Background

Many industrial geometries such as gasket surfaces, lap joints, and bolt heads create narrow gaps between two surfaces which are in contact with a liquid environment. Localized corrosion can be initiated on such surfaces when they are metallic. Only one of the surfaces has to be a metal or alloy, the other can be wood, plastic, rubber, etc. Such corrosion can be in the form of pits localized to certain areas within that gap region to more general corrosion across the entire surface within that gap region. These narrow gaps can have points at which the two opposing surfaces touch each other. These gaps are called crevices and the resulting corrosion is called crevice corrosion. This type of corrosion has been called under deposit corrosion when the narrow gap is formed by a dirt or corrosion product deposited on an otherwise non-obstructed surface. Sometimes crevice corrosion has been called occluded cell corrosion.

The picture below shows a two flanges with a gasket between them. This geometry is commonly found in plant equipment such as pipes and tanks where two pieces of equipment are connected by bolts. Since the contact between the gasket and metal is not perfect, this region can be exposed to the fluid because of seepage. In this case, each flange-gasket geometry creates a crevice between the flange and the gasket on each side of gasket.

Mechanism

The picture below shows an overview of the what is happening within and at the mouth of the crevice region as crevice corrosion occurs. The picture is meant to be a magnification of the narrow area.

If stainless steel or some nickel based alloys are used as the example alloy, initiation and propagation of corrosion within the crevice from the time the alloy is immersed in the fluid are as follows.

1. Initially all surfaces of the stainless steel both inside and outside of the crevice region are passivated the same. Initially the passivity is fairly high.
2. Oxygen reduction and reaction at the surface serves as the cathodic contribution to the passive current. As long as sufficient oxygen is present in the crevice region all surfaces remain at the same potential.
3. Oxygen diffusion into the crevice region is severely limited by the crevice dimensions. The passive current (rate of oxygen reduction) can be greater than the rate of diffusion of unreacted oxygen into the crevice. Oxygen becomes depleted within the crevice. The potential within the crevice becomes more active relative to that of the metal beyond the crevice. The result is that electrons begin to flow through the metal from the crevice to the outer metal surface where oxygen is reduced (see figure). The electron flow results in the further oxidation of alloy constituents. Metal-containing ions are released within the crevice region.
4. The metal ions are hydrolyzed by reaction with water. The result is a decrease in pH as hydrogen ions are a product of the hydrolysis reaction.
5. Simultaneously with hydrolysis, the counter ion concentration (more than likely chloride ions) increases within the crevice region to maintain charge neutrality. This concentration can increase with distance into the crevice because the rate of metal ion formation can increase with distance into the crevice.
6. A potential difference exists between the base of the crevice and its mouth driving corrosion within the crevice. The potential difference can be so great that hydrogen might evolve near the base. The cathodic area for oxygen reduction outside of the crevice is much greater than the anodic or actively corroding area further enhancing the corrosion.

The above steps also apply to other alloys such as titanium and aluminum. In the case of titanium, the hydrolysis can result in a corrosion product of TiO₂ or Ti(OH)₄ which acts as a porous corrosion product layer. At low pH the product can be Ti(OH)₃. Alloying with palladium tends to eliminate crevice corrosion in titanium. Aluminum crevice corrosion can be accelerated by chloride and bromide ions creating an autocatalytic effect.

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