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However, many of the things engineers think they know about stainless steel aren’t true.
Unfortunately, the term “stainless steel” is responsible for another myth: the belief that stainless steel doesn’t rust.
(In languages other than English, the confusion can be even worse; for example, in Spanish, stainless steel is called
“acero inoxidable” -- translation: unrustable steel.)
Many times, I’ve heard people complain that their supplier has swindled them if a stainless part shows signs of rusting. In fact, while stainless steels are corrosion-resistant, they are not immune to corrosion. The term used in the US military -- “corrosion-resistant steel,” or CRES for short -- is much more accurate, even though it may not roll off the tongue as easily.
In order to understand why stainless steels can rust, it’s important to understand what makes them corrosion-resistant
in the first place. Stainless steels are iron alloys that contain a minimum of 10.5% chromium.
The chromium reacts with oxygen in the atmosphere to form a thin, protective layer of chromium oxides.
This passive oxide layer protects the steel from corrosion.
However, certain chemical species, such as chloride ions, can attack and break down this passive layer.
This can happen in salt water, for example, since salt is sodium chloride. Once the passive layer is gone,
the steel can rust, just like any other steel. This is called “pitting corrosion,”
since it typically occurs in localized pits.
Since the passive layer requires oxygen to form, it can also break down in areas that are depleted of oxygen.
For example, this can take place beneath gaskets, under bolt heads, or in screw threads.
Corrosion occurs rapidly in these oxygen-starved areas. This is called “crevice corrosion.”
Another form of corrosion can occur when the chromium in the steel combines with carbon in the steel to form carbides.
When this happens, the chromium is no longer available to form a protective oxide layer.
Carbides can form when the steel is heated to high temperatures; for example, in the heat-affected-zone of a weld.
The carbides tend to form along the grain boundaries of the material, so the corrosion follows the grain boundaries.
Therefore, this phenomenon is called “intergranular corrosion.”
In the specific case of welds, it is also called “sensitization” or “weld decay.” This form of corrosion can be minimized
by using stainless steels with low carbon contents. These alloys are identified with the letter L:
for example, 304L and 316L.
This, by the way, busts another myth: the idea that all stainless steels are created equal. In fact,
different stainless alloys have different levels of corrosion resistance.
Martensitic (400-series) stainless steels are very strong, but are not as corrosion-resistant as austenitic (300-series)
stainless steels. Even among austenitic stainless steels, corrosion resistance varies.
Let’s consider three austenitic stainless alloys: 303, 304, and 316. Type 304 is the standard grade.
It consists of approximately 18% chromium and 8% nickel; the rest is iron. It’s widely used, and has fairly good
corrosion resistance.
Type 303 has the same amount of chromium and nickel as type 304; however, it also has some sulfur added.
The added sulfur increases the machinability, but decreases the corrosion resistance.
Type 316 also has the same amount of chromium and nickel, but also has 2% to 3% molybdenum.
The molybdenum addition increases the corrosion resistance.
Finally, it’s a myth that stainless steels are non-magnetic.
Some martensitic stainless steels are equally as magnetic as carbon steel. Austenitic stainless steels tend to
have lower magnetic permeabilities; in some cases, so low that they are not attracted to a handheld magnet.
However, cold-working operations such as stamping, forging, wiredrawing, and heading can increase the magnetic
permeability of these materials.
It’s common for 300-series stainless-steel fasteners to be at least slightly magnetic. So using a magnet to test
whether steel is stainless or not is unreliable, at best.
Most of us don’t consider stainless steel to be an exotic material. Unlike the latest cutting-edge biomimetic materials,
nanocomposites, or quantum dots, it’s a material we think we know something about. However, as with any other material,
when designing a product with stainless steel, make sure the things you know are actually true!
