chap03

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  Graphical representations of thermodynamic and electrochemical equilibria between metal and water, indicating thermodynamically stable phases as a function of electrode potential and pH ;   - predicts the spontaneous direction of reactions. - estimates the composition of corrosion products. - predicts environmental changes that will prevent or reduce corrosion attack. 3.1 M/H2O system   For metal M immersed in the pure water (25°C) in which the metal M is divalent and forms solvated M2+ ions and the simple oxide is MO. M2+ +2e- = M EM2+/M = E°M2+/M + 0.059/2 log aM2+ = E°M2+/M +0.0295 log10-6 (1) MO + 2H++ 2e- = M + H2O EMO/M = E°MO/M + 0.059 log aH+ = E°MO/M - 0.059pH (2) 3. Potential - pH (Pourbaix) Diagrams

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- Water itself is sensitive to electrode potential, decomposing to hydrogen below a potential EH2 given by : 2H+ + 2e- = H2 EH2 = 0.00 - 0.059/2 [log PH2 - log aH+2] = 0.00 - 0.059 pH at 1 atm = -0.03 - 0.059 pH at 10 atm and decomposing to oxygen above EO2 given by : 1/2 O2 + 2H+ + 2e- = H2O EO2 = 1.23V EO2 = 1.23 + 0.059/2 log PO21/2 aH+2 = 1.23 - 0.059 pH at PO2= 1atm = 1.23 + 0.015 - 0.059 pH at PO2 =10 atm - It is evident that the increase of pressure increases the range of stability of water

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3.2 General equil. reactions in Metal/H2O system aR + bH+ + ne- = pD + wH2O G = G° + RT ln (aDp/aRa aH+b) Case 1 : for straight chemical reaction, n =0 at equil., 0 = G° + RT ln (aDp/aRa aH+b).....vertical line on E - pH diagrams. Case 2 : for electrochemical reaction, n  0 i) if b = 0, aH+b does not exist ; -nFE = -nFE° + RT ln (aDp/aRa) or E = E° - RT/nF ln (aDp/aRa) : horizontal line ii) if b  0 : sloping line

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3.3 Pourbaix diagram of Fe-H2O system 25°C Fe2+, FeO, Fe2O3, Fe3O4,Fe(OH)2, Fe(OH)3. Fe3+ H2, O2 H+, OH- 1) Fe2+ + 2e- = Fe E = E°+ 0.059/2 log aFe2+, E = -0.44 + 0.059/2 log 10-6 = -0.62 V   2) Fe3+ + e- = Fe2+ E = E° - 0.059 log (aFe2+/aFe3+) assuming that aFe2+ = aFe3+ = 10-6 , E = 0.77 V 3) Fe2O3 + 6H+ + 2e- = 2Fe2+ + 3H2O E° = -G°/2F = -(2G°Fe2+ + 3G°H2O - G°Fe2O3)/2F = -{2[-20300] + 3[-56700] - [-177103]}/2x(-23604) = 0.728 V E = E°-0.059/2 log {(aFe2+2 aH2O3)/(aFe2O3 aH+6) = 0.728 - 0.059/2 x (-12) - 0.177 pH = 1.082 - 0.177 pH.............sloping line using the data : G°Fe2O3 = -177,103 cal/mole G°Fe2+ = -20,300 cal/mole G°H2O = -56700 cal/mole

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4) Fe2O3 + 6H+ = 2Fe3+ + 3H2O (straight chemical reaction or no charge involved.) G = G°+ 2.303 RT log (aFe3+2/aH+6) = 0...at equil. pH = 1.76 5) Fe3O4 + 8H+ + 2e- = 3Fe2+ + 4H2O E°= 0.98 V, G°Fe3O4 = -238,500 cal/mol E = 1.511 - 0.236 pH 6) Fe3O4 + 8H+ +8e- = 3Fe + 4H2O E = -0.0636 - 0.059 pH 7) HFeO2- + 3H+ + 2e- = Fe + 2H2O E = 0.493 - 0.0887 pH 8) Fe3O4 + 2H2O +2e- = 3HFeO2- + H+ E = -1.82 + 0.0295 pH 9) Fe2O3 + H2O +2e- = 2HFeO2- E = constant   10) 6Fe2O3 + 4H+ + 4e- = 4Fe3O4 + 2H2O E = 0.0583 - 0.059 pH

