Why is Stainless Steel Corrosion Resistant?

Stainless steelis known for itscorrosion resistancein many environments in which carbon and low-alloytool steelswould corrode. (Background reading:An Introduction to Stainless Steels.) The corrosion resistance is a result of a very thin (about 5 nanometers)oxide layeron the steel’s surface. This oxide layer is referred to as apassive layersince it renders the surface electrochemicallypassivein the presence of corrosive environments.

The passive layer forms because of thechromiumadded to stainless steel. Stainless steel must have at least 10.5% chromium in order for the passive layer to form. The more chromium that is added, the more stable the passive layer becomes, and the better the corrosion resistance. (For more on chromium, seeThe Role of Chromium in Intergranular Corrosion.) Other elements such as nickel, manganese and molybdenum can be added to enhance stainless steel corrosion resistance.

Another requirement for the formation and maintenance of the passive layer is that the steel surface must be exposed to oxygen. Corrosion resistance is greatest when the steel is boldly exposed and the surface is maintained free of deposits. Ifpassivityis destroyed under conditions that do not permit restoration of the passive film, then stainless steel will corrode much like a carbon or low-alloy steel. For example, covering a portion of the surface—for example, by biofouling, painting or installing a gasket—produces an oxygen-depleted region under the covered region. The oxygen-depleted region isanodicrelative to the well-aerated boldly exposed surface, possibly resulting in the corrosion of the covered region.

Pitting in 304 stainless steel.

Under certain circumstances, the passive layer can break down at localized spots on a well-exposed stainless steel surface. When this happens, the metal can corrode in the localized spots. This is calledpitting corrosion. One common cause of pitting corrosion is exposure to aqueous environments that contain chloride. Examples are coastal atmospheres, road salt combined with rainwater, and even tap water containing high levels of chloride.

Intergranular corrosion of 304 stainless steel.

During the fabrication of stainless steel components or structures, it is possible to degrade the corrosion resistance. This occurs whenaustenitic stainless steels(e.g.,304 grade) are exposed to temperatures between about 797°F (425°C) and 1598°F (870°C). If the exposure time is too long, then the areas near the metal’sgrain boundarieslose their corrosion resistance and can be preferentially attacked when exposed to a corrosive environment. The grains fall out and the metal loses strength. The increased susceptibility to corrosion by this change in microstructure is calledsensitization.

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