Corrosion Principles: Oxidation and Reduction Reactions

1. CHAPTER 2 CORROSION PRINCIPLES Chapter Outlines 2.1 Oxidation and Reduction Reactions 2.2 Standard Electrode Half- Cell Potentials 2.3 Standard EMF Series 2.4 Galvanic Cells With 1 Molar Electrolytes 2.5 Galvanic Cells Not 1Molar Electrolytes

2. 2.1 Oxidation and Reduction Reactions • In metal, corrosion process is normally electrochemical @ electrochemistry (a chemical reaction in which there is transfer of electrons from one chemical species to another) • 2 reactions that occur during corrosion process: • Oxidation reaction • Reduction reaction Asyadi

3. Oxidation reaction @ anodic reaction • Definition: the removal of one or more electrons from an atom, ion or molecule • Equation: (in which M becomes an n+ positively charged ion and in the process loses its n valence electrons; e- is used to symbolize an electron) • Example: • Anode is the side at which oxidation takes place. M Mn+ + ne- Asyadi

4. Reduction reaction • Definition: the addition of one or more electrons to an atom, ion or molecule (because the electrons generated from each metal atom that is oxidized must be transferred to and become a part of another chemical species = reduction reaction) • Equation: (some metals undergo corrosion in acid solutions, which have a high concentration of hydrogen (H+) and hydrogen gas (H2) is evolved) • Cathode is the side at which reduction occurs M+ + e- M(n-1)+ Asyadi

5. There are 3 possibilities of reaction that can occur at cathode (reduction): • First possibilities Cathodic half- cell reaction : Condition : if the electrolyte is an acid solution Reaction : hydrogen ions in the acid solution will be reduced to hydrogen atom to form diatomic hydrogen gas • Second possibilities Cathodic half- cell reaction : Condition : if the electrolyte also contain oxidizing agent Reaction : oxygen will combine with hydrogen ions to form water molecules Asyadi

6. Third possibilities Cathodic half- cell reaction : Condition : if the electrolyte is basic or neutral and oxygen is present Reaction : oxygen and water molecules will react to form hydroxyl ions

7. Overall Electrochemical Reaction • Consist of at least one oxidation (half reaction) and one reduction (half reaction), and will be the sum of them • Example: (Zinc metal immersed in an acid solution) At some regions on the metal surface, zinc will experience oxidation or corrosion Asyadi

8. Oxidation half reaction: Since Zn is a metal and good electrical conductor, these electrons may be transferred to an adjacent region at which the H+ ions are reduced. • Reduction half reaction: • Total electrochemical reaction: Zn + 2H+ Zn2+ + H2 (gas) Asyadi

9. Chemical reaction: Zn + 2HCl ZnCl2 + H2 Ionic form: Zn + 2H+ Zn2+ + H2 Half- cell reaction: Zn Zn2+ + 2e- (oxidation) 2H+ + 2e- H2 (reduction) Zinc metal hydrochloric acid Fig. Reaction of hydrochloric acid with zinc to produce hydrogen gas Asyadi

10. 2.2 Standard Electrode Half- Cell Potentials • Every metal has a different tendency to corrode in a particular environment • E.g. ‘zinc is chemically attacked or corroded by dilute hydrochloric acid, whereas gold is not’ • Method for comparing the tendency for metals to form ions in aqueous solution is to compare their half- cell oxidation or reduction potentials (voltages) to a standard hydrogen- hydrogen ion half- cell potential. Asyadi

11. Experimental Setup for the Determination of Half- cell Standard Electrode Potentials Experimental setup for the determination of the standard emf of zinc. In a beaker a Zn Electrode is placed in a solution of 1 M Zn2+ ions. In the other there is a standard hydrogen reference electrode consisting of a platinum electrode immersed in a solution of 1 MH+ ions which contains H2 gas at 1 atm. Asyadi

12. Standard Hydrogen Electrode • Represent only differences in electrical potential and thus it is convenient to establish a reference point/ reference cell to which other cell halves may be compared. • It consist of an inert platinum electrode in a 1M solution of H+ ions, saturated with hydrogen gas that is bubbled through the solution at a pressure of 1 atm and temperature of 25°C. • The platinum itself does not take part in the electrochemical reaction: it acts only as a surface on which hydrogen atoms may be oxidized or hydrogen ions may be reduced. Asyadi

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14. 2.2 Standard EMF Series • Electromotive force (EMF) series: is generated by coupling to the standard hydrogen electrode, std half- cells for various metals and ranking them according to measured voltage. • Table 17.1- show the list of the standard half- cell potentials of some selected metals which represents the corrosion tendencies for the several metals Asyadi

