Surgical stainless steel (SSS) is a metal alloy widely used in medical devices and consumer products, such as jewelry, due to its durability and resistance to breakdown. While the term “stainless” suggests complete immunity, SSS is not entirely impervious to all forms of degradation. It is highly resistant to corrosion, but under extreme or prolonged chemical exposures, it can experience surface damage. SSS does not typically “rust” in the common sense of iron oxidation (the reddish-brown flaking seen on ordinary steel). Instead, it can suffer localized forms of corrosion that appear as pitting or staining.
The Composition and Mechanism of Resistance
Surgical stainless steel is an iron-based alloy that derives its superior properties from the addition of other elements, primarily chromium. The most common grade used for body-contact applications is 316L, where the “L” signifies a low carbon content that enhances corrosion resistance. The composition of 316L typically includes 16% to 18% chromium, 10% to 14% nickel, and 2% to 3% molybdenum.
The resistance of stainless steel is due to the chromium content, which forms a thin, protective surface layer called a passive film. When chromium is exposed to oxygen, it instantly reacts to create an extremely dense, invisible layer of chromium oxide (Cr2O3). This passive film acts as a stable barrier, tightly adhering to the metal and sealing the underlying iron from the environment.
This chromium oxide layer is self-repairing. If the surface is scratched or damaged in the presence of oxygen, the exposed chromium immediately reacts to rebuild the protective film. This mechanism prevents the iron content from reacting with oxygen and moisture to form iron oxide, or true rust. The addition of molybdenum in 316L enhances the stability of this passive film, especially against chloride ions.
Environmental Factors Causing Corrosion
Despite its passive layer, SSS can corrode when exposed to aggressive environments that overwhelm the self-repairing mechanism. The primary threat comes from high-chloride solutions, which break down the chromium oxide film and cause localized attacks. Chloride ions found in saltwater, concentrated cleaning agents (like bleach), and saline sweat can penetrate the passive layer, leading to pitting corrosion.
Pitting is a localized form of corrosion where small, deep holes develop on the surface, often appearing as dark spots or staining. This failure is not generalized rust but occurs when the protective film is compromised in a small area. High temperatures can accelerate corrosion rates, making the metal more susceptible to attack when combined with corrosive agents.
Another form of attack is crevice corrosion, which occurs when oxygen is deprived in a tight space, such as under a washer, a screw head, or debris buildup. In these confined areas, the oxygen concentration is too low to reform the passive chromium oxide layer, allowing corrosive agents to attack the base metal. Contact with carbon steel tools can also transfer iron particles onto the stainless steel surface. These particles then rust and initiate a corrosive process on the host material, a phenomenon called ferrous contamination.
Practical Care and Longevity
Maintaining the longevity of surgical stainless steel items, such as jewelry and instruments, centers on preserving the integrity of the passive oxide layer. For routine cleaning, a mild soap solution and warm water are effective for removing surface contaminants. Use a soft cloth or brush and thoroughly dry the item afterward, as standing water can leave deposits.
Users should avoid exposing the metal to strong household chemicals, particularly those containing chlorine, such as bleach or pool chemicals. Chloride ions in these substances are the primary cause of pitting and staining corrosion. If the metal is exposed to a high-chloride environment, like ocean water or a chlorinated pool, rinsing it immediately with fresh water and drying it mitigates the risk of damage.
When cleaning surgical instruments, use neutral pH detergents and avoid mixing them with dissimilar metals like aluminum or copper in the same cleaning bath. This separation prevents galvanic corrosion, which occurs when two different metals are electrically connected in a corrosive electrolyte. Galvanic corrosion damages the less noble metal and can discolor the stainless steel. Proper storage in a clean, dry location also prevents the buildup of debris or moisture that could lead to crevice corrosion.