Description of the Rusting Process - Essay Example

The Rusting Process Rusting or the formation of rust is also referred to as corrosion in metals or more commonly in iron materials. It is a chemicalreaction which occurs on the surfaces of these materials and is characterized by production of fine particles with distinct discoloration, more commonly in brown or brick red hue. It is observed when metals come into contact with moisture or when submerged in water even for a short period of time. It has been a common sighting since the introduction of iron in human civilization. What seems like a trivial occurrence in everyday tools and structures is actually a special concern for the industrial sector since it contributes to a significant loss to the world's economy. This is due to the need of replacing corroded metal tools and structures to preserve functionality and safety (Roberge; Lancashire). The rusting process is a complex reaction involving various stages upon the contribution of different compounds and production of derivatives. Rusting is an electrochemical process that occurs in the presence of water or moisture and source of electrolyte. This process cannot proceed to any considerable extent if any of the said requirements is not present. When the metal is in the air, there should be more than 50% relative humidity while above 80% relative humidity results to severe rusting of bare metal (Lancashire 3). Above are the conditions that are conducive to corrosion of metals but the reason behind why metals or any other substance corrode in the first place need to be answered. Corrosion is an opportunity for metals to deteriorate. Metals like most substances need to undergo this process to be able to combine electrochemically with other substances in order to form new compounds. Many environmental conditions provide this opportunity and places with high moisture or relative humidity is one of these conditions, called rusting, although corrosion occurs when metal come into contact with various chemicals such as acids, bases, ammonia gas and other vapors and substances (Roberge). A model (Figure 1) illustrates the chemical process that occurs during rusting. The first stage of the process is called anodic reaction. During this stage, the metal dissolves by generating electrons as shown in the upper portion of the model. The second stage is called cathodic reaction, during which the electrons that were produced in the first stage are then consumed. These two stages of the rusting process can occur adjacently or far apart (Roberge). Figure 1. A schematic representation of the rusting process showing the generation and consumption of electrons (from Corrosion Doctors). There are six chemical reactions that occur during the rusting process. First is the oxidation of the iron (Fe) which has come into contact with moisture or water droplet. The second chemical reaction involves the formation of water from the absorption by dissolved oxygen of the electrons produced from the first reaction. The third chemical reaction includes the generation of hydrogen gas through the consumption of electrons by hydrogen ions. The fourth chemical reaction is the production of the insoluble iron or rust through the reaction of hyrdroxide ions with ferric ions. Another reaction involves the production of rust from the combination of hydrogen ions and oxygen with iron ions. And another chemical process includes the formation of rust through the formation of iron hydroxides with the interaction between hydroxide irons with iron ions (Tarr). The first chemical reaction is illustrated by the following process (Fig. 2). This chemical reaction occurs when water moisture comes into contact with a metal surface. The solid iron or Fe(s) oxidizes in the presence of water to produce aqueous iron or Fe2+(aq), producing two free electrons (Baldragon Academy). Figure 2. Oxidation of iron. The second reaction is the formation of water through consumption of electrons by hydrogen ions and dissolved oxygen as shown by Figure 3. Four electrons (e) are absorbed by four hydrogen ions in aqueous form or 4H+(aq) and dissolved oxygen as O2(aq) to produce two molecules of water in liquid form or 2H2O(l) (Lancashire). Figure 3. Formation of water. The third chemical reaction increases the rate of rusting as indicated in the equation (Fig. 4). Hydrogen ions consume the free electrons which results to the formation of hydrogen gas in place of water in the second chemical reaction. Two hydrogen ions react with two electrons to generate a gaseous form of hydrogen or H2(g) (Tarr). Figure 4. Formation of hydrogen gas. The fourth chemical reaction results to the formation of rust (Tarr). When the hydrogen ion concentration decreases in the water, hydroxide ions are produced. These hydroxide ions react with iron ions to produce rust in the form of insoluble iron (II) hydroxides (Figure 5). Figure 5. Formation of rust particles. Another chemical reaction which produces iron ions and water molecules includes the equation below (Figure 6). This chemical process involves four iron (II) ions which react with four aqueous hydrogen ions and aqueous oxygen to generate four aqueous iron (III) ions and two water molecules (Lancashire). Figure 6. Formation of iron ions and water molecules. The other remaining chemical reaction produces hydrated iron oxides. Iron (III) ions react with three hydroxide ions which generates iron (III) hydroxides (Figure 7) (Tarr). Figure 7. Formationof hydrated iron oxides during the rusting process. Temperature has a direct and significant effect on rust formation. Temperature serves as additional kinetic energy which differentially affects the redox reaction that is rusting. Lower temperature appears to have higher positive effect on the rusting chemical reaction. For example, 5oC significantly increases the rate of rust formation in an experiment by McNeill & Edwards (74). There was a drastic increase in the formation of rust starting from 90 days in the experiment which shot up to its highest peak after 99 days. At 20oC, there was almost equal rust formation at 90 but lagged behind the 5oC setup after 99 days. 25oC setup was the lowest which lagged significantly from the very start up to the end of the trial. Figure 8. Graphical representation of the formation of rust at different temperatures for 99 days. A Pourbaix equilibrium (Vannerberg 1832) is very useful in the establishment of equilibrium in iron and water system. But during iron corrosion, the iron and water system is never in equilibrium or close to this point. The current kinetics of the system is important for the process of rusting than its resulting status. In other words, the active dynamics between the iron and water system lends useful energy to the rusting process which cannot be achieved if these two attains equilibrium. Figure 9. Pourbaix equilibrium showing iron and water system at 25oC from Vannerberg (1832). Another environmental or physical factor that has an effect on the rusting chemical process is irradiation. Irradiation apparently induces the corrosion process as illustrated by the figure below from Lapuerta et al. (15) (Figure 10). This is due to the formation of radical species near the metal surface albeit differently in disaerated and aerated water as shown below. Nonetheless, irradiation during corrosion still occurs even in disaerated condition. Aerated condition achieved maximal iron dissolution thickness at approximately 45 minutes while disaerated condition started production of dissolved iron thickening approximately 10 minutes later and continued 30 minutes more than aerated setup to achieve equal dissolved iron thickness. Figure 10. Effect of irradiation time on dissolved iron thickness from Lapuerta et al. (15). Works Cited Baldragon Academy. Corrosion. Baldragon Academy Department of Chemistry. 21 November 2007 Lancashire, R.J. Some chemistry of iron. 1 November 2006. University of the West Indies Department of Chemistry. 21 November 2007 Lapuerta, S., Millard-Pinard, N., Moncoffre, N., Bererd, N., Jaffrezic, H., Brunel, G., Crusset, D. and Th. Mennecart. Origin of the hydrogen involved in iron corrosion under irradiation. Institute de Physique Nucleaire de Lyon. 21 November 2007 McNeill, L.S. and M. Edwards. Temperature effects on iron corrosion. 21 November 2007 Roberge, P.R. Corrosion. Corrosion Doctors. 21 November 2007 Tarr, M. Corrosion. University of Bolton. 21 November 2007 Vannerberg, N.G. Solution chemistry and corrosion. Pure & Appl. Chem. 60.12 (1988): 1831-1840. 21 November 2007 < www.iupac.org/publications/pac/1988/ pdf/6012x1831.pdf > Read More