The addition of a small amount of tin 10 and arsenic 004 stops this type of

The addition of a small amount of tin 10 and arsenic

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The addition of a small amount of tin (1.0%) and arsenic (0.04%) stops this type of corrosion. Selective dissolution of iron from cast irons is called graphitization. The corrosion is detrimental largely because it yields a porous metal with poor mechanical properties. The remedy involves use of non-susceptible alloys.
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~ Page 6 of 73 ~ 7. CELLS CONCENTRATION CORROSION Variations in concentration of electrolyte in contact with a metal can lead to corrosion. The part of the metal in contact with more concentrated electrolyte acts as a cathode while the part in contact with more dilute electrolyte acts as an anode and so corrodes most. Such a cell is known as a concentration cell. For example, a drop of water corrodes a polished iron surface. The part of metal in contact with the water having the greatest concentration of oxygen acts as a cathode while the part of metal in contact with water having the least concentration acts as an anode and so corrodes most. A drop of water on a steel surface is likely to have a higher concentration of dissolved oxygen near its surface where it is in contact with air than in the centre of the drop. The metal at the centre of the drop acts as an anode and so corrodes most. Fig. 2 below shows the mechanism of cell concentration corrosion. The case of water line corrosion is similar to a tank that is kept only partly filled with water. It corrodes more rapidly than a completely filled one. Corrosion takes place at the water line because of the gradient in oxygen concentration that develops between the solution at the water surface and that at some depth. The greatest attack is just below the water line, where there is a pronounced drop in oxygen concentration as well as a short electron path to the water line, where hydroxyl ions are formed. 8. INTER-GRANULAR CORROSION To understand inter-granular corrosion we have to go back to basic metallurgy. When a molten metal is cast, its solidification begins at many randomly distributed nuclei. Each of these grows in a regular atomic array to form what is known as a grain. The atom arrangements, and the spacings between the layers of atoms, are the same in all the grains of a given metal. However, because of random nucleation, the planes of atoms in neighbouring grains do not match up. The area of mismatch between the grains is usually along the grain boundaries. Grain boundaries are sometimes attacked by corrodent. The attack is usually related to segregation of specific elements or the formation of a compound in the boundary. Corrosion generally occurs because the corrodent attacks the grain boundary phase, or a zone adjacent to it which has lost an element necessary for adequate corrosion resistance. Fig. 2
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~ Page 7 of 73 ~ In a severe case of grain boundary corrosion, entire grains are dislodged due to complete deterioration of their boundaries.
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