Corrosion Risk Identification and Assessment - Report Example

Introduction Corrosion is formally defined as the chemical or electrochemical reaction between a material, usually a metal, and its environment that produces a deterioration of the material and its properties. It is been defined by reefEd as “the process by which metals are “eaten” away by salt, water and air” present in our environment. Wikipedia defines Corrosion as the “deterioration of useful properties in a material due to reactions with its environment” We can, therefore, say that Corrosion is the process by which a material deteriorates as a result of its reaction with its surroundings. It is commonly thought to be synonymous with rust, however, the latter is actually a form of Corrosion. Rust refers to the specific deterioration of metals with their reaction to the atmosphere, whereas corrosion may include ceramic surfaces as well. Corrosion mostly occurs due to exposure to moisture in air, however, it can also occur due to exposure to certain substances. Oxidation & Reduction Oxidation and Reduction are the two processes that evolve while a chemical reaction takes place. A reaction which involves both oxidation and reduction is commonly called a redox reaction. Oxidation and Reduction can both be explained in terms of: Oxygen transfer Hydrogen transfer (old definition) Electron transfer Oxygen Transfer: In terms of Oxidation is the gain of oxygen whereas reduction is the loss of oxygen. Hydrogen Transfer: In terms of Hydrogen transfer, Oxidation is the loss of hydrogen while Reduction is the gain of hydrogen. Although this is an old definition used to differentiate between oxidation and reduction, I felt it pertinent to mention this in my assignment. Electron Transfer: In terms of Electron transfer, Oxidation involves the loss of electrons whereas Reduction involves the gain of electrons. Thus, to summarize, we can say that: Oxidation involves the - gain of oxygen - loss of hydrogen - loss of electrons, and Reduction involves the – loss of oxygen - gain of hydrogen - gain of electrons. OXIDATION Sn2+ + 2Fe3+ Sn4+ + 2Fe2+ REDUCTION The Sn2+ has lost 2 electrons and is therefore oxidized. When both oxidation and reduction occurs simultaneously in a reaction, corrosion occurs. If either one of these is absent from the reaction, corrosion cannot begin. Likewise, if during a redox reaction, if either one of the oxidation and reduction reactions is stopped, then the corrosive process will also come to a stop. Consider the following reaction, illustrated by the equation Fe + 2H+ Fe2+ + H2 This chemical reaction comprises of two partial reaction, an anodic and a cathodic reaction. This occurs when iron is immersed in some HCl and as a consequence corrodes, hence explaining the redox reaction. ELECTROCHEMICAL CONSIDERATION: A typical characteristic of most corrosion processes is that the oxidation and reduction steps occur and different locations on the metal. As metals are conductive, their electrons can flow through the metal from the anodic to the cathodic regions. As this theory suggests, the presence of water is necessary for this process to take place, and thus, even a small, thin film of any moisture absorbed can be enough to trigger the process. Corrosion usually begins at that location in the metal where the latter is under stress (e.g. at a bend or weld), or is isolated from air (this would be the case where, for example, two metal pieces are joined or under a loosely-adhering paint coating). The metal ions dissolve in the layer of moisture and cause the electrons to move to another location. Corrosion also often occurs where the metal is exposed to a “depolarizer”, such as Oxygen. ELECTRODE POTENTIAL: An electrode is defined as “an electric conductor which is used to make contact with a metallic part of a circuit”. Electrodes can be of two types, namely, anodes and cathodes. When a metal is placed in a solution of its ions, an equilibrium is established between: (a) the tendency of the metal to lose electrons and pass into solution as ions, and (b)the opposing tendency for the ions in solution to gain electrons and be deposited on the metal. M (s) Mn+ (aq) + ne- The metal aquires an electrical charge (usually negative) and a potential difference is set up between the metal and the solution called the electrode potential of the metal.   The size of this electrode potential will depend on the position of this equilibrium and this depends on: 1. the metal concerned 2. the concentration of the metal ion in solution 3. temperature FORMS OF CORROSION The various forms of corrosion are as follows: Uniform Attack Uniform attack is the most common form of corrosion. It is normally characterized by a chemical or electrochemical reaction which spreads uniformly over the entire exposed surface or over a large area. The metal becomes thinner and eventually fails. For example, a piece of steel or zinc immersed in dilute sulfuric acid will normally dissolve at a uniform rate over its entire surface. A sheet iron roof will show essentially the same degree of rusting over its entire outside surface. Uniform attack, or general overall corrosion, represents the greatest destruction of metal on a tonnage basis. Uniform attack can be prevented or reduced by (1) proper materials, including coatings, (2) inhibitors, or (3) cathodic protection. Galvanic or Two-Metal Corrosion As the name suggests, this form of corrosion