Oxidation Reactions. This oxide will from only if thermodynamics favour a reaction of the form: M + O 2 = MO 2. Which must form rapidly (favourable(

Transcription

1 Oxidation of s Oxidation is a general term used to define the reaction between a metal or alloy and its environment. s or alloys are oxidised when heated to elevated temperatures es in air or highly oxidised atmosphere with excess air or oxygen Oxidation can also take place in reducing environments (low oxygen potential) In industrial practice, metals and alloys are rarely exposed to pure oxygen. The atmosphere may contain oxidising species in addition to oxygen. The corresponding atmospheres consist mainly of, N 2, H 2 O, C, S 2, HCl, Cl 2, NH 3, S, etc.. Thus, H 2 O and C may play an important role in affecting the oxidation behaviour of a metal. Understanding oxidation of metals and alloys has been behind the successful development of alloys that resist environmental attack k at high temperatures Oxidation resistance of these alloys is due to the presence of reactive r alloying elements, such as Cr, Al and Si, which can form stable, protective external oxide s on the metal surfaces and so prevent further corrosion Oxidation Reactions During the initial stages of oxidation reaction, a gas- adsorbed film will form and develop by nucleation and growth to form a thicker oxide. (g) o o o o o 1 Adsorption (g) Oxide nucleation & growth Oxide dissolution This oxide will from only if thermodynamics favour a reaction of the form: M + = M Which must form rapidly (favourable( kinetics) - e - 3 Mn+ n+ Scale growth Internal oxidation 2 (g) M n+ Growth of porosity and microcracks 4

2 A metal can also be oxidised by either water vapour (H 2 O) or C. M + H 2 O = MO + H 2 Thermodynamics of oxidation Thermodynamics predict the relative stability of an oxide with respect to its metal, but does not predict how protective an oxide will be and M + C = MO + CO An oxide will form on a metal surface when the oxygen potential in the environment is greater than the oxygen partial pressure in equilibrium with the oxide Kinetics of Oxidation (Oxidation Rates) The basis for developing high temperature oxidation- resistant alloys is the formation of a protective oxide layer between the metal and the environment. Reaction kinetics are controlled by the rates of diffusion of metal ions and / or across the barrier oxide. Since the diffusion processes are temperature-dependent, oxidation rates increase rapidly with increase in temperature according to: D = D 0 exp (-Q/RT)(

3 Three laws describe the oxidation rates for most metals and alloys: 1. Linear growth x = k l t + x 0 2. Parabolic growth x 2 = k p t + x 0 3. Logarithmic growth x = k e log(at + 1) Weight Gain Time Linear Parabolic Logarithmic x: oxide thickness, t: time, k l, k p, k e, and a are constants. x 0 is oxide thickness at t = 0 The linear rate law is applicable to the formation and development of non-protective oxide layers at high temperature The rate constant, k l, changes with temperature according to an Arrhenius type relationship: x = k l t + x 0 The parabolic rate law assumes that the diffusion of metal cations or oxygen anions is the rate controlling step and is derived from FickF ick s first law of diffusion. This law is applicable to uniform, continuous and protective oxide layers The rate constant, k p, changes with temperature according to an Arrhenius type relationship: x 2 = k p t + x 0 The logarithmic rate law is mainly applicable to thin oxide s (less than 100 nm) ) formed at low temperatures, and therefore rarely applicable to high temperature engineering problems. x = k e log (at + 1) Ideally, the oxidation rate should be relatively slow (parabolic growth law). Such a requirement depends on: 1. Rate of diffusion of reactants (metal cations and through the oxide layer 2. Rate of supply of to the outer surface of the oxide 3. Molar volume ratio of oxide to metal anions) The slowest process at a given temperature will control the rate of oxidation.

4 Types of Oxides Two types of metal oxides 1. Non-protective (unstable) oxides Formation of a porous and cracked oxide film on the metal surface. Because of porosity, penetrates to the metal surface and reacts with the metal to form more oxide Ultimately, the entire metal will be consumed 2. Protective (stable) oxides Formation of a non-porous film on the metal surface can only react with the metal ions through diffusion Growth rate decreases as the oxide thickness increases. This is parabolic growth rate law and is associated with thick oxides An indication whether an oxide film is protective or not is given n by the Pilling Bedworth ratio. This is linear growth rate law and occurs at high temperatures Pilling Bedworth ratio (PB) volume of oxide produced PB = = volume of metal consumed Wd ndw Where: W: : is the molecular weight of oxide, D: : is the density of oxide w: : is the atomic weight of metal, d: : is the density of metal n: : is the number of metal atoms in the oxide (e.g; in Al 2 O 3, n = 2) 1. PB < 1 porous and non-protective oxide (the oxide film is insufficient to fully cover the metal surface 2. PB > 1 protective oxide Oxidation rates at later stages depends on whether the thick film remains continuous and protective as it grows or whether it contains pores or cracks (non- protective) As the oxide film grows and becomes thicker (PB > 2), however, compressive stresses develop in the oxide leading to cracking and then spalling (flake off). This is called breakaway corrosion.

5 List of protective and non-protective protective oxide s Linear Protective oxides Non protective oxides Be 1.59 K 0.45 Cu 1.68 Ag 1.59 Al 1.28 Cd 1.21 Weight Gain Parabolic Breakaway corrosion Cr 1.99 Ti 1.95 Mn 1.79 Mo 3.40 Fe 1.77 Hf 2.61 Co 1.99 Sb 2.35 Ni 1.52 W 3.40 Time Pd 1.60 Ta 2.33 Pb 1.40 U 3.05 Ce 1.16 V 3.18 Some possible forms of oxide films Some possible forms of oxide films Scale Scale Compact and uniform Porous Cracked Compact and uniform Porous Cracked Compound formation in Non-adherent and cracked Compound formation in Non-adherent and cracked

6 Oxidation in Alloys Follow the same principles as for oxidation of pure metals. However, oxidation of alloys is more complex because: An example of the effect of Cr on oxidation is illustrated by one of the common high temperature alloys (Fe Cr). 1. Alloying elements have different affinities for 2. Alloying elements have different diffusion rates in oxides and in i alloy 3. Higher and complex oxides may form 4. Possibility of solid solubility between the oxides Source: D. A. Jones (Principles and Prevention of Corrosion)