Material Science > QUESTIONS & ANSWERS > Practice Exam 3.21 - Stony Brook University ESG 332 (All)

Practice Exam 3.21 - Stony Brook University ESG 332

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1) Define the following terms and provided illustrations to support your answer: a) anodic and cathodic processes in aqueous corrosion b) pitting corrosion c) galvanic corrosion d) intergranular corrosion e) atmospheric oxidation of metals (25pts) a) anodic and cathodic processes in aqueous corrosion The anodic process involves the oxidation of a metal layer that is more susceptible to corrosion in an aqueous environment than the metal that it is covering. The oxidation supplies electrons to the underlying metal, making it a cathode and protecting it from corrosion. b) pitting corrosion Pitting is a form of very localized corrosion attack in which small pits or holes form. Oxidation occurs within the pit itself, with complementary reduction at the surface. It is supposed that gravity causes the pits to grow downward, the solution at the pit tip becoming more concentrated and dense as pit growth progresses. c) galvanic corrosion Galvanic corrosion occurs when two metals or alloys having different compositions are electrically coupled while exposed to an electrolyte. The less noble or more reactive metal in the particular environment will experience corrosion (i.e., oxidation); the more inert metal, the cathode, will be protected from corrosion (experience reduction). anode cathode Intergranular corrosion is corrosion that occurs preferentially along grain boundaries for some alloys and in specific environments. The net result is that a macroscopic specimen disintegrates along its grain boundaries. This situation can happen in otherwise corrosion-resistant alloys, when the grain boundaries are depleted of the corrosion-inhibiting elements. This is known as grain boundary depletion. A precipitate containing the corrosion-resistant element diffuses away from the bulk and to the grain boundary and forms a precipitate along the grain boundary. As a result, depleted zones form adjacent to the grain boundaries (this process is called sensitization). These depleted zones are no longer composed of the corrosion-resistant element, making these regions around the grain boundary highly susceptible to corrosion. d) intergranular corrosion e) atmospheric oxidation of metals Metals experience corrosion when exposed to ambient gases. Generally, metals would undergo oxidation reactions when exposed to oxygen and water vapor, while oxygen is reduced. As a result, a precipitate forms at the interface of the metal and air. E.g., Iron exposed to air 1) 2Fe + O2 + 2H2O → 2Fe(OH)2 2) 2Fe(OH)2 + ½ O2 + H2O → 2Fe(OH)3 and 4Fe + 3O2 → 2Fe2O3 2) a) What is the Galvanic series and explain how it is used in engineering design of a metal can used for food containment? (12pts) b) What factors influence conductivity in metals and alloys? (13pts) 2a) What is the Galvanic series and explain how it is used in engineering design of a metal can used for food containment? (12pts) Galvanic series- A ranking of metals and alloys as to their relative electrochemical reactivity in seawater For food containment purposes, a metal can will be exposed to salt solutions, namely those with relatively high concentrations of sodium chloride. This environment is similar to seawater, which frequently produce pitting and crevice corrosion. To avoid corrosion within the metal can, one should look at the galvanic series for the most

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