occurs when two dissimilar metals are involved, and electric current exists. A potential difference exists between these metals when they are immersed in a corrosive or conductive solution. If they are then placed in contact with each other, this potential difference produces an electron flow between them. Corrosion of the less corrosion-resistant metal is usually increased and attack of the more resistant material is decreased, as compared with the behavior of these metals when they are not in contact. Consequently, the less resistant metal becomes anodic in nature whereas the more resistant metal becomes cathodic in nature. Usually, the cathodic metal corrodes very little or sometimes even not at all, in this type of couple. This type of corrosion is generally more difficult to keep track of all over the world. By simply adding a screw which is of the wrong type, this form of corrosion can be triggered. Pitting Like the name suggests, Pitting is used to describe the formation of small pits on the surface of a metal or alloy, thereby resulting in holes in the metal. These holes can be small or large, and are sometimes so close together that they make the surface appear rough. It generally occurs on flat surfaces. Pitting can go on undetected for a very long period of time, until eventually when a failure occurs. This form of corrosion is one of the most destructive and vicious forms, and is widespread across the world. This is because it causes equipment to fail owing to its ability to perforate with only a small percent of weight loss of the entire structure. Pits are usually difficult to detect because of their small size and because they are often covered with corrosion products, hence they go undetected until they become more visible and have spread out. Crevice Corrosion This is a form of electrochemical corrosion. It is a very severe, localized corrosion which occurs frequently with crevices and other shielded areas (on metal corrosives), which are exposed to corrosives. This type of attack is usually associated with small quantities of stagnant solution caused by holes, lap joints, gasket surfaces, surface deposits, and crevices under bolt and rivet heads. It occurs in much the same way as pitting, except that the latter forms on flat surfaces. Intergranular Corrosion Localized attack at and adjacent to grain boundaries, with relatively little corrosion of the grains, is intergranular corrosion. In this, the alloy disintegrates, and may even fall out, hence losing its strength. Intergranular corrosion can also be caused by the following: some impurities at the grain boundaries, enrichment of one of the alloying elements, or depletion of one of these elements in the grain-boundary areas. Selective leaching Selective leaching is the form of corrosion whereby one element is removed from a solid alloy by corrosion processes. The most common example is the selective removal of zinc in brass alloys (also known as “dezincification”). Selective leaching is also used in other alloy systems in which aluminum; iron, cobalt, chromium, and other elements are removed. Erosion Corrosion This exists when there is an increase in rate of deterioration or attack on a metal because of relative movement between a corrosive fluid and the metal surface. This movement is quite rapid, and mechanical wear-and-tear effects or abrasion are involved. Metal is removed from the surface as dissolved ions, or it forms solid corrosion products which are mechanically swept from the metal surface. Almost all alloy or metals are susceptible to some type of erosion-corrosion as this form of corrosion is very dependent on the fluid. This form of corrosion is typically characterized in appearance by grooves, gullies, waves, rounded holes, and valleys. A directional pattern is also seen in Erosion Corrosion. In many cases, failures because of erosion corrosion occur in a relatively short time.  RATE OF CORROSION: One of the main causes of corrosion is the moisture present in the environment. Rusting occurs much more rapidly in moist conditions as compared to a dry environment like a desert. Several other factors, which affect the rate of corrosion of an element/substance, exist. For example, the presence of salt in the environment/atmosphere greatly enhances the rusting of metals. This is primarily due to the fact that the dissolved salt increases the conductivity of the aqueous solution formed at the surface of the metal and consequently enhances the rate of electrochemical corrosion. This is one reason why iron or steel tend to corrode much more quickly when exposed to salt, or moist, salty air near the ocean. CONCLUSION: It is common knowledge that unless the correct steps are taken, corrosion will damage or effectively destroy a large proportion of metal goods manufactured and produced around the world. Corrosion of items during storage and transportation is usually prevented by the use of 'temporary protectives' (which are ultimately removed by the customer). Corrosion is a widespread issue of concern, and a large chunk of the Governments’ revenue is spent on countering corrosion-related issues globally. REFERENCES: 1. High School Chemistry by William Yu Wang, Dana R. Freeman, Publisher – Kaplan 2. Chemistry – Concepts and Problems: A self-teaching Guide by Clifford C. Houk and Richard Post (Second Edition, 1996) 3. Inorganic Electrochemistry: Theory, Practice & Application by P. Zanello (2