Protecting Metal Surfaces from Corrosion

Noble metals such as gold and platinum have high resistance to general environmental conditions. Corrosion is a type of chemical reaction that converts the surface of most metals into their oxides or hydroxides through the action of water and oxygen. This new formation adversely changes many properties of metals in pure or alloy form, such as durability, conductivity and service life [1-6]. Corroded metals become more brittle and have higher solubility, leading to undesirable consequences such as reduction in metal quantity and environmental pollution (Figure 1). When these disadvantages are combined, they will cause disruptions in work as well as losses worth billions of lira and loss of life.

Figure 1. Corrosion not only worsens the appearance of metal products, but also reduces their durability, conductivity and service life, and causes environmental pollution.

Corrosion

A surface can be defined as the boundary between two phases that do not mix or mix only limitedly (Figure 2). Depending on whether these phases are solid, liquid, gas or vacuum, the resulting surface will be solid-gas, solid-liquid, solid-solid, liquid-liquid, liquid-gas or solid-vacuum interface.

Figure 2. Corrosion is an undesirable reaction that occurs between a metal surface (phase 2) and a second phase containing oxygen (air) and water (humid air or water).

Corrosion on solid metal will occur at the solid-gas or solid-liquid interface. Three basic methods are applied to prevent corrosion: alloy formation, cathodic protection and protective surface coating. Selection among these is made according to the type of metal and their application areas. More than one principle can be utilized, such as applying protective coating to an alloy product. For example, if a piece of iron is immersed in water or kept in humid air, the water and air will want to take electrons from the iron to form hydroxide ions. If these two reactions showing the oxidation of iron and the reduction of oxygen in the same environment are combined: Fe + H2O + ½ O2 → Fe(OH)2 with one more oxidation step, Fe(OH)3 is formed. Their dehydrated forms are FeO and Fe2O3 respectively. Alloys are homogeneous new products created by melting certain metal mixtures together. In addition to applications aimed at improving many properties of metals such as hardness, elasticity and wear resistance, they are also a widely used method to increase their corrosion resistance. For example, chromium-nickel stainless steel created by combining iron containing 18% chromium and 8% nickel. Cathodic protection is achieved by connecting a chemically more active metal with a conductor to the metal that is to be protected from corrosion. Since the active metal will corrode first, the metal attached to it will not be affected by corrosion. An example of this method is the protection of underground steel water or fuel tanks, natural gas or oil pipelines by connecting them with wire to magnesium rods, which are much more active than iron. Protective coating is applied at the finishing stage of metal product manufacturing or to prevent corrosion of existing metal products. Here a protective coating is applied to the metal surface through painting, enameling, oiling or galvanizing operations, and it is intended to cut off contact between the metal surface and water and air (Figure 3). Protective coating can serve many purposes, such as coloring, patterning, beautification, reducing light effects or warning, as in the case of paint.

Figure 3. Protective coating will prevent corrosion by cutting off contact between metal and air and water.

Metal-Water Systems Study

When a metal comes into contact with water, many changes occur on its surface, while many species are released into the water. All of these are affected by many factors such as the pH value of the water, temperature, and the types and concentrations of other ions in the environment [7-13]. For this reason, knowing the equilibrium of metals in aqueous solutions will be important for corrosion studies. Determining conditions that will provide passivity on the metal surface will be another approach to preventing corrosion. While some information can be obtained experimentally, theoretical findings based on literature knowledge will also be very useful. The stability regions of a metal in aqueous solution can be studied with potential-pH diagrams using thermodynamic data. For example, let a related general reaction be as follows: aA + bB + ne- = cC + dD (oxidized form) + ne- = (reduced form) The Nernst equation can be written for this. Part of the work being conducted in the Industrial Chemistry Department at Erciyes Üniversitesi on surface properties of materials is to determine iron's equilibrium in aqueous solution using thermodynamic data and possible reactions. The pH-potential graph related to calculations performed for two activity values can be given as an example of the subject (Figure 4). Approximations can be made using values given to other activity values related to ionic species. Areas where solid species are dominant, that is, where ionic species are present in low concentration, are passive areas for corrosion. This simplified graph represents a single metal-water system. In most real applications, the presence of other species in the environment will make the systems much broader in scope.

