Lecture 28: Kinetics of Oxidation of Metals: Part 1: rusting, corrosion, and

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Lecture 8: Kinetics of xidation of etals: Part 1: rusting, corrosion, and the surface rotection, all about chemistry Today s toics hemical rocesses of oxidation of metals: the role layed by oxygen.

1 Lecture 8: Kinetics of xidation of etals: Part 1: rusting, corrosion, and the surface rotection, all about chemistry Today s toics hemical rocesses of oxidation of metals: the role layed by oxygen. How to inhibit the oxidation deends on how effectively break u the contact between oxygen and the metals: the ways of anti-rusting and anti-corrosion. The kinetics of oxidation over the surface layer of metals can be described by the diffusion of metal cations ( n+ ) and oxide anions ( - ): diffusion flux as learned from Lecture 7. Understand how the Nernst otential forms between the outside and inner side of the metal oxide layer: solely caused by the difference in artial ressure of oxygen. Basics of oxidation of metals: () air x : etal; : metal oxide The initial stages of oxidation include oxygen adsortion and incororation to the metal surface, oxide islands nucleation, and growth into a continuous film. Such a rocess reresents a secial case of hase transition that involves not just hysical roerty change (as tyical observed for most of the hase transformations we learned so far), but ermanent chemical change of the comonents. Further oxidation requires the transort of either oxygen through the film to the metal/metal oxide interface, or that of the metal to the metal oxide/air interface. Thus, it is readily to see that the diffusion of metal and/or oxygen through the oxide film must somehow determine the kinetics of oxidation. The oxidation rocesses have been actively studied and are now understood relatively well. The kinetics of thick (> Å) oxide films aroximately follows the arabolic law. t can be described, at least qualitatively, by the Wagner model of oxidation, in which diffusion across the oxide film is the rate limiting rocess. However, the situation is much more comlicated in the case of very thin (<1 Å) films. The kinetic equations in this region cannot be integrated exactly, and only aroximations for the oxide growth rate as a function of time can be obtained. etal oxidation rocesses are extensively used in technology for rotection of materials against rusting and corrosion, for which two major rotection ways are mentioned here, for examle, 1) rotective coatings: 1

2 e.g., lating, ainting, and cladding, roviding a barrier of corrosion-resistant material between the air or water (containing oxygen) and the metal surface --- reventing the surface contact with oxygen. ) corrosion inhibitors: one such examle is acting as oxygen scavenger, for examle hydrazine is a strong reducing agent, it is used as an oxygen scavenger to revent corrosion in boiler water and hot-water heating systems. Note: The bonding in most oxides is ionic, although some oxides do exhibit artially covalent bonding. For simlicity, let s assume ionic bonding for the metal oxide we will treat below. Thus, it is imerative that the diffusing secies are electrically charged, i.e., + and/or -, rather than the neutral atoms of metal or molecules of oxygen. Air () / + e h + e e +V, Nernst Potential, created by difference in artial ressure of xygen Where h + stands for hole, ositive charge. At the /air interface, the chemical otential of oxygen is given by µ = µ + RT ln where is the oxygen artial ressure in air. At the / interface, the equilibrium / is established, so, () Then, µ + µ = µ Assuming and to be ure (i.e., concentration is constant, and defined as unit), we have µ = µ + RT ln = µ µ = µ + RT ln = µ Also, µ = µ + RT ln where () () () is the oxygen artial ressure at the / interface. Then, 1 () µ + µ = µ 1 1 µ + µ + RT ln = µ ()

3 r, r, As 1 () 1 RT ln = µ ( ) µ µ = G < G ex[ ] RT () = <, the chemical otential of oxygen at the / interface, () µ, is lower than that at the () /air interface, µ. That is, µ µ <. () Also, µ = µ > µ = µ + RT ln, where () << 1 Thus, oxygen transort is favored from air/ interface to / interface, and metal transort is favored form / interface to air/ interface. Since the bonding of is ionic, transort of and must occur as + and -, as deicted in the diagram above. We can see that there is a gradient in (with air as the atmoshere), and at the / interface P otential across the film given by across the oxide film, where at the air/ interface P.1atm () () << 1 atm. This creates a so-called Nernst RT P V = ln{ } P. This otential (with the air/ side more ositive) reresents a chemical equilibrium established by the reverse diffusion of + and - as deicted in the diagram above. Transort of charged secies occurs down an electrochemical otential gradient ( d η ). Then we have the diffusion flux for the two ions (Fick s first Law), as we learned from Lectures 5 and 9, D + dη + D + dη + + = + (in unit er atom), or + = + (er mole) k T RT B D dη = (in unit er atom), or kbt D dη = (er mole) RT concentration mobility where η 's are the electrochemical otentials of the ions as defined below: η = µ + Φ = + FΦ (er mole) --- as we learned in Lecture ZF µ + Where Φ electrostatic otential, Z is the valence, F is Faraday constant = coulombs/mole = e N =

4 Similarly, η = µ Φ (valence of - is -) F Note: the concentration of + and - are uniform, constant throughout the oxide film. Thus, we have dµ + dµ ; d η d F d F d + µ + Φ Φ = + = Similarly, d η =F Now by substituting the electrochemical otential gradient ( d η ) back to the diffusion flux for the two ions as listed above: We have, D = F (er mole) RT D =+ F (er mole) RT Since the secies are charged, when they transort, there must be electric current generated by each of the ions as defined below: = e ; =e + + Where, e = electronic charge = F/N, N is the Avogadro s constant, #/mole. So, + D d RTN + + Φ = (amere /cm ) D = RTN Now, oxygen ions ( - ) move from /air interface to / interface (), and + move from () to. Thus, both of the electrical currents due to the transort of ions are of the same sign. We know that, Also, we have E = electric field, as also learned in Lectures 9 and 3. 4

5 = D ; σ = D defined as ionic conductivities (ohm -1.cm -1 ). RTN σ RTN Thus, + = σ + E; and = σ E The total ionic current density caused by the diffusion of both ions is: = + ion + = ( σ + ) + σ E = σ E = σ ion ion Assuming if only ionic current flows through the oxide film, the transort will quickly sto as soon as enough + ions are transorted to /air interface, and/or enough - ions are transorted to / interface to RT P reach the equilibrium as defined by the Nernst Potential V = ln{ } () P as mentioned above. However, that the oxide film also conducts electrons and/or electron holes, and flow of these electrons and holes neutralize the flow of the + and - ions --- the overall result combining these two airs of diffusion is that moves to the / interface and moves to the air/ interface, to kee the oxide layer growing and maintain the Nernst Potential (i.e., the thermodynamic driving force for the hase growth) until the oxide layer becomes too thick, i.e., the diffusion across the whole layer takes forever (now becoming Kinetics limited). --- details to be continued in next Lecture. 5

Lecture 29: Kinetics of Oxidation of Metals: Part 2: Wagner Parabolic

Lecture 29: Kinetics of Oxidation of Metals: Part 2: Wagner Parabolic Lecture 9: Kinetics of Oxidat of Metals: Part : Wagner Parabolic Mod Today s topics Oxidat of metals: controlled by both the ic diffus (carried by M + and O - ) and the ectronic diffus (carried by e -