16 Briefly describe the techniques generally used to control corrosion of

16 briefly describe the techniques generally used to

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16. Briefly describe the techniques generally used to control corrosion of metals. 17. Define corrosion and briefly describe two (2) different mechanisms of corrosion. 18. Name the type of corrosion which may take place even at room temperature. 19. (a) Discuss the electro-chemical phenomenon of corrosion. 20. (b) in light of your discussion in (a), describe: (i) cathodic protection; and (ii) anodic protection. 21. Explain the effect of pH value on corrosion. 22. Describe the criteria to be used to reduce corrosion when two dissimilar metals are joined together. 23. Recommend a material to be used to contain 70% sulphuric acid temperatures between 40 and 90 °C. 24. Describe the method of galvanic protection for a steel water storage tank. 25. Explain why: (a) The deaeration of water in a boiler reduces corrosion.(b) Food cans cannot be made of galvanised steel.(c) Zinc or magnesium is fastened to a ship hull. 26.
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~ Page 13 of 73 ~ RECRYSTALLISATION Recrystallization is a process by which deformed grains are replaced by a new set of undeformed grains that nucleate and grow until the original grains have been entirely consumed. Recrystallization is usually accompanied by a reduction in the strength and hardness of a material and a simultaneous increase in the ductility . Thus, the process may be introduced as a deliberate step in metal processing or may be an undesirable byproduct of another processing step. The most important industrial uses are the softening of metals previously hardened by cold work , which have lost their ductility, and the control of the grain structure in the final product. During cold-working processes, the grain of the metal becomes distorted and internal stresses are introduced into the metal. If the temperature of the cold-worked metal is now raised sufficiently, nucleation occurs and ‘seed’ crystals form at the grain boundaries at points of maximum internal stress. The more severe the cold working and the greater the internal stress, the lower will be the temperature at which nucleation occurs for a given metal. The principle of nucleation is shown in Fig. 1. The minimum temperature at which the reformation of the grain occurs is called the temperature of recrystallisation . At temperatures above the recrystallisation temperature, the kinetic energy of the atoms on the edges of the distorted grains increases. This allows these edge atoms to move away and attach themselves to the newly formed nuclei which will then begin to grow into grains. This process continues until all the atoms of the original, distorted crystals have been transferred.
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~ Page 14 of 73 ~ Since, after severe cold working, more nuclei form than the number of original grains, the grain structure after recrystallisation is usually finer than the original grain structure before cold working.
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