nickel, such as in the 300 series nickel-chromium stainless steels. The most common is type 304.
Adding molybdenum improves resistance to corrosion, useful in marine atmospheres or when air pollution is present. The more expensive type 316 stainless steel contains at least 2% molybdenum, distinguishing it from type 304, which has no added molybdenum.
Nickel is expensive, so manufacturers use other alloying elements to replace some of it. Several stainless steels, sometimes designated as 200 series, use combinations of nickel, manganese, copper and nitrogen to duplicate some of the properties of types 304 and 316.
These 200 series stainless steels are not inferior to 300 series grades, but they are different and do not appear to have sufficient corrosion resistance in exterior use in the humid, polluted and marine atmospheres typically found in urban New Zealand.
If you are offered a cheaper stainless steel, ask the supplier for the material or heat certificate for the alloy to make sure it is the grade you want.
Steps to successful performance
The performance of a stainless steel cladding depends on a combination of interrelated factors, so it’s essential to consider each at an early stage of the project.
Contamination in the environment causes corrosion. Chlorides from windborne sea salt are the most common aggressive contaminant in New Zealand, but air contaminants from industry and traffic can also accelerate corrosion.
Areas of steel and stainless steel not washed by rain are particularly prone to corrosion by these contaminants. If unwashed areas can’t be avoided in the design, use a more corrosion-resistant stainless steel or plan for regular cleaning.
Use simple shapes that do not collect dirt or moisture and avoid ponding on the surface. Avoid crevices where water and contamination can collect – the protective film relies on oxygen to keep it stable, and anything that can interfere with this can enhance corrosion.
Ideally, provide a washing system or other means of access for cleaning exterior stainless steel cladding.
Beware of putting dissimilar metals in contact, as this can cause galvanic corrosion. Stainless steel is noble relative to most other common building metals, so other metals can corrode when in contact with it.
Ensure different metals are not in contact or that the surface area of stainless steel is small compared with the surface area of the other metal. For this reason, rapid fastener corrosion can occur if fasteners made of aluminium or galvanised steel are used to fix stainless steel.
Stainless steel grade selection
Select the right stainless steel for the specific engineering and construction purpose (see Table 1). Most engineering applications in New Zealand use types 304 or 316 stainless steel.
Type 304 is suitable for most indoor applications, except industrial buildings, and for a range of less demanding external applications. It is the most commonly used grade.
Type 316 is generally chosen where the external environment is more corrosive. For severe marine environments and areas suffering from heavy industrial pollution even type 316 may not be satisfactory, and it may be necessary to use a more highly alloyed stainless steel such as type 904L, duplex grade 2205 or even a superaustenitic 6% Mo grade. If this is the case, seek specialist advice.
Generally, the smoother the surface of the stainless steel, the better its corrosion performance. It’s more difficult for contamination to stick and build up on smooth surfaces and easier for rainwater or manual cleaning to remove any deposits. Bright, smooth surfaces such as 2B, BA and mirror polish have better corrosion performance than rougher polished finishes, such as a No.4 finish, but are quite reflective.
Sometimes it is preferable to use a textured sheet that will have lower reflectivity, still with a smooth finish such as 2B or BA to enhance cleanability.
Fabrication and installation
The fabrication of stainless steel is not difficult, but it is different from mild steel. Protect the surface of stainless steel at all stages of transportation, storage and installation, and after installation, make sure the surface is thoroughly cleaned.
Any plastic film used to protect the surface of the stainless steel should be removed once installed and all adhesive residues cleaned from the surface.
It is very important to ensure that no particles of iron – steel – are embedded in the surface of the stainless steel. This may occur:
- when stainless steel is cut using a shear blade that has been used on carbon steel
- when stainless steel is ground using a grinding wheel that has been used on carbon steel and is therefore contaminated with steel particles
- when fabrications come into contact with steel chains or wire slings
- if weld spatter or grinding particles from site work deposit on to the surface of the stainless steel.
It is also important to remove the heat tint – discolouration – produced during stainless steel welding since the layer immediately beneath the discolouration is depleted in chromium and therefore has inferior corrosion resistance. This can be done chemically by pickling or electropolishing, or mechanically, by fine grinding.
Some maintenance needed
Stainless steel is a low-maintenance material, but it is not maintenance-free. External stainless steel should be cleaned as frequently as window glass and in a similar fashion – with detergent or ammonia in water, washed off with clean water.
The cleaning frequency depends on the nature of the finish, the severity of the environment and how often the surface is washed by rain. A particularly bad situation is when there is regular fog but little rain, and deposits form on the surface. Surfaces can be damp for significant periods, creating significantly elevated corrosion rates.
Visit the NZ Stainless Steel Development Association at www.nzssda.org.nz.
Articles are correct at the time of publication but may have since become outdated.