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6Sep/130

Ask Dr. Steel: “Why is Stainless Steel stainless?”

So what do we mean when we say that stainless steel is stainless? Does that mean if we spill our coffee on it that it won't stain? Not exactly. The name gets it origin from the fact that stainless steel corrodes or rusts less easily than ordinary steel. In this article we'll look into the chemistry behind stainless steel and find out what makes it stainless.

Stainless steels (there are many varieties depending on the applications) have a higher resistance to oxidation (which causes rust) and corrosion in a wide variety of environmental conditions. They are often used where both the structural integrity is important as well as the finish on the steel itself, such as in skyscrapers, ornamental finishing and appliances.

Stainless steels are made with a minimum of 10.5% chromium content. It is this element that gives stainless steel its ability to “stain less” when compared to other steel types. To further enhance the steels ability to resist oxidation and corrosion nickel, molybdenum and niobium can be found in modern stainless steels.

It is the chromium in the steel that combined with oxygen to form a thin, invisible layer of chrome-containing oxide. This layer is only a few atoms thick due in part to the ability of the chromium atoms and oxide atoms to pack together tightly. This layer protects the steel underneath and has a unique self-healing ability. Should the layer become scratched or cut, the newly exposed steel will form another protective area with oxygen in the air. It is for this reason that stainless steels need an oxygen-rich environment.

It is important to know that stainless steel is not rust and corrosion proof, just resistant. As we talked about above, stainless steel ‘stains less' than other steels but that does not mean it won't corrode or rust. If the wrong type of stainless steel is used for the wrong application then it will behave like any other iron-based metal and start corroding when exposed to oxygen. If we were to take a piece of stainless steel and place is in salt water the salt would destroy the protective layer faster than it could be repaired and result in corrosion.

Another process gaining popularity is to dip the steel in an oxidant, such as nitric acid or citric acid, to remove any free iron from the top surface of the steel. The steel can then be painted or plated once the surface area is clean. However, this process has one fatal drawback in that all of the iron must be removed. This can be difficult in crevices or corners where the steel joins together. These tight corners tend to corrode because of the inability to remove all of the free iron.

Many times, especially in household use, metals may be stainless steel plated. While this plate provides corrosion resistance and rust protection it does not fully protect the metal should it become deeply scratched or cut. In addition, plates can improperly applied and wear off over time and use. It is important when purchasing any item that claims to be made of stainless steel to ask if the steel is completely stainless or if it is just top-level plating with a cheaper steel or metal underneath.

Planet earth and our oxygen-rich atmosphere guarantee us that any iron-based metal will never be 100% rust and corrosion proof. However, thanks to chemical advances in metallurgy we have a high-endurance, corrosion and rust resistant close second in stainless steel. It's wide use in outdoor and indoor applications such as home appliances and outdoor sculptures have proven that it will hold up in a wide range of environmental factors and everyday use. Today stainless steel appliances grace the kitchens of both commercial eateries as well as our homes. And after all, who would want a rusty refrigerator?

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