Oxidation is a natural process where a material reacts with oxygen, often leading to degradation, such as the rusting of iron. This raises the question of whether durable materials like stainless steel can also oxidize. Stainless steel is known for its corrosion resistance and is used in many applications.
The Passive Layer
Stainless steel’s resistance to oxidation stems from its composition, particularly the presence of chromium. This element, at least 10.5%, reacts with oxygen to spontaneously form a thin, invisible layer on the surface. This protective film is primarily composed of chromium oxide, known as the passive layer.
The passive layer acts as a barrier, preventing oxygen from reaching the iron components within the steel, inhibiting rust formation. This film is stable and possesses a self-repairing capability; if scratched or damaged, it can re-form. Higher chromium content and other alloying elements like molybdenum and nickel enhance this protective layer.
Forms of Stainless Steel Oxidation
Despite its inherent resistance, stainless steel can oxidize under specific conditions that compromise its passive layer. The protective chromium oxide film requires sufficient oxygen to form and maintain itself. When conditions prevent this, or when aggressive substances are present, the layer can break down, leading to various forms of corrosion. Contaminants like free iron particles introduced during fabrication can also disrupt the passive layer and initiate rust.
Pitting Corrosion
One common form is pitting corrosion, which manifests as small, localized holes on the surface. This typically occurs in environments containing chloride ions, found in substances like salt, bleach, or seawater, which can attack and penetrate the passive film. Pits can be difficult to detect initially as they may be small on the surface but extend deeply into the material, compromising structural integrity.
Crevice Corrosion
Crevice corrosion is another localized attack, occurring in tight spaces where oxygen levels are limited, such as under gaskets, in bolted connections, or beneath deposits. In these confined areas, the oxygen supply necessary for passive layer repair is restricted, leading to a breakdown of the protective film and accelerated corrosion within the crevice. The environment within the crevice can become more acidic, further intensifying the attack.
General Corrosion and High-Temperature Effects
General corrosion, while less common in stainless steel, can occur under highly aggressive conditions, particularly with strong acids. In such instances, the passive layer is uniformly attacked, resulting in metal loss across the entire surface. High temperatures can also lead to heat tint or sensitization, where the steel discolors due to surface oxidation. Sensitization occurs when stainless steel is exposed to certain high temperatures, causing chromium to combine with carbon and deplete the surrounding areas of chromium, making them susceptible to corrosion.
Preventing and Treating Oxidation
Preventing oxidation in stainless steel involves careful selection and maintenance. This includes choosing the correct grade for a specific environment, especially those with high chloride exposure or extreme temperatures. Grades with higher molybdenum content, for instance, offer increased resistance to pitting and crevice corrosion. Proper design that minimizes crevices and ensures good drainage also helps prevent localized corrosion.
Regular cleaning helps remove contaminants that can initiate corrosion, such as dirt, grime, and iron particles. Mild soap or detergent and warm water are effective for routine cleaning, followed by thorough rinsing and drying to prevent water spots. Avoid harsh chemicals, especially those containing chlorides, and abrasive tools like steel wool, which can damage the passive layer or introduce foreign iron particles.
When minor rust spots or surface discoloration appear, specific treatments can be applied. For light rust staining, mild, non-scratching creams or polishes can be used. For more significant issues, chemical passivation treatments can be performed. This process involves cleaning the surface to remove free iron and contaminants, followed by an acid bath, which promotes the re-formation of the chromium oxide passive layer. This helps restore the material’s inherent corrosion resistance.