10 FATIGUE CORROSION This is a special form of stress cracking corrosion

10 fatigue corrosion this is a special form of stress

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10. FATIGUE CORROSION This is a special form of stress cracking corrosion. Fatigue failures occur in the absence of corrodents and are caused by repeated cyclic stressing. Such failures are common in structures subjected to continued vibration. When susceptibility to fatigue is increased by the presence of a corrodent, the resulting failure is attributed to fatigue corrosion. A metal that has failed because of fatigue corrosion will usually have visible corrosion products on the fracture surface. This can be a misleading indication in some cases, because a regular fatigue fracture surface will sometimes corrode before being inspected. There will generally be less crack-branching than in a regular stress crack corrosion.
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~ Page 8 of 73 ~ 11. UNDERGROUND CORROSION This type of corrosion occurs in the underground pipes due to the corrosive nature of the soil and causes severe damage to the pipes in case the protective measures are not provided. Corrosive nature of soil depends upon a number of factors such as acidity, degree of aeration, electrical conductivity, moisture, salt content, presence of bacteria, micro-organism and soil texture. The corrosion takes place due to the formation of galvanic concentration or concentration cells between different metals. (See Fig. 3 below) In order to find out just how aggressive soil is, in addition to the type of soil involved, one also needs to know the following: (a) p H of the soil (b) Water content and dry mater content. (c) Water holding capacity. (d) Total acidity or total alkalinity. (e) Percentage of carbon content. (f) Physical examination of the soil, particularly the specific electric resistivity of the soil. (g) Chemical analysis of the acqueous soil extract. Pipes and structures buried in the soil containing sulphates and organic matter are attacked by the sulphate reducing microbes or bacteria. It forms iron sulphide. Attack by bacteria is fast and localised. These micro-organisms are: (i) Sulphate reducing bacteria. (ii) Micro-organisms forming micro-biological films. (iii) Sulphur bacteria or microbes; (iv) Corrosion due to microbes or bacteria is also known as micro-biological corrosion. 12. HIGH TEMPERATURE CORROSIONThe process involved for high temperature corrosion reactions is very complex.At elevated temperature the rate of oxidation of metal increases.So the components exposed for prolonged time at high temperature are exposed to this type ofcorrosion.The oxides are volatile or stable.Volatile oxides evaporate at the elevated temperature exposing new surface for corrosion.On the other hand, stable oxide forms an oxide layer and further prevents corrosion.Stable oxides can be again divided into two groups. Fig. 3
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~ Page 9 of 73 ~ Minor group evaporates at relatively lower temperature whereas major group normally remains on the metal surface, unless they are physically or chemically removed. Molybdenum is a classic example of the evaporating oxide. At 300 ˚C Outer layer – MoO 3 Inner layer – MoO 2 Above 500 ˚C MoO 3 starts evaporating At 700 ˚C Rate of volatisation equals rate of corrosion.
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