Stainless steel is an alloy of iron that resists corrosion because it contains a minimum of 10.5% chromium. When this metal is exposed to air, the chromium reacts with oxygen to form a layer of chromium oxide on the surface, known as the passive layer. This film acts as a protective barrier. While this passive film is capable of self-repairing if scratched or damaged, certain aggressive substances and environmental conditions can overwhelm its protective capacity, leading to localized or widespread corrosion.
The Ubiquitous Threat of Chlorides and Halides
The greatest threat to the passive layer of stainless steel comes from the presence of chloride ions (Cl⁻), which are widely found in common substances like table salt, sea salt, bleach, and many cleaning products. These chloride ions are small and highly mobile, allowing them to penetrate microscopic defects or weak points in the chromium oxide film. Once they breach the protective layer, they initiate a localized electrochemical attack.
This attack results in a highly specific form of damage called pitting corrosion, characterized by small, deep holes in the metal surface. Within the microscopic pit, the chemistry becomes self-sustaining and aggressive, as metal dissolution concentrates the chloride ions and creates a highly acidic environment, with the pH often dropping below 2. The acidic conditions prevent the passive layer from reforming, accelerating the metal loss within the pit while the surrounding surface remains protected.
Common household exposure occurs when chloride-containing solutions are left to dry on the surface, concentrating the corrosive ions. Examples include road salt residue on outdoor fixtures, prolonged contact with foods that contain salt and vinegar, or using cleaning agents containing sodium hypochlorite (bleach). Even small, residual amounts of chloride can initiate damage, making thorough rinsing and drying of stainless steel surfaces after cleaning necessary.
Corrosion Caused by Stagnant Conditions and Oxygen Deprivation
Stainless steel’s corrosion resistance relies on oxygen being readily available to continuously repair and maintain the passive layer. When the metal is in a stagnant environment where oxygen cannot freely circulate, its self-healing mechanism is compromised. This condition frequently occurs in narrow gaps, under fastener heads, beneath gaskets, or in tight joints where water or debris can accumulate, leading to crevice corrosion.
Within a crevice, the trapped liquid quickly consumes the available dissolved oxygen, creating an oxygen-depleted zone. The lack of oxygen prevents the passive film from repairing itself, and the localized corrosion process begins. As metal dissolves in the confined space, positively charged metal ions are released, which naturally attract negatively charged chloride ions from the surrounding solution.
The influx of chloride ions causes the environment inside the crevice to become highly acidic, which further accelerates the breakdown of the protective film and the corrosion of the metal. Crevice corrosion is particularly insidious because the damage occurs in hidden areas, often causing structural failure with minimal visible warning. Designing systems to avoid tight spaces or ensuring they are fully sealed is the primary method to prevent this type of geometrically driven corrosion.
Chemical Attacks from Strong Acids and Alkaline Cleaners
While chlorides cause highly localized pitting, concentrated strong acids and certain alkaline substances can cause more generalized corrosion by dissolving the entire passive film rapidly. The protective chromium oxide layer is stable across a wide range of pH levels, but it is vulnerable to extremes. Highly acidic solutions, particularly those with a pH below 2, actively attack and dissolve the oxide layer, exposing the underlying metal.
Aggressive agents include concentrated sulfuric acid or hydrochloric acid, which can quickly degrade the metal surface. Even milder acids, like the acetic acid in vinegar or citric acid in lemon juice, can cause visible surface etching if left in prolonged contact, particularly on lower-grade stainless steels. This type of chemical attack results in a uniform thinning or dulling of the surface rather than isolated holes.
Highly concentrated alkaline cleaners, with a pH of 13 or 14, can also be corrosive, although the risk is lower than with strong acids. These substances are often found in industrial degreasers or drain cleaners, and extended exposure can compromise the passive film. Proper selection of cleaning agents and minimizing contact time are necessary to avoid stripping the protective layer.
Galvanic Corrosion and Contact with Other Metals
Galvanic corrosion is an electrochemical process that occurs when two different metals are in electrical contact and exposed to an electrolyte, such as moisture or salt water. The difference in electrical potential, or “nobility,” between the two metals creates a galvanic cell. The more active (less noble) metal becomes the anode and corrodes preferentially, while the more noble metal (the cathode), often stainless steel, is protected.
Stainless steel is a noble metal, meaning it is typically the cathode in a dissimilar metal pairing. This accelerates the corrosion of the other, less noble metal, such as carbon steel, aluminum, or zinc (galvanized steel). Using carbon steel screws to fasten a large stainless steel fixture, for example, can lead to the rapid deterioration of the screws because the small anode area is forced to protect the large cathode area.
Although stainless steel is usually protected, the corrosion products from the rapidly dissolving anode can sometimes stain the stainless steel surface. If the stainless steel is already in an actively corroding state due to the presence of chlorides or crevice conditions, its electrochemical potential can shift, increasing its susceptibility to galvanic effects. Insulating the two metals or choosing alloys with similar nobility is necessary to prevent this condition.