15. Increasingly inert (cathodic) Increasingly active (anodic) Asyadi

16. those metals at the top (Au & Pt) --- are noble or chemically inert • Moving down --- metals become increasingly more active (more susceptible to oxidation) (sodium & potassium) • The voltages --- are for the half- reactions oxidation reaction: electron on the right hand side reduction reaction: electron on the left hand side (sign of the voltage changed) V1º M1 Mn+ + ne- V2º M+ + e- M(n-1)+ Asyadi

17. Overall cell potential ΔV° is: ΔVcell° = V° 1 + V° 2 Asyadi

18. Galvanic couple: Two metals electrically connected in a liquid electrolyte wherein one metal becomes anode and corrodes, while the other acts as a cathode GALVANIC CELLS Asyadi

19. 2.4Galvanic Cells With 1 Molar Electrolytes • Can be constructed with two dissimilar metal electrodes each immersed in a 1M solution of their own ions • The two solutions are separated by a porous wall to prevent their mechanical mixing, and an external wire in series with a switch and a voltmeter connects the two electrodes • E.g.: zinc electrode immersed in a 1 M solution of Zn2+ ions and another of copper immersed in a 1 M solution of Cu2+ ions with the solutions at 25°C Asyadi

20. A macroscopic galvanic cell with zinc and copper electrodes. When the switch is closed and the electrons flow, the voltage difference between the zinc and copper electrodes is -1.10V. The zinc electrode is the anode of the cell and corrodes. Asyadi

21. Calculation of electrochemical potential of Zn- Cu galvanic cell • From the Standard emf Series: Zn Zn2+ + 2e- E° = -0.763 V Cu Cu2+ + 2e- E° = +0.340 V • Oxidation half- cell reaction: (ANODE) Zn Zn2+ + 2e- E° = -0.763 V°1 • Reduction half- cell reaction: (CATHODE) Cu2+ + 2e- Cu E° = -0.340 V°2 • Overall reaction (by adding): Zn + Cu2+ Zn2+ + Cu E°cell = V°1 + V°2 = -0.763 + (-0.340) = -1.103 V Asyadi

22. Problem 1: A galvanic cell consist of an electrode of zinc in a 1M ZnSO4 solution and another of nickel in a 1 M NiSO4 solution. The two electrodes are separated by a porous wall so that mixing of the solutions is prevented. An external wire with a switch connects the two electrodes. When the switch is just closed: • At which electrode does oxidation occur • Which electrode is the anode of the cell? • Which electrode corrodes? • Write the equation for the half- cell reaction at the anode? • Write the equation for the half- cell reaction at the cathode? • What is the emf of this galvanic cell when the switch is just closed? Asyadi

23. Answer: • Oxidation occurs at the zinc electrode since the zinc half- cell reaction has a more negative E° potential of -0.763 V as compared to -0.250 V for the nickel half- cell reaction. • The zinc electrode is the anode since oxidation occurs at the anode • The zinc electrode corrodes since the anode in a galvanic cell corrodes. • Zn Zn2+ + 2e- E° = -0.763V • Ni2+ + 2e- Ni E° =+0.250V • The emf of the cell is obtained by adding the half- cell reactions together: Anode reaction: Zn Zn2+ + 2e- E° = -0.763 V Cathode reaction: Ni2+ + 2e- Ni E° = +0.250 V Overall reaction:Zn + Ni2+ Zn2+ + Ni E°cell = -0.513 V Asyadi

24. 2.5Galvanic Cells Not 1 Molar Electrolytes • Most electrolytes for real corrosion galvanic cells are not 1 M, but are usually dilute solutions that are much lower than 1 M. • If the concentration of the ions in an electrolyte surrounding an anodic electrode is less than 1 M, the driving force for the reaction to dissolve or corrode the anode is greater since there is a lower concentration of ions to cause the reverse reaction Asyadi

25. Nernst equation: E = E° + 0.0592 log Cion n Where: E = new emf of half- cell E° = standard emf of half- cell n = number of electrons transferred (for example, M Mn+ + ne-) Cion = molar concentration of ions Asyadi

26. Problem 2: A galvanic cell at 25ºC consist of an electrode of zinc in a 0.10 M ZnSO4 solution and another of nickel in a 0.05 M NiSO4 solution. The two electrodes are separated by a porous wall and connected by an external wire. What is the emf of the cell when a switch between the two electrodes is just closed? Asyadi

27. Answer: • Half cell reactions: Zn Zn2+ + 2e- E° = -0.763V (ANODE) Ni Ni2+ + 2e- E° = -0.250V (CATHODE) • Apply Nernst Equation: Ecell = E° + 0.0592 log Cion n Anode reaction: EA= -0.763 V + 0.0592 log 0.10 = -0.793 V 2 Cathode reaction: Ec= - (- 0.250 V + 0.0592 log 0.05) = +0.289 V 2 Asyadi

28. Emf of the cell (Ecell) = EA + EC = -0.793V + 0.289 V = -0.505 V Asyadi