Corrosion Galvanic Corrosion

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  • Topic: Corrosion, Cathodic protection, Electrochemistry
  • Pages : 17 (4743 words )
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  • Published : March 7, 2011
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Corrosion
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For the hazard, see corrosive.
Materials failure modes|
Buckling|
Corrosion|
Creep|
Fatigue|
Fouling|
Fracture|
Impact|
Mechanical overload|
Thermal shock|
Wear|
Yielding|
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Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen. Formation of an oxide of iron due to oxidation of the iron atoms in solid solution is a well-known example of electrochemical corrosion, commonly known as rusting. This type of damage typically produces oxide(s) and/or salt(s) of the original metal. Corrosion can also refer to other materials than metals, such as ceramics or polymers, although in this context, the term degradation is more common. In other words, corrosion is the wearing away of metals due to a chemical reaction. Many structural alloys corrode merely from exposure to moisture in the air, but the process can be strongly affected by exposure to certain substances (see below). Corrosion can be concentrated locally to form a pit or crack, or it can extend across a wide area more or less uniformly corroding the surface. Because corrosion is a diffusion controlled process, it occurs on exposed surfaces. As a result, methods to reduce the activity of the exposed surface, such as passivation and chromate-conversion, can increase a material's corrosion resistance. However, some corrosion mechanisms are less visible and less predictable.

Rust, the most familiar example of corrosion.

Volcanic gases have sped the corrosion of this abandoned mining machinery.

Corrosion on exposed metal.
Contents[hide] * 1 Galvanic corrosion * 1.1 Galvanic series * 2 Resistance to corrosion * 2.1 Intrinsic chemistry * 2.2 Passivation * 3 Corrosion in passivated materials * 3.1 Pitting corrosion * 3.2 Weld decay and knifeline attack * 3.3 Crevice corrosion * 4 Microbial corrosion * 5 High temperature corrosion * 6 Methods of protection from corrosion * 6.1 Surface treatments * 6.1.1 Applied coatings * 6.1.2 Reactive coatings * 6.1.3 Anodization * 6.2 Controlled permeability formwork * 6.3 Cathodic protection * 6.3.1 Sacrificial anode protection * 6.3.2 Impressed current cathodic protection * 6.4 Anodic protection * 7 Economic impact * 8 Corrosion in nonmetals * 8.1 Corrosion of glasses * 8.1.1 Glass corrosion tests * 9 See also * 10 References * 11 External links | [edit] Galvanic corrosion

Main article: Galvanic corrosion
Galvanic corrosion occurs when two different metals electrically contact each other and are immersed in an electrolyte. In order for galvanic corrosion to occur, an electrically conductive path and an ionically conductive path are necessary. This effects a galvanic couple where the more active metal corrodes at an accelerated rate and the more noble metal corrodes at a retarded rate. When immersed, neither metal would normally corrode as quickly without the electrically conductive connection (usually via a wire or direct contact). Galvanic corrosion is often utilized in sacrificial anodes. What type of metal(s) to use is readily determined by following the galvanic series. For example, zinc is often used as a sacrificial anode for steel structures, such as pipelines or docked naval ships. Galvanic corrosion is of major interest to the marine industry and also anywhere water can contact pipes or metal structures. Factors such as relative size of anode (smaller is generally less desirable), types of metal, and operating conditions (temperature, humidity, salinity, etc.) will affect galvanic corrosion. The surface area ratio of the anode...
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