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Codes and Standards |
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Steel corrodes in reaction with its environment because of the thermodynamically unstable condition of iron after it has been extracted from its ores. Reduction of iron from its state as an oxide in ore requires energy in the reduction process. The fundamental laws of nature govern-mg conservation of energy require that. eventually, balance must be restored by return of the unstable metal to its oxidized state, in the case of iron [steel) the oxidized state usual y appears as rust. Rust is similar in appearance and practically identical in composition (Fe203) to the most common form of iron ore (hematite). Appropriate conditions yield two other oxidized forms, one of which has the same chemical composition as a principle form of iron ore magnetite (Fe3O4) The other is the lowest oxide of iron, FeO. Alt three of these oxides are components of the "milt scale" formed on steel by oxidation at temperatures encountered in the manufacture of steel into structural shapes and plates. Effects of such mill scale must be taken into account in preparing and painting steel to prevent corrosion. The principal difference, in terms of energy, between reduction from ore and eventual conversion into rust by corrosion is not the amount of energy required but the rate of reaction. Fortunately, ambient environmental corrosion of iron proceeds much more slowly than high temperature oxidation. The principal function of a ca(et coating is to reduce the rate of corrosion in the environment end the area of the metal involved as much as possible(e, ideally to zero. Understanding the process of corrosion provides the key to steps that may be taken to prevent the reaction from occurring and to identify the role that saint can play in achieving this result. Obviously, if the metal can be isolated from a corrosive environment, no corrosion reaction can occur. Such isolation is the most important function of a paint coating. In addition, some constituents of a coating can suppress the rate of corrosion reactions where complete isolation is not achieved either generally or locally, as at pores. scratches or other discontinuities [holidays) in a coating, Consideration must be given, also. to the possibility that a constituent of a coating might actually accelerate a corrosion reaction. Experience has shown that corrosion in the presence of a paint coating is likely to be much more serious where it is localized at discontinuities in a coating rather than where it occurs in a more general attack under a coating. This is True even if a coating is unable to isolate the metal from its environment. Consequently, what happens at discontinuities in a coating as related to the processes of corrosion requires special consideration. III. THE MECHANISM OF CORROSION It has been well established by experimental demonstration (1,2,3,4,5,6,7) that corrosion is the result of an electrochemical process involving an anodic reaction. Here, the metal goes into solution as an ion, and a cathodic reaction takes place where the electrons released by the anodic reaction are discharged to maintain electrical neutrality by reaction with ions in solution, e.g. hydrogen ions m acid solutions, or by reduction of oxygen in solution n neutral or alkaline solutions. The anode in a corrosion cell is analogous to the negative zinc electrode in an ordinary dry cell battery. The cathode is analogous to the positive careen electrode in such a cell. The current flows in the electrolyte inside the battery cell from the anode, zinc. to the cathode, carbon. The electrons generated by the cell move in the external circuit from the zinc electrode (-) to the carbon electrode k (+). By convention, the flow of current in the external circuit is opposite the electron movement. Whether a particular area of a steel surface will act as an anode or a cathode will be determined by a number of factors. One factor is the condition of the thin. air-formed oxide films that exist on dry steel. Such films when they are intact induce a modest level of passivity that makes the film-covered surfaces more noble than. and therefore cathodic to, adjacent surfaces where a less protective film may exist. Observations of steel surfaces after immersion in water for several days have shown that, ordinarily, about 50% of the surface has been corroding as an anode with the remaining cathodic surfaces showing little or no evidence of attack. As time progresses, there is breakdown of the protective films on the original cathodic surfaces so that corrosion spreads eventually over the whole surface, But a division between anodic and cathodic surfaces persists with tee cathodic areas at any time being protected from corrosion by currents flowing from adjacent anodic areas. Other factors in establishing anodic areas are differences in crystal orientation, the presence of contamination on the surface of the steel, and, in exceptional cases, the effects of stresses above the elastic limit of the metal. which cause rupture of protective oxide films by plastic deformation, Anodic areas can be established also by variations m the dissolved oxygen concentration of a solution in different zones on the steel surface. These variations can give rise to what is called an oxygen concentration cell in which current will flow from an anodic area in contact with the solution having the low concentration of dissolved oxygen to a cathodic area in contact with the solution having me higher concentration. The difference in corrosion potential that can be created by this mechanism on a steel surface can exceed 100 mV, The anodic and cathodic reactions in the corrosion of iron can be written as follows:
The hydroxyl ions generated by cathodic reactions can contribute to degradation of paints subject to attack by alkali. Figure I helps to illustrate the process of corrosion, iron ions (Fe++) released by the anodic reaction interact with hydroxyl (OH-) ions generated by cathodic reactions to form Fe(OH)2 near the boundaries of anodic and cathodic areas. Oxygen reaching the precipitated Fe(OH)2 reacts with it to form Fe(OH)3 and, eventually, rust Fe203. The essential requirements for the electrochemical reactions in corrosion are, therefore, a thermodynamically unstable metal, iron; an electrolytic conductor of ions, water or another conductive solution; an electrical conductor, the metal; and an electron acceptor, hydrogen ions or dissolved oxygen. We have the metal that we wish to protect from corrosion. What we need to control, therefore, is the availability of an electrolyte. This is best accomplished by an isolating barrier such as paint, or by reducing the concentration of electron acceptors such as hydrogen ions or dissolved oxygen.
It may be possible under some circumstances to prevent corrosion by interfering with the anodic reaction by a process called passivation or reduction of the tendency of the iron to go into solution. In the case of steel, passivation usually is accomplished by very thin adherent oxide films which change the corrosion potential of the iron in the more noble direction (towards gold in the electromotive series). Galvanic Corrosion Induced by Passivation The change in potential of steel as a result of passivation, achieved for example by contact with passivating pigments such as red lead and chromates, can create galvanic couples between the passivated iron under the paint film and adjacent unpassivated iron at bare spots. The result would be galvanic acceleration of corrosion of the exposed iron. For this reason it has been proposed that passivating pigments be excluded from paints used to protect steel under conditions of continuous or frequent, complete or partial immersion. However, with no more than the thin film of electrolyte with limited electrical conductivity that will exist on surfaces exposed only to the atmosphere, a significant galvanic effect on a bare spot need not be anticipated. The benefit of passivating the bare spot by a pigment will more than offset the galvanic effect of passivation under the paint film. For this reason passivating pigments such as zinc chromate are beneficial rather than harmful in paints used for protection of steel in atmospheric exposures. In view of the fact that an electrolyte (water or moisture) must be present for corrosion to occur, the principal function of a paint coating is to provide a barrier to penetration of water or moisture to the underlying metal surface. Transfer of water or moisture through a paint coating can occur by water absorption by a coating or by transfer of water vapor through a coating. Details of these processes will be described in other chapters of this book. For the present it will suffice to note that penetration of water or moisture is accompanied by poor adhesion of the coating to the metal. This permits osmotic effects to operate through the coating acting as a membrane and thereby results in the development of blisters. Such action may be accentuated further by the superimposed effects of electrical currents created by corrosion, leading to the phenomenon of electroendosmosis with resulting blisters adjacent to cathodic areas. |
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