Mechanically damaged surface

Cavitation Formation of transient voids or vacuum bubbles in a liquid stream passing over a surface is called cavitation. This is often encountered arouna propellers, rudders, and struts and in pumps. When these bubbles collapse on a metal surface, there is a severe impact or explosive effec t that can cause considerable mechanical damage, and corrosion can be greatly accelerated because of the destruction of protective films. Redesign or a more resistant metal is generally required to avoid this problem. 

Fretting Corrosion This attack occurs when metals shde over each other and cause mechanical damage to one or both. In such a case, frictional heat oxidizes the metal and this oxide then wears away or the mechanical removal of protective oxides results in exposure of fresh surface for corrosive attack. Fretting corrosion is minimized by using harder materials, minimiziug friction (via lubrication), or designing equipment so that no relative movement of parts takes place. 

Ceramic or carbon-brick linings are frequently used as facing linings over plastic or membrane linings when surface temperatures exceed those which can be handled by the unprotected materials or when the membrane must be protec ted from mechanical damage. This type of construction permits processing of materials that are too corrosive to be handled in low-cost metal constructions. 

Glass-reinforced aluminum foil with either a bright polished or white lacquer surface is utilized with most types of insulant. Primarily it is used as a vapor control layer or as a means of upgrading the fire properties of plastic foams, but it does give a semi-decorative finish to the insulation. It is therefore often use where the insulation is open to view but located away from direct risk of mechanical damage. 

At higher flow rates cavitation is a serious degradation mechanism, where vapor bubbles created by pressure fluctuations brought about by the flow of liquid past the surface collapse on the metal surface with tremendous force. This damages any protective oxide which may be present, leading to pitting corrosion. It also causes mechanical damage to the metal. 

In addition to the mechanical damage of the protective film, velocity or movement will also bring the cathode reactant more rapidly to the metal surface thus decreasing cathode polarization. 

Film is removed and underlying metal surface is mechanically damaged which contributes to overall metal loss i.e. erosion corrosion rate is equal to bare metal dissolution rate plus possibly synergistic effect of mechanical damage. 

If infra-red heating or any other radiation curing method is employed, areas which are shaded from the rays or are outside the area of greatest flux density, cannot dry as hard as the fully irradiated surfaces, and may form weak spots susceptible to mechanical damage and consequent corrosion. 

Compressive forces on enamel applied to a convex surface are less than when a concave surface is coated, and it is therefore apparent that the sharper the radius of the metal the weaker the enamel applied to it will be. This fact is also relevant to mechanical damage. 

Blood platelets are key players in the blood-clotting mechanism. These tiny fragments of cytoplasm are shed into the circulation from the surface of megakaryocytes located in the bone marrow. When the lining of a blood vessel is injured, activated platelets release clotting factors, adhere to each other and to damaged surfaces, and send out numerous filopodia. The shape changes that occur in activated platelets are the result of actin polymerization. Before activation, there are no microfilaments because profilin binds to G-actin and prevents its polymerization. After activation, profilin dissociates from G-actin, and bundles and networks of F-actin filaments rapidly appear within the platelet. 

Notches on the pipe surface can be caused by mechanical damage in manufacture, transportation, handling, or installation, and when determined to be mechanically caused, shall be treated the same as gouges and grooves in (a) above. 

When a compression wave travels into materials such as rock or concrete, no damage is inflicted on the materials because of their high compressive strength. However, when an expansion wave travels within the same materials, mechanical damage results near B. This is because rock and concrete are materials of low tensile strength. Fig. 9.7 shows a pair of photographs of the surface (A) and the reverse... 

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