Stainless steel, like all types of steel, is not a single metal but an alloy, a material made from two or more separate elements alloyed or melted together. What all steels have in common is that their major ingredient or alloying element is the metal iron (Fe), to which a small amount of carbon (C) has been added 12. Stainless steel is an iron alloy that has a minimum chromium (Cr) content of 10.5% 3.

Characteristic of Stainless Steel

Stainless steel is often specified for its corrosion resistance, which is directly correlated to its chromium content. The chromium in stainless steel reacts with oxygen in the atmosphere to form a thin layer of chromium oxide, an insoluble film that is instantaneously formed on the surface. This natural chemical reaction forms a passive layer which acts to protect the material and has the ability to self-repair. Whereas steel, with no chromium content forms a layer of iron oxide, or rust, when exposed to air or water. See illustration below 4.

Protective Passive Layer on Stainless Steel

Molybdenum (Mo) can be added to the alloy to further increase corrosion resistance. Grades of stainless steel are characterized by the composition of alloying elements. The higher percentage of chromium and molybdenum elements the higher the corrosion resistance of the alloy.

Selecting the wrong alloy for your project could lead to premature corrosion and even structural failure. Learn more about stainless steel categories and the breakdown of alloy composition and elements in the complete Metal Usage Guide(pdf).

The naturally occurring passive layer of stainless steel may be attacked by the presence of corrosive material or if there is insufficient oxygen available to the surface disabling its selfrepairing nature. Pitting and crevice corrosion are the most common attacks on stainless steel.

Pitting and crevice corrosion

Pitting is usually characterized by very tiny dark brown pits on the surface. Crevice corrosion is essentially the same but on a larger scale and is usually caused by contact with a non-stainless steel material, like a washer, that creates a crevice on the surface. Pits or crevices form pockets in the surface of the material where oxygen cannot circulate adequately for the protective passive layer to reform and disrupts the micro structure of the metal.

Atmospheric substances, like air and water, and chlorides in airborne sea spray, rain, and dry salt particles carried by wind, may cause pitting and rusting of stainless steels. Earth’s natural environment of oxygen and condensed water vapor is itself sufficient to cause gradual corrosion of iron and steel surfaces through the creation of iron oxide unless a sufficiently corrosion resistant grade is chosen.

Corrosion alters the micro structure of the metal and its passive layer and drastically reduces the mechanical strength and useful life of the metal. This is why all stainless steel requires some level of maintenance depending on the alloy and where it is installed 6.

Understanding the environment and site conditions and then choosing the right alloy is critical to the success of your project.

Environmental and Atmospheric Conditions

The chlorides in rainwater are primarily marine salts carried inland by weather patterns. Generally, locations within 10 to 20 km of salt water are considered at risk for chloride-related corrosion, but local weather patterns and the performance of metals near the site should be evaluated prior to material selection. In some locations salt could be carried much further inland. The MAP shows the average chloride concentration in rainfall across North America, milligrams per liter (mg/L) 8.

Microclimates

In addition to known environmental factors, local site-specific microclimates and potential changes to the environment should be considered before a final stainless steel grade selection is made.

A microclimate is a local set of atmospheric conditions restricted to a very small area that differs from those in the surrounding area 9. Areas particularly impacted by these microclimates are coastal locations or near chemical plant chimneys 10, where unexpected acid condensates can form.

In contrast, factors such as low temperatures and low humidity reduce the risks of corrosion and can mean that a steel grade perhaps not thought suitable for a particular site may be worth considering.

Architectural sites are categorized as the following11:

• Urban: Residential, commercial or light industrial areas with low to moderate pollution from vehicular traffic and similar sources.
• Industrial: Locations with moderate to heavy atmospheric pollution usually in the form of sulphur and nitrogen oxides, a bi-product of burning coal, oil, diesel and natural gas from power plants.
• Rural and Suburban: Unpolluted, inland sites away from industrial atmospheric discharges or coastal or de-icing salts. Areas with low population densities and light, non-polluting industry may also be categorized as rural.
• Marine/Deicing Salt: Areas where windborne sea spray and dry salt particles containing concentrated chlorides are present.

After classifying a site, it is important to consider any changes that may occur during the design life of a proposed building or project. For example, will the environment get more polluted or cleaner over time? These considerations can have impactful implications and could potentially change the way in which a site should be classified.