Materials and Corrosion - Essay Example

Crevice Corrosion Corrosion occurring within or adjacent to crevices, which are commonly left at joints, due to small volumes of stagnant corroding fluids is known as crevice corrosion. The most important feature of this type of corrosion is the contact with stagnant solution which takes place in interstices, near seals and in cervices made for nuts and rivet heads. The metals and alloys used in chemical plants and other industrial applications are covered with passive films of protective coatings in order to prevent contact with high concentrations of Cl- and H+ ions.

When such places are left unclean for long durations after long usages of the plant, sand and other substances also get deposited near metals and alloys which prevent the proper application of protective coatings. In such a scenario the crevices and interstices are more likely to come in direct contact with corroding media. The corroding fluids which commonly contains high concentrations of chloride ion is likely to get accumulated in the crevices made for nuts, rivets or any other type of hole.

The prolonged contact with such chemical fluids starts the oxidation of the metals. The direct contact with aerated chloride rich media creates a differential aeration cell which destroys the passivity of the material. The oxidized metal starts dissolving into the chemical which is in contact with the metal, in the presence of oxygen and the prolonged contact can corrode the material completely. The best way to prevent crevice corrosion is to prevent crevice corrosion is to prevent crevices. This can be done by using welds in place of bolts and rivets in joints.

The design of various components should be done in such a way that there is always a drainage mechanism for the accumulating fluid so that the harmful corroding liquids do not come in contact with the metals for long duration of time. Other effective means of preventing crevice corrosion is the use of corrosion inhibitors and coatings, some of which are discussed below. Inhibitors 1. VpCl-337 which is a vapor phase corrosion inhibitor is also very effective for preventing crevice corrosion. The crevices are fogged by the material and thus large areas of material are coated against corrosion. 2. Ecoline 3690 which is a biodegradable and bio based inhibitor is also very effective for using for crevice corrosion prevention because of its ability to displace moisture and provide protection against aggressive environments.

It is very suitable for use in marine environment and high humidity conditions. 3. VCI (Vapor Corrosion Inhibitor) Foam is also a very useful inhibitor for preventing crevice corrosion. It can be used in the form of a foam pad cut into the size of the space where it is to be used. It is very easy to apply as compared to other vapor inhibitors. 4. VCI 2000 is particularly useful in pipes as it can be applied on the crevices on pipes during fabrication. Protective Coatings 1. The most commonly used coating for crevice corrosion is a mixture of PdO and TiO which is applied in the form of a 0.

4 pm thick coating. The mixture is found effective in all kinds of conditions where there is a prolonged contact with chemical fluids. 2. Another coating which is present with a brand name of Reactive Gel (RG 2400) is also suitable for prevention of crevice