Homework 11 - Besides changing the crystal structure to FCC...

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MSE 230 HW11 (due 04/15, 04/16) Spring 2010 1. "Rust" is the solid hydroxide of iron, Fe(OH) 3 , which is only sparingly soluble in water. Write the balanced anode and cathode reactions for the process of iron rusting by reaction with water containing dissolved oxygen. 2. Alloying iron with at least 10 wt.% chromium imparts an almost magical resistance to rusting and forms the basis for the class of steels known as stainless steels. The solubility of Cr in Fe increases from about 5 wt.% at room temperature to complete solid solubility above ~820ºC. (a) Compare Cr and Fe (in table form) in terms of the four atomic characteristics that govern solid solution formation (p. 84 of Callister) and explain why Cr is so soluble in solid iron. (b) Is chromium more or less anodic than iron and by how much? (c) Explain the mechanism by which Cr imparts resistance to rusting of stainless steels. (d) Adding Ni to Fe-Cr alloys produces the “Austenitic” class of stainless steels (see Table 11.4).
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Unformatted text preview: Besides changing the crystal structure to FCC, the Ni improves the corrosion resistance further. Justify the improved corrosion resistance on the basis of the standard emf series. 3. Compare the merits of using tin and zinc as a protective coating on steel for: (a) food cans and (b) outdoor fencing. 4. (a) List the thermoplastics in Table 17.4 that would be suitable for making gasoline tanks. Take gasoline to be octane, C 8 H 10 . (b) Adding the additional constraint that the tanks must be impact resistant down to -20°C, which of the polymer(s) would be suitable? Justify your choice(s) using the glass transition temperature as an approximate measure of the temperature of transition between ductile (tough) and brittle behavior. 5. Briefly explain why silicone rubber has such high resistance to weathering, sunlight, oxidation and ozone compared to most other elastomers. Base your explanation on molecular structure differences....
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