4324q5fall2004_key4324

4324q5fall2004_key4324 - Quiz 5 EMA 4324, Stability of...

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Quiz 5 EMA 4324, Stability of Materials, LecSec 3009 KEY Monday, November 22, 2004 Electrochemical data for the JRA - H 2 O system are presented on the following table: 1. [10 points] The corrosion potential and corrosion current density for a 1 cm 2 area of JRA exposed to an oxygenated solution of pH 4 [p(O 2 ) = 1 atm] were determined by Tafel extrapolation to be -0.138 v and 1.97x10 -4 A/cm 2 , respectively. The figure below is an LPR [linear polarization resistance] plot for JRA in the same environment. Calculate polarization resistance [R p ] and corrosion current density for JRA and compare with that obtained by Tafel extrapolation. j corr = B/R p ; B = 0.15*0.1/[2.3(0.15+0.1)] = 0.026; j corr = 0.026/126.6 = 2.05x10 -4 A/cm 2 , pretty doggone close! 0.10 1.0x10 -15 [on JRA] 0.993 O 2 + 2H 2 O + 4e - = 4OH - 0.15 5x10 -7 -0.528 J +2 + 2e - = J Tafel constant, β , volts exchange current density, j o , A/cm 2 equilibrium [Nernst] potential, volts SHE process E o [J +2 /J] = -0.350 v; [J +2 ] = 10 -6 M; assume all j o values to have been determined at respective E N values; solution pH = 4 -0.170 -0.155 -0.140 -0.125 -0.110 -0.095 electrode potential, volts SHE -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 current, milliamperes [A/1000] R R p p = = E/ E/ I = 30/0.237 I = 30/0.237 = 126.6 ohm = 126.6 ohm
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2. [10 points] Briefly explain the limitations of the LPR method in determining experimental values for corrosion current density. From text:
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4324q5fall2004_key4324 - Quiz 5 EMA 4324, Stability of...

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