Did you know that stainless steel stains less easily than other iron-based metals? Like the word itself, stainless steel isn't really stainless, the material can get marked up by all kinds of different things; fingerprints, grease, it can develop discoloration and even eventually rust.
The difference in different types of stainless steel is how resilient it is.
Stainless steel can take a lot more abuse than other metals. It can withstand a lot more before it shows signs of wear.
If you remember in your chemistry classes, all steels have the same basic iron and carbon compositions. The difference with stainless steel; is that it contains what is called chromium.
Stainless steel must contain at least 10 percent chromium. Chromium is the element that reacts with oxygen in the air to form a complex chrome-oxide surface layer that is invisible, but strong, to prevent further oxygen from staining (rusting) the surface.
Higher levels of chromium and the addition of other alloying elements such as nickel and molybdenum enhance this surface layer and improve the corrosion resistance of the stainless material.
Two of the most common, or the ones you here most about, are 304 and 316. The difference between the two is molybdenum. Molybdenum is the chemical element of atomic 42, a brittle silver-gray metal of the transition series, used in some alloy steels. The difference between 304 and 316 is that 316 stainless steel contains molybdenum, while 304 does not.
304 Stainless Steel
For things like rails, stainless steel is an ideal corrosion-resistant material, but it’s only going to withstand long-term exposure for some long. While 304 is the choice of many, it is an economical and practical choice for most environments. It doesn’t, however, have the chloride resistance of 316. In some cases, the higher price point of 316 is well worth it in areas where there is high chloride exposure.
Just in case you didn’t know this already, stainless steel is the most common form of steel used around the world, mostly because of its excellent corrosion resistance and value. Stainless steel contains between 16 and 24 percent chromium, and up to 35 percent nickel – as well as small amounts of carbon and manganese.
The most common form of 304 stainless steel is 18-8 or 18/8, which means it contains 18 percent chromium and 8 percent nickel. One of the nice things about 304 stainless steel is that it can withstand corrosion from almost any oxidizing acids. Because of that durability, 304 stainless steel makes is easy to sanitize – which makes it ideal for kitchen and food applications.
There are draw backs, however, 304 stainless steel is susceptible to corrosion from chloride solutions. The ions in chloride can cause localized areas of corrosion, which is called pitting. Pitting is a form of extremely localized corrosion that leads to the creation of small holes in the metal.
The driving power for pitting corrosion is the de-passivation of a small area, which becomes anodic while an unknown but potentially vast area becomes cathodic leading to very localized galvanic corrosion. The corrosion penetrates the mass of the metal, with a limited diffusion of ions. The mechanism of pitting corrosion is probably the same as crevice corrosion.
Solutions with as little as 25 ppm of sodium chloride can begin to have a corrosive effect.
316 Stainless Steel
As we stated earlier, the 316 stainless steel grade it the second most common form out there today. It looks at acts just like its friend stainless steel with the same physical and mechanical properties. It also contains a similar material make-up as the 304.
The key difference that separates the 316 from the 304 is the fact that the 316 incorporates about 2 to 3 percent of molybdenum. The extra molybdenum increases corrosion resistance – especially against chlorides and other industrial solvents.
When you think of 316 stainless steel, you will notice that it is commonly used in a lot of industrial applications involving processing chemicals – as well as high-saline environments such as coastal regions and outdoor areas where de-icing salts are normally common. 316 stainless steel is also used in the manufacturing of medical surgical instruments.
Alternative 300-series grades can contain up to 7 percent molybdenum. They provide even better chloride resistance, but such heavy-duty resistance is only necessary in industrial or high concentration exposure conditions.