| The Scientific Basis of Our Reactive Metals Based Civilization Digby Macdonald1; 1UNIVERSITY OF CALIFORNIA AT BERKELEY, Berkeley, United States; PAPER: 380/Corrosion/Regular (Oral) SCHEDULED: 14:25/Mon. 28 Nov. 2022/Andaman 2 ABSTRACT: Our civilization is based upon the reactive metals, such as aluminum, iron, nickel, chromium, titanium, and so forth. All these metals and their alloys react with oxygen and water with considerable negative changes in the Gibbs energy, indicating that the reactions are thermodynamically spontaneous and many of the reactions occur at considerable rates; some violently so (e.g., the burning of Al or Mg in air). In other cases, the reaction is muted, allowing the metals and their alloys to be used in fabricating machines that require close tolerances. The resulting corrosion exacts an enormous cost on society that has been estimated at about 3.5 % of the GDP for industrialized countries like the US. Given that the GDP of the US is about $21 trillion ($21x1012), the annual cost of corrosion is approximately $735 billion. On a worldwide basis, the cost is estimated to be $2.2 trillion. Corrosion is an electrochemical process comprising at least two partial reactions, one of which is the electrodissolution (destruction) of the metal or alloy substrate to produces electrons that are quantitatively consumed by a cathodic partial reaction, such as the reduction of oxygen or the evolution of hydrogen via the reduction of water. The rate of the electrodissolution reaction in the active state increases exponentially with the electrochemical potential, so that even modest changes in the potential can result in massive changes in the rate. Fortunately, once the potential exceeds a critical value, known as the Flade or passivation potential, the rate drops precipitously to values that are sufficiently low (< 1 µm/a) that the metals and their alloys may be used to fabricate machines that retain their precise dimensions over useful service lifetimes (40 – 100 a). This if known as the passive state in which the thermodynamically highly reactive metals attain kinetic stability because of the formation of a oxide film on the surface that is commonly no more than 1-3 nm thick and that separates the reactive substrate from the corrosive environment. In this presentation, I will review the scientific basis for the phenomenon of passivity within the framework of the Point Defect Model (PDM) and define precisely the condition that must be achieved for passivity to occur. Indeed, the occurrence of our metals-based civilization can be expressed as a simple inequality that has profound implications for life as we know it. I will also discuss how the PDM predicts the breakdown of passivity that is responsible for the $2.2 trillion annual cost of corrosion. These predictions will be illustrated with practical examples, such as the corrosion and failure of airframes, the failure of oil/gas pipelines, and the failure of nuclear reactor coolant piping. |
