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Stainless Steel Does “Rust” – Sometimes!

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One of the common misconceptions about stainless steel is that it does not corrode when exposed to water or the elements – that it’s rustproof. The term “rustproof” as commonly used implies that the metal can be safely and effectively applied in wet, damp or otherwise harsh environments without risk of corrosion or long-term integrity failure. Unfortunately, it isn’t that simple!

For the most part, stainless steel does not readily corrode, rust or stain when exposed to water the way ordinary steel does. However, it is not fully corrosion resistant in high saline or poor air circulation environments. There are certain types of stainless steel corrosion that can occur in spite of the metal’s highly favorable properties.

Galvanic Corrosion occurs when there are two metals with different corrosion potentials present and there is direct metal-to-metal electrical contact. A conductive solution such as water connects the two metals at regular galvanic-corrosionintervals such as happens when there is condensation, rain, fog or other moisture sources making contact. As an example, bolts and other connectors of different metals applied directly on stainless steel in a regularly wet environment can cause both metals to corrode and even fail at the connection point.

Crevice Corrosion occurs mostly in seawater applications due to the low PH of salt water. Chlorides create pits where the low PH saltwater attacks the passivated surface of the exposed metal. Lacking sufficient oxygen to re-passivate the area, significant corrosion can take place.

General Corrosion of stainless steel occurs when there’s widespread breakdown of the protective passive film allowing the entire surface to experience active corrosion. This usually requires an aggressive environment such as acids or bases to produce this surface breakdown. The corrosion rate usually increases with higher temperatures and chemical flow rates. General corrosion of stainless steel most often occurs when using an alloy with insufficient resistance for its environment.

Various grades and surface finishes of stainless steel are commonly made available to suit the specific environment in which the metal is most often used. Design architects and construction contractors need to understand the different grades and surface finishes of stainless steel and in what unique circumstances each should be specified.

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The fundamental properties of the most common stainless steel alloys are specified by an industry recognized numbering system. The Specialty Steel Industry of North America (SSINA) provides some handy guidelines for choosing the appropriate grade:

  • 303 stainless offers improved machinability while still being corrosion resistant. Sulphur is added to the steel composition to make it highly machineable. However, the sulphur presence lowers the alloy’s corrosion resistance in certain harsh environments.
  • 304 is the common “18-8” alloy (18% chromium, 8% nickel) which is the most readily available grade, and most often specified for “all purpose” applications. It has excellent corrosion resistance and very good formability.
  • 316 is basically a 304 grade with the addition of 2 to 3% molybdenum. It has extremely high corrosion resistance and is usually preferred for long-term application in aggressive industrial, chemical and marine environments.

architectural-stainless-steelThere are several other specialty grades of stainless steel alloys that are formulated for highly specialized and precision applications, but for the architect, 303, 304 and 316 are the grades suited for most structural work. The exposed surface finish desired will also factor into which grade should be chosen. For example, if intricate machining is required and the surface will be colored or textured, specifying a 303 grade and powder coating the entire surface may be the best bet.

If you have questions about custom fabricating with stainless steel or any other alloys, feel free to call us at 603-402-3022.