Corrosion is a natural, destructive process where refined metal converts to a more chemically stable form, such as an oxide, hydroxide, or sulfide. Crevice corrosion represents a form of localized attack. It is insidious because it occurs hidden from view in confined spaces, often leading to sudden component failure without visible external warning.
Defining the Phenomenon
Crevice corrosion is defined by the physical conditions required for its initiation. It attacks a metal surface that is part of a narrow gap or occluded space where a stagnant solution can collect. This gap, or crevice, must be wide enough to permit the entry of an electrolyte, such as water or moisture, but narrow enough to ensure the fluid remains trapped and non-circulating.
The geometrical requirement involves two surfaces in close contact, which can be metal-to-metal, such as in a bolted joint, or metal-to-nonmetal, as seen with gaskets or seals. The most sensitive gap width for this type of corrosion typically falls between 25 and 100 micrometers. Gaps much wider than this range allow for fluid circulation, which prevents the necessary concentration changes that drive the corrosion process. The presence of a stagnant electrolyte, like water or a salt solution, is a prerequisite for the chemical mechanism to begin.
The Chemical Mechanism
The localized attack is driven by sequential chemical changes that create a highly corrosive micro-environment inside the confined space. The initiation begins with the consumption of dissolved oxygen within the stagnant electrolyte trapped inside the crevice. Oxygen is used up quickly by the normal cathodic corrosion reaction, but the narrow geometry severely restricts its replenishment from the bulk solution outside the crevice.
This lack of oxygen creates a differential aeration cell. The metal surface inside the crevice becomes the oxygen-depleted anodic site, and the surrounding, oxygen-rich surface outside the crevice becomes the cathodic site. The anodic reaction involves the dissolution of the metal, releasing positively charged metal ions (\(M^{n+}\)) into the crevice solution.
To maintain electrical neutrality within the crevice solution, the positively charged metal ions attract negatively charged anions from the bulk electrolyte. These anions often include hydroxide (\(OH^-\)) and chlorides (\(Cl^-\)). The metal ions then react with water in a process known as hydrolysis, which produces hydrogen ions (\(H^+\)) and results in a significant drop in the crevice’s pH.
The solution inside the crevice can become highly acidic, with the pH often dropping to values as low as 2, which is sufficient to destroy the protective passive film on many corrosion-resistant metals. The concentration of aggressive anions, particularly chlorides, further accelerates the attack in a self-sustaining, or autocatalytic, cycle. The concentrated acid environment continuously dissolves the metal surface, driving the localized corrosion forward rapidly until a structural failure occurs.
Identifying Susceptible Environments and Materials
Susceptible Environments
Crevice corrosion is a pervasive threat across many industrial and marine environments where mechanical joints or surface obstructions are common. Typical locations include the interfaces of bolted connections, under washers, and within the overlaps of lap joints in fabricated structures. Gaskets and seals frequently form the second surface of a metal-to-nonmetal crevice.
Artificial crevices can also be formed by surface deposits, such as sludge, dirt, marine fouling, or corrosion products, which create occluded areas where an electrolyte can stagnate. In piping systems, this attack is common under pipe supports (CUPS) where the pipe contacts a beam or clamp. Even insulation can trap moisture against a surface, leading to corrosion under insulation (CUI).
Susceptible Materials
Materials that rely on a passive oxide layer for their corrosion resistance are the most vulnerable. These include stainless steels, aluminum alloys, and titanium, all of which are designed to be highly corrosion resistant in open environments. The acidic, low-oxygen conditions generated inside the crevice destroy the protective film, leaving the base metal exposed to rapid dissolution. Austenitic stainless steels, such as the 304 and 316 grades, are especially susceptible because their resistance depends heavily on the stability of this thin, chromium-rich oxide layer.
Strategies for Prevention and Mitigation
The most effective approach to preventing crevice corrosion involves eliminating the conditions required for its formation during the initial design phase. Engineers should prioritize design modifications that remove occluded areas where moisture can collect and stagnate. For instance, using continuous welding instead of bolting or riveting for joints significantly reduces the number of potential crevices.
Mitigation Strategies
When joints are unavoidable, selecting appropriate materials is a key mitigation strategy. Choosing highly resistant alloys, such as super-austenitic or duplex stainless steels with high molybdenum content, enhances the material’s tolerance for aggressive environments. Molybdenum increases the stability of the passive film against chloride attack and acidic conditions.
For interfaces involving non-metallic parts, such as gaskets, it is important to select non-absorbent materials like solid polytetrafluoroethylene (PTFE or Teflon). This prevents the gasket from acting as a sponge that retains corrosive electrolytes against the metal surface. Regular maintenance, including the cleaning of surfaces to remove deposits and sludge, also helps to eliminate the formation of artificial crevices.