Definition - What does Occluded Cell mean?
An occluded cell is a narrow gap that exists between two opposite surfaces that come in contact.
Cell occlusion is considered the main culprit in premature and unexpected failures. It is one of the most dangerous and researched corrosion problems. The mechanism of occluded cell corrosion is not completely understood, but it is regarded as the key to crack propagation.
An occluded cell is also known as crevice corrosion.
Corrosionpedia explains Occluded Cell
A great number of industrial geometries like lap joints, gasket surfaces and bolt heads have narrow gaps between two surfaces that are connected by a surrounding liquid. In such circumstances, a localized form of corrosion may initiate, especially on metallic surfaces. In this setting, only one surface should be alloy or metal, and the other agent can be rubber, plastic, wood or another material.
Occluded cell corrosion can take the form of pits that are localized in particular regions and can expand to more generalized forms of corrosion throughout the entire surface within the location of the gap. The narrow gaps have points opposing each other.
This corrosion type is classified as deposit corrosion, particularly when the narrow gap that exists is created through corrosion product deposits or dirt in a non-obstructed object.
In the case of nickel or stainless steel, the initiation and proliferation of corrosion in the crevice from the point the alloy is submerged in a liquid takes place as follows:1. There will be an increased passivity in the inside and outside surface of the stainless steel.
2. Oxygen redox at the surface initiates a cathodic reaction that contributes to the passivity of the current.
3. Oxygen dispersed in the crevice area is seriously restricted by the dimensions of the crevice.
4. Metal ions become hydrolyzed by water reaction.
5. Ion concentration is elevated in the crevice area to keep the neutrality of the charge.
6. A potential difference occurs between the crevice base and the mouth region, pushing corrosion into the crevice.
The potential difference that occurs can be tremendous, making hydrogen arise proximal to the base. These steps are also applicable to other types of alloys such as aluminum and titanium.