Fall 2011 Corrosion Rate

1. Fall 2011 Corrosion Rate

2. THERMODYNAMICS OF CORROSION As you learned in Chemistry there is a free energy change with reactions. This change in free energy must be negative for the reaction to be spontaneous. There are two reactions involved in the corrosion process; the total free energy change must be negative for corrosion to occur. These reactions are an oxidation and a reduction. Oxidation occurs at the anode Mg = Mg+2 + 2e- Reduction occurs at the cathode 2H+ +2e- = H2 For a total reaction: Mg +2H+ = Mg+2 +H2 The Free energy can be calculated by: DG=-nFDE Where: n is the number of electons exchanged ( two in the above example) F is the Faraday Constant (96500 Coulombs/equivalent) E corresponds to the energy change in the reaction.

3. Each reaction ( anodic and cathodic) has a half-cell potential associated with it: given as- E* The sum of the anodic and cathodic half-cell potentials is the total potential: given as- E* Further considering concentration effects of the hydrogen ion and reaction species concentration yields the Nerst Equation E=Eo-(RT)/(nF)*ln{[B]b [H2O]d}/{[A]a[H+]m} Where: Eo is the defined value for the potential referenced to a standard* R is the gas constant T is temperature n and F are described above The terms of within the natural log are the concentrations of the products over the reactants. That was certainly brief and very cursory but the important thing to take away are the types of things that can play a role in the corrosion reactions. Hopefully you have seen most of this in a freshman chemistry course. * The reference is the standard hydrogen electrode (SHE) which is defined as Eo=0 Volts.

4. Summary of Electrochemical Theory Electrochemical Reactions An electrochemical reaction is a reaction involving the transfer of charge as a part of a chemical reaction. Typical electrochemical reactions in corrosion are metal dissolution and oxygen reduction: In contrast a chemical reaction, such as the precipitation of a metal hydroxide, does not involve a transfer of charge: Faraday's Law Faraday's Law relates the amount of charge involved in an electrochemical reaction with the number of moles of reactant reacting and the number of electrons required for the reaction. In addition to Faradaic processes that obey Faraday's Law, non-Faradaic processes may also occur. Typically these are processes such as adsorption that do not involve a complete transfer of charge from the solution to the metal.

5. Electrochemical Half Cells A half cell is an electrochemical reaction that results in a net surplus or deficit of electrons, and it corresponds to the smallest complete reaction sequence (while it may proceed as a sequence of simpler reactions, the intermediate stages are not stable). Oxidation or anodic reactions are those that result in a surplus of electrons, and for corrosion these typically correspond to the various metal dissolution reactions, such as: Reduction or cathodic reactions result in the consumption of electrons, and for corrosion these typically correspond to the oxygen reduction or hydrogen evolution reactions: Note that the above reactions have been shown going in one direction only. While the reverse reactions are perfectly possible, they reverse of an anodic reaction is a cathodic reaction and vice versa.

16. Kinetics of Corrosion Thermodynamics are fundamental and show that corroison will occur in most environments. It is also important to know how fast corrosion will occur. As shown in the thermodyanmics, the the reactions in the corrosive process produce and consume electrons. Electron flow can be quantified as current; Faraday first found the relationship between electrons exchanged and mass reacted: m=(Ita)/(nF) m= mass reacted I=Current t=time A=atomic weight n=number of equivalents F= Faraday's Constant (96500 Coulombs/equivalent) To find the corrosion rate (and equalize for area) divide through by time and Area yielding:

17. CR=(ia)/nF CR=corrosion rate i= current density Each reaction has a characteristic equilibrium current density this is termed io Electrochemical Polarization is a method used to better understand the corrosion behavior of various materials by varying the potential above or below the equilibrium potential (E). This polarization drives the reaction either in the noble ( anodic) or active ( cathodic direction). While carrying the potential the current produced is recorded, as can be seen in the equation above the higher the current density, the higher the corrosion rate. Discuss Chapt 3, Table

20. Polarization Behavior Metallic surfaces can be polarized by the application of an external voltage or by the spontaneous production of a voltage away from equilibrium. This deviation from equilibrium potential is called polarization. The magnitude of polarization is usually described as an overvoltage (h) which is a measure of polarization with respect to the equilibrium potential (Eeq) of an electrode. This polarization is said to be either anodic, when the anodic processes on the electrode are accelerated by changing the specimen potential in the positive (noble) direction or cathodic when the cathodic processes are accelerated by moving the potential in the negative (active) direction. There are three distinct types of polarization in any electrochemical cell, the total polarization across an electrochemical cell being the summation of the individual elements: E(applied) - Eeq = htotal = hact +hconc +iR�� (see a typical fuel cell polarization curve) where: hact is the activation overpotential, a complex function describing the charge transfer kinetics of the electrochemical processes. hact is predominant at small polarization currents or voltages. hconc is the concentration overpotential, a function describing the mass transport limitations associated with electrochemical processes. hact is predominant at large polarization currents or voltages. iR is often called the ohmic drop. iR follows Ohm's law and describes the polarization that occurs when a current passes through an electrolyte or through any other interface such as surface film, connectors ... The ohmic drop is the simplest of the three polarization terms and can be evaluated either directly with a conductivity cell or using conductance data.