Figure 4. Potential-pH diagram obtained in a study of iron's ionic species in aqueous solution based on two activity values. (T= 25°C)

Other studies related to metal corrosion are also being conducted in the research unit mentioned above. Rapid determination of corrosion stages using spectroscopic methods; effect of surface roughness on corrosion rate; comparison of protective coating methods and making enamel coating more effective with boron compounds are a few examples of these studies [1-4]. Corrosion stages can be measured comparatively within minutes by examining light reflection from the surface. It has been observed that corrosion on a smooth surface progresses more slowly than on a rough surface. Adding boron compounds to enamel raw materials reduces the temperature required for ceramic formation by several hundred degrees, providing energy savings and practicality in the process. Preventing or delaying metal corrosion will have important contributions to reducing numerous accidents, injuries and loss of life. While it is not possible to prevent corrosion completely, controlling it will provide very large economic benefits. The damage corrosion causes to a metal can be defined as direct cost. Indirect cost is defined as the damage that would result from a metal product's corrosion not being noticed or being neglected, and can be hundreds or even thousands of times greater than the direct cost. Such as a car losing control and having an accident due to a small corroded part worth a few lira, or disruptions in energy production or transmission affecting transportation, numerous settlements and factories.

Conclusion

Most metals undergo a change called corrosion when exposed to water and air. When moisture and oxygen in the air or oxygen-containing water comes into contact with a metal, reactions occur between them, resulting in metal oxides or hydroxides forming on the surface. This product loses the original metal's properties such as strength, conductivity and service life to a great extent. Due to its brittle structure and generally high solubility, it also has the potential to cause environmental pollution. The corrosion resistance of metals can be increased through methods such as protective coating, cathodic protection and alloy formation. Detailed experimental and theoretical study of the corrosion resistance properties of metals to be used will also be effective in reducing future damage from corrosion. Ömer Edip Kuzugüdenli - Head of Chemistry Department / Faculty of Science - Erciyes Üniversitesi

References 1. Kuzugüdenli, Ö.E., Role of Protective Surface Covering against Metal Corrosion, IX th International Chemical Physics Congress, 14-16 Oct. 2010, İzmir. 2. Kuzugüdenli, Ö.E., Effect of Boron Compounds on Enamel Coating of Metals," 12th International Corrosion Symposium, 06-09 Oct. 2010, Eskişehir. 3. Kuzugüdenli, O. E. and Ulgen, A., Use of Spectrometric Techniques to Measure Progress of Metal Corrosion, 8th International Electrochemistry Meeting, Antalya, Turkey, 8-11 October, 2009. 4. Kuzugüdenli, Ö. E. and Ülgen, A., Effect of Surface Smoothness on Corrosion Resistance of Metals, 11th International Corrosion Symposium, 22-25 Oct. 2008, İzmir. 5. Jones, D.A., Principles and Prevention of Corrosion, 2nd ed., Prentice-Hall, Inc, Upper Saddle River , New Jersey, 1996. 6. Shreve, R. N. and Brink, J. A., Chemical Process Industries, McGraw-Hill Book Co., New York, 1986. 7. Jones, M. M., Netterville, J. T., Johnston, D. O. And Wood, J. L., Chemistry, Man and Society, 2nd ed. W.B. Saunders Co., Philadelphia, USA, 1976. 8. Bockris, J. O. M. and Reddy, A. K. N., Modern Electrochemistry, Plenum Press, New York, 1970. 9. Pourbaix, M., Atlas of Electrochemical Equilibria in Aqueous Systems, Pergamon Press, London, 1966. 10. Garrels, R. M. and Christ, C. L., Solutions, Minerals and Equilibria, Freeman, Cooper & Company, San Francisco, 1965. 11. Latimer, W. M., The Oxidation States of Elements and Their Properties in Aqueous Solutions, 2nd ed. Prentice-Hall, Englewood Cliffs, NJ, 1956. 12. Stumm, W. and Morgan., J. J., Aquatic Chemistry – An introduction Emphasizing Chemical Equilibria in Natural Waters, 2nd ed., John Willey & Sons, New York, 1981. 13. Adamson, A.W., Gast, A.P., Physical Chemistry of Surfaces, John Wiley and Sons, Toronto, 1977.