CREVICE CORROSION Crevices commonly exist at gaskets lap joints bolts rivets

Crevice corrosion crevices commonly exist at gaskets

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4.CREVICE CORROSIONCrevices commonly exist at gaskets, lap joints, bolts, rivets, etc.They are also created by dirt deposits, corrosion products, scratches in paint films, etc.This type of corrosion is usually attributed to one or more of the following: (a)Change in acidity in the crevice.(b) Lack of oxygen in the crevice.(c)Build-up of a detrimental ion species in the crevice.(d) Depletion of an inhibitor in the crevice. Like all forms of localized corrosion, crevice corrosion does not occur in all metal-corrodent combinations. Some materials are more susceptible to it than others; namely those which depend on an air formed oxide film to achieve their corrosion resistance, such as stainless steel, titanium. These materials can be alloyed to improve their resistance. 5. PITTING CORROSION Pitting corrosion is the formation of holes in an otherwise relatively unattacked surface. These holes can have various shapes. The shape of a pit is often responsible for its continued growth, for the same reasons mentioned under crevice corrosion. A pit can, in effect, be considered a self-formed crevice. Pitting corrosion results from an electro-chemical reaction between the broken film as anode and the surrounding unbroken film as cathode which forms a small galvanic cell and produce pits. To minimize pitting, a clean, homogeneous surface is desirable. For example, a pure homogeneous metal with a highly polished surface will generally be much more resistant than one with many inclusions, defects and a rough surface. Pitting is usually a slow process (taking several months or years to become visible) but it can still cause unexpected failures. The small size of a pit and the small amount of metal that has to be dissolved to form pitting and make detection difficult in the early stages. Surface cleanliness and selection of materials known to be resistant to pitting in the given environment are usually the safest ways of avoiding the problem. 6. EXFOLIATION AND SELECTIVE LEACHING Exfoliation is sub-surface corrosion that begins on a clean surface but spreads below it and differs from pitting. In exfoliation the attack has a laminated appearance. Whole layers of material are eaten away. Their attack is usually recognizable by a flaky and sometimes blistered surface. The end of a sample has the appearance of a pack of cards in which several cards are damaged. It is well known in aluminium alloys. It can be combated by heat treatment and alloying Selective leaching (sometimes called Parting) is the removal of one element in an alloy. The most common example is the removal of zinc in copper-zinc alloys. This is known as dezincification. It takes place in brasses containing more than 15% zinc. This type of corrosion is common in brass pipes.
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