21. Activation Overpotential Both the anodic and cathodic sides of a reaction can be studied individually by using some well established electrochemical methods where the response of a system to an applied polarization, current or voltage, is studied. A general representation of the polarization of an electrode supporting one redox system is given in the Butler-Volmer equation: where: i is the anodic or cathodic current; b = charge transfer barrier or symmetry coefficient for the anodic or cathodic reaction. b values are typically close to 0.5; hact = Eapplied - Eeq, i.e. positive for anodic polarization and negative for cathodic polarization; n = number of participating electrons; R = gas constant; T = absolute temperature; F = Faraday = 96485 C mol-1

27. The curve above shows slightly different behavior. This is called passivity: an increased corrosion resistance in oxidizing conditions due to a thin barrier film formed in these conditions or at anodic potentials. Passivity can be seen by the lowering of the current density as the potential is increased. A good example of this is stainless steel. The reason that stainless steel does resist corrosion so well is that a thin chromium oxide film naturally forms on the surface preventing an anodic reaction form occurring on the surface of the steel. It is desirable to stay in the passive region ( above the critical point) because of the reduced current density which correlates to a reduced dissoluton rate. The passive region shows a rather steady current density for a wide range of potential values, until the passive film breaks down, causing an increase in the current denisty. This is occurs at relatively high potential and is usually called trans-passive breakdown.

30. Reversibility of Electrochemical Reactions A reaction is said to be reversible if it can proceed easily in either direction as conditions change (typically as the electrochemical potential is changed). There are several aspects of reversibility. � Chemical reversibility relates to the chemical feasibility of the reaction, with a chemically irreversible reaction being one in which the reverse reaction is prevented by the occurrence of competing reactions, A thermodynamically reversible reaction is a chemically reversible reaction for which the reaction will change direction as a result of an infinitesimal change in potential. � A practically reversible reaction is a thermodynamically reversible reaction that occurs at a significant rate with small overpotentials.

31. Electrochemical Equilibria Thermodynamically reversible reactions will adopt an equilibrium potential which is described by the Nernst equation: � Example of an Electrochemical Equilibrium If we consider copper in equilibrium with copper ions in solution: Consequently the equilibrium potential will go up as the concentration of copper ions in solution goes up

32. Reference Electrodes Reference electrodes are needed to convert from the charge carriers in the metal (electrons) to the charge carriers in solution (ions) in a reproducible fashion. They must be practically reversible. The Normal Hydrogen Electrode (NHE) is used as the (arbitrary) standard. This consists of hydrogen at unit activity (i.e. solution in equilibrium with hydrogen gas at 1 atmosphere) in equilibrium with unit activity of hydrogen ions in solution (1.19 M HCl solution). The equilibrium potential is detected with a platinum electrode that is coated with platinum black (finely divided platinum) to enlarge the effective surface area. The NHE is inconvenient for general-purpose use, and a range of secondary reference electrodes have been developed (Table 1).

34. Electrochemical Kinetics The Electrochemical Double Layer There is a tendency for charged species to be attracted to or repelled from the metal-solution interface. This gives rise to a separation of charge, and the layer of solution with different composition from the bulk solution is known as the electrochemical double layer. There are a number of theoretical descriptions of the structure of this layer, including the Helmholtz model, the Gouy�Chapman model and the Gouy�Chapman�Stern model. � As a result of the variation of the charge separation with the applied potential, the electrochemical double layer has an apparent capacitance (known as the double layer capacitance).

41. The Rate Determining Step Real electrochemical reactions tend to occur as a sequence of very simple steps. For example, even a very simple reaction such as hydrogen evolution occurs as two steps, with two alternatives for the second step: The rate of the overall reaction is controlled by the rate of the slowest reaction, and this is known as the rate controlling step. This may be an electrochemical reaction (such as Step 1 above) or a chemical reaction (such as Step 2a). Different rate controlling steps will typically give a different Tafel slope for the reaction and a different reaction order (dependence on concentration of reactants). Electrochemical measurements may be used to help to determine the reaction mechanism and the rate-controlling step.