Stainless steel is an iron alloy known for its remarkable resistance to corrosion, making it suitable for countless applications. While often considered “rust-proof,” this is misleading; stainless steel can corrode under certain circumstances. It is better described as highly corrosion-resistant rather than impervious to degradation. Understanding the protective mechanism and challenging environments is key to ensuring its longevity.
How Stainless Steel Achieves Corrosion Resistance
The corrosion resistance of stainless steel comes from a unique, microscopically thin surface layer. This layer requires a minimum of 10.5% chromium in the alloy. When the steel is exposed to oxygen, the chromium rapidly reacts to form a layer of chromium oxide.
This layer is known as the passive film and is only a few atoms thick, measuring just one to three nanometers. Unlike the flaky iron oxide that forms on regular steel, this chromium oxide film is dense, stable, and highly adherent to the metal surface. It acts as an inert barrier, preventing oxygen and moisture from reaching the underlying iron and stopping corrosion. The passive layer can also self-repair; if the surface is damaged, the exposed chromium reacts with oxygen to quickly reform the protective film.
Environments That Cause Stainless Steel to Corrode
The passive layer can be compromised by external factors, leading to localized forms of corrosion. Pitting corrosion occurs when the passive film is attacked by specific chemical species, most notably chloride ions from salt or bleach. Once the layer is breached, corrosion rapidly creates small, deep cavities on the surface, often causing significant damage.
Crevice corrosion attacks stainless steel in tight, oxygen-starved spaces, such as under washers, bolts, or in poorly welded joints. When oxygen cannot freely circulate, the passive layer cannot reform. Trapped moisture becomes acidic and rich in chlorides, accelerating the breakdown of the protective film and causing corrosion within the confined area.
A third form of localized attack is stress corrosion cracking (SCC), which requires a specific combination of three factors. SCC happens when the steel is under tensile stress, exposed to a corrosive environment, and subjected to elevated temperatures. These conditions cause microscopic cracks to initiate and propagate through the metal, potentially leading to sudden failure.
Understanding Different Stainless Steel Grades
Stainless steel refers to a family of alloys whose corrosion resistance varies significantly based on composition. Different grades are formulated for specific environments. The two most common grades are 304 and 316, both belonging to the austenitic class.
Grade 304 is the most widely used type, offering excellent corrosion resistance for general applications like kitchen appliances and indoor architecture. However, its performance is limited in environments with high concentrations of chlorides. To enhance resistance, other elements are added to create superior grades.
Grade 316 stainless steel, often called marine-grade, includes a substantial addition of Molybdenum, typically between two and three percent. This alloying element significantly improves resistance to pitting and crevice corrosion, especially in environments exposed to salt water, de-icing salts, or harsh acids. Choosing the appropriate grade is necessary to ensure the material performs as expected.
Maintaining Stainless Steel to Ensure Longevity
Protecting the passive layer is the most effective way to ensure stainless steel longevity. Routine cleaning is necessary to remove surface contaminants like dirt, grease, or iron dust. Allowing these particles to remain can disrupt the passive layer and initiate localized rusting.
When cleaning, avoid harsh cleaners, especially those containing high concentrations of chlorine or chloride ions, as they directly attack the protective film. Abrasive tools like steel wool should never be used, as they can scratch the surface and embed reactive carbon steel particles.
If the passive layer is compromised, a process called passivation can restore it. This involves treating the surface with a mild acid solution, such as citric acid. Passivation removes contaminants and promotes the rapid reformation of the chromium oxide layer.