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 Confusion in the interpretation of E-pH diagrams The terms of corrosion and passivity in the E-pH diagram are strictly kinetics in nature, and thus should not be used to describe regions of a equilibrium diagram. The problem is best illustrated by corrosion product film on the surface. However, passivity will be achieved only if the film is protective. Only "immunity" has a practical significance in terms of thermodynamics, since in this region the metal cannot corrode, regardless of time.  Corrosion control using Pourbaix diagram For the state point A in Pourbaix diagram for iron, 1) Increase pH ----- passivation 2) Remove O2 or deaeration------- Reduction in cathodic reaction rate leads to simultaneous reduction in anodic reaction rate of metals (corrosion rate). 3) Decrease E to immunity region------ Cathodic protection. 4) Increase E at the same pH----- Anodic protection depending on the protectiveness of passive film.

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3.4 Dependence of electrochemical reaction direction on electrode potential

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3.5 Determination of E-pH diagram by potentiokinetic experiments

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3.6 E - pH diagram of Fe-H2O system at high temperature The principal effects of increasing temperature : 1. The decrease in the thermodynamic stability of water 2, An expansion in the stability domain of the Fe(III) oxyanion(HFeO2-) at high pH 3. The shift in the predominance domains for Fe2+ and Fe3+ to lower values 4. The increased stability of anionic species at the expanse of cationic species at elevated temperatures is a fairly general phenomenon for metal-water system.

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3.7 Resistance of metals to pure water Generally, the metals showing perfect resistance to pure water at 25 C will be those having an E - pH diagram on which the perpendicular from pH=7 cross only the immunity or passivation domains at potentials between -0.8 and +0.7 V. The metals that can be passivated by oxidation and activated by reduction are those which have a higher oxide less soluble than a lower oxide and thus will present a triangular corrosion domain. Pourbaix diagram for aluminium Al Is highly corrosion resistant in neutral solution. A decrease in the electrode potential into the region of immunity (cathodic protection)in closed system may leads to alkaline corrosion. 2H+ + 2e-  H2 ( pH ) Al + 2OH- AlO2- + H2 +e-

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Pourbaix diagram for copper Based on the data in table 1, draw the E- pH relationships for the equilibrium reaction occurring on the surface in pure water at 25°C and plot the Pourbaix diagram of Cu/H2O Table 1. Standard free energies G° in cal./mole of the various chemical species involved in the Cu-H2O system. Copper will not be corroded in deaerated water since the value of Ecu2+/cu is higher than that of EH+/H2. Thus Cu is not corroded in nonoxidizing acids like dil. HCl and H2SO4 etc. But in oxidizing acids like HNO3, Cu corrodes. The addition of oxidizing species Fe3+ to nonoxidizing acid can give rise to severe corrosion attack.

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Pourbaix diagrams of Ti Ti is thermodynamically reactive, but is nevertheless very corrosion resistant because of a highly resistant passive film that is stable at all pHs in oxidizing potentials. It corrodes only at low pH in solutions without oxidizers. Ti does not corrode in HNO3 due to the oxidising nature of HNO3, but corrodes in dilute HCl and H2SO4. By adding some oxidising agent such as Fe3+, corrosion reaction of Ti in HCl or H2SO4 can be reduced.

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3.8 Limitation of Pourbaix Diagram Tell us what can happen, not necessarily what will happen. No information on the rate of reaction. limited to pure elements, pure water and 25°C. passivation criteria.