6/2/20161 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

6/2/2016Asyadi2  In metal, corrosion process is normally electrochemistry (a chemical reaction in which there is transfer of electrons from one chemical species to another)  2 reactions that occur during corrosion process: i.Oxidation reaction ii.Reduction reaction 2.1 Oxidation and Reduction Reactions

6/2/2016Asyadi3 i.Oxidation 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 M n+ + ne -

6/2/2016Asyadi4 ii.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 (H 2 ) is evolved)  Cathode is the side at which reduction occurs M + + e - M (n-1)+

6/2/2016Asyadi5  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

6/2/20166  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

6/2/2016Asyadi7 iii.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

6/2/2016Asyadi8  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 + Zn 2+ + H 2 (gas)

6/2/2016Asyadi9 Fig. Reaction of hydrochloric acid with zinc to produce hydrogen gas Chemical reaction: Zn + 2HCl ZnCl 2 + H 2 Ionic form: Zn + 2H + Zn 2+ + H 2 Half- cell reaction: Zn Zn e - (oxidation) 2H + + 2e - H 2 (reduction) Zinc metal hydrochloric acid

6/2/2016Asyadi10  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. 2.2 Standard Electrode Half- Cell Potentials

6/2/2016Asyadi11 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.

6/2/2016Asyadi12 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.

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6/2/2016Asyadi14  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 show the list of the standard half- cell potentials of some selected metals which represents the corrosion tendencies for the several metals 2.2 Standard EMF Series

6/2/2016Asyadi15 Increasingly inert (cathodic) Increasingly active (anodic)

6/2/2016Asyadi16  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) M 1 M n+ + ne - M + + e - M (n-1)+ V1ºV1º V2ºV2º

6/2/2016Asyadi17  Overall cell potential ΔV° is: ΔV cell ° = V° 1 + V° 2

6/2/2016Asyadi18 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

6/2/2016Asyadi19  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 Zn 2+ ions and another of copper immersed in a 1 M solution of Cu 2+ ions with the solutions at 25°C Galvanic Cells With 1 Molar Electrolytes 2.4 Galvanic Cells With 1 Molar Electrolytes

6/2/2016Asyadi20 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.

6/2/2016Asyadi21 Calculation of electrochemical potential of Zn- Cu galvanic cell  From the Standard emf Series: Zn Zn e - E° = V Cu Cu e - E° = V  Oxidation half- cell reaction: (ANODE) Zn Zn e - E° = V° 1  Reduction half- cell reaction: (CATHODE) Cu e - CuE° = V° 2  Overall reaction (by adding): Zn + Cu 2+ Zn 2+ + Cu E° cell = V° 1 + V° 2 = (-0.340) = V

6/2/2016Asyadi22 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: (a)At which electrode does oxidation occur (b)Which electrode is the anode of the cell? (c)Which electrode corrodes? (d)Write the equation for the half- cell reaction at the anode? (e)Write the equation for the half- cell reaction at the cathode? (f)What is the emf of this galvanic cell when the switch is just closed?

6/2/2016Asyadi23 Answer: (a)Oxidation occurs at the zinc electrode since the zinc half- cell reaction has a more negative E° potential of V as compared to V for the nickel half- cell reaction. (b)The zinc electrode is the anode since oxidation occurs at the anode (c)The zinc electrode corrodes since the anode in a galvanic cell corrodes. (d)Zn Zn e - E° = V (e)Ni e - Ni E° =+0.250V (f)The emf of the cell is obtained by adding the half- cell reactions together: Anode reaction:Zn Zn e - E° = V Cathode reaction: Ni e - NiE° = V Overall reaction:Zn + Ni 2+ Zn 2+ + Ni E°cell = V

6/2/2016Asyadi24  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 Galvanic Cells Not 1 Molar Electrolytes 2.5 Galvanic Cells Not 1 Molar Electrolytes

6/2/2016Asyadi25  Nernst equation: E = E° log C ion n Where: E = new emf of half- cell E° = standard emf of half- cell n = number of electrons transferred (for example, M M n+ + ne - ) C ion = molar concentration of ions

6/2/2016Asyadi26 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?

6/2/2016Asyadi27 Answer:  Half cell reactions: Zn Zn e - E° = V (ANODE) Ni Ni e - E° = V (CATHODE)  Apply Nernst Equation: E cell = E° log C ion n Anode reaction: E A = V log 0.10 = V 2 Cathode reaction: E c = - ( V log 0.05) = V 2

6/2/2016Asyadi28 Emf of the cell (E cell ) = E A + E C = V V = V