ECHE 571/Fall 2010
PASSIVITY 1
Problem Set #7
1. If copper and iron are in contact determine:
a) Which meal will corrode
b) If the corrosion rate is 2.4x10-7 cm/s calculate the minimum cathodic current which must be applied to the couple in order to passivate the metal (to stopt the corrosion).Take the area of Cu=4.5 dm2 and area if Fe=0.8 dm2
Hint: Cathodic Current needed= Area x I C.R.=0.129
where M is molecular weight, n=# of electrons and =s. gravity
c) Calculate the minimum current which must be applied to avoid copper dissolution in 1 M H2SO4 if [Cu2+]= 1M, pH=0, T=25oC, IoCu=0.4 A/m2, anodic Tafel Slope = 0.0227 =IoH=1.2x10-6 A/m2 and cathodic Tafel slope = -0.059.
2. From the standard Flade potential for iron, calculate the apparent free energy of formation of the passive film per gram atom of oxygen. Similarly for nickel and chromium.
Given:
Hint: Assume that the reaction that occurs on the surface takes the following form:
M+H2O
The Flade potential is equal to the equilibrium potential for this reaction
3. Corrosion of passive metals occurs by pitting in chloride solutions. This pitting can be prevented by the addition of anionic inhibitors. At a critical ratio of adsorbed Cl- to adsorbed anion, the passive film is displaced by Cl-, allowing a pit to initiate.
a) Derive an equation relating the minimum amount of anion necessary to inhibit pitting of a passive metal in chloride solutions to the actiovity of chloride ions (Cl-).
Hint: assume the amount of ion adsorbed per unit area follows the Freundlich adsorption isotherm.
b) A set of experimental data relating the critical activity of nitrate anions inhibiting pitting of 18-8 stainlsess steel in FeCl3 solutions andchloride activity is given bellow. Check if these experimental data fit the equation derived by you in (a)
Activity of nitrate anions / Activity of chloride ions0.049 / 0.069
0.103 / 0.265
0.16 / 0.62
0.27 / 1.23
c) relate the critical ratio of adsorbed Cl- to adsorbed anion to the Freundlich adsorption isotherm constants.
4. Calculate the Flade potential at 25oC for iron for the following passivation reaction:
M +H2O= OM +2H+ +2e-
Assume that the film substance represented schematically as Om in the above example is (a) Fe2O3, (b) Fe3O4, (chemisorbed oxygen).
Hint: Go for formation for Fe2O3 = -177.1 kcal/mole; for Fe3O4=-242.4 kcal/mole; for H2O (l) =-75 kcal/mole O2; So=-46.2 cal/mol O2/oC (estimated from value for chemisorbed N2)
5. From the standard Flade potential for iron, calculate the free energy of formation of the passive film per gram atom of oxygen. Similarly for nickel and chromium.
6. Given active-passive alloys A and B having the following electro-chemical parameters:
Icorr, A / ba, V / Epp, V / Ipass, (A) / Ecorr / Etr AAlloy A / 1x10-6 / +0.1 / 0 / 1x10-5 / -0.400 / +0.7
Alloy B / 7x10-7 / +0.1 / +0.3 / 1x10-6 / -0.200 / +1.2
a) Which will be more corrosion resistant in reducing conditions (active state )?
b) Which will be the more corrosion resistant in the passive state? Why
c) Which is more easily passivated by dissolved oxidizers? Why
d) Which would be more easily protected by anodic protection? Why
7. (a) Use the equation relating the pH to the critical current density necessary to passivity metals in the presence of oxygen (given in Uhligs' Ref. J. Electrochem. Soc., 108, No 4, (1961) to estimate
a) The dependence of pH (crit) vs the concentration of the dissolved oxygen.
b) The dependence of pH (crit) to the critical current density for different diffusion layer thicknesses.
c) The dependence of the critical current density on the pH and to the limiting diffusion current for reduction of oxygen.
The anodic exchange current density is dependent on the hydrogen ion activity k (H+)- where k and are constants. Further the anodic concentration polarization is given by:
= )
where is the activity of the metal ions in the bulk of the solution, D is the diffusion coefficient of the action, is diffusion layer thickness and t+ is the transference number fro the metal ion.
: where and all other constants are defined in the above Ref.
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