Why is Stainless Steel Corrosion – Resistant?
Stainless Steel Corrosion Resistance
Stainless steel is known for its corrosion resistance due to its unique protective layer, a thin layer of chromium oxide that forms when exposed to oxygen. This layer, called a “passive layer” or “passive film,” is highly stable and adheres tightly to the metal. If damaged or scratched, it can self-repair with oxygen.
Other elements like nickel, molybdenum, and nitrogen can enhance corrosion resistance. However, a characteristic that makes stainless steel a reliable material is that it is not immune to corrosion, as it can break down under low-oxygen, high-salinity, or high-temperature conditions.
Additionally, depending on the type and environment, stainless steel can be susceptible to other forms of corrosion, such as crevice corrosion, stress corrosion cracking, or galvanic corrosion.
It is well known that stainless steel resists corrosion in various settings where carbon and low-alloy tool steels would deteriorate. This corrosion resistance is due to a very tiny oxide coating on the surface of the steel (approximately 5 nanometers thick). Since this oxide layer makes the surface electrochemically inert in the presence of corrosive conditions, it is known as a passive layer.
The chromium addition to stainless steel creates the passive layer. The chromium content in stainless steel must be at least 10.5% for the passive layer to develop. When more chromium is applied, the passive layer becomes more stable and exhibits improved corrosion resistance. To increase the corrosion resistance of stainless steel, additional elements can be added, including nickel, manganese, and molybdenum.
The steel surface must be exposed to oxygen in order for the passive layer to form and remain in place. When the steel is openly exposed, and the surface is clear of deposits, corrosion resistance is at its highest. Stainless steel will corrode similarly to carbon or low-alloy steel if passivity is lost in circumstances that do not allow the passive film to be restored. For instance, covering a piece of the surface, whether with paint, a gasket, or biofouling, results in an oxygen-depleted area beneath the covered area. The covered area may corrode because the oxygen-depleted region is anodic compared to the well-aerated, boldly exposed surface.
304 Stainless Steel Pitting
A well-exposed stainless steel surface may occasionally experience localized passive layer breakdown. When this occurs, the metal may corrode in specific locations. Pitting corrosion is the term for this. Exposure to chloride-containing aqueous conditions is one of the main causes of pitting corrosion. Examples include coastal environments, road salt mixed with rainwater, and even tap water with high chloride levels.
Corrosion Between Granules
It is possible for stainless steel components or structures to lose some of their corrosion resistance during manufacture. Austenitic stainless steels (like 304) that are subjected to temperatures between approximately 425 °C (797 °F) and 870 °C (1598 °F) experience this. Long-term exposure causes the metal’s grain boundaries to lose their corrosion resistance, making them more susceptible to attack when exposed to corrosive conditions. The metal becomes weaker as the grains are lost. Sensitization describes the increased susceptibility to corrosion caused by this change in microstructure.
