Stainless steel is a family of engineering materials that is widely used in industry. Its properties make it well suited for a wide range of applications, from surgical instruments to table knives, and from marine propellers to acid-resistant tubing in chemical plants, among many others. However, many of the things engineers think they know about stainless steel aren’t true.
Even the discovery of stainless steel is shrouded in myth. According to legend, stainless steel was accidentally discovered in 1913 by British metallurgist Harry Brearly, when he noticed that some samples of an experimental alloy he was planning to throw away hadn’t rusted after sitting on a shelf for several months. In fact, Brearly knew what he was doing; it had been known since the 1800s that adding chromium to steel increased its corrosion resistance, and a number of inventors around the world had already developed materials that would be considered stainless steels today before 1913. Brearly’s main contribution was realizing that these steels could be used for table cutlery. It was Ernest Stuart, one of Brearly’s associates in the cutlery business, who came up with the term “stainless steel.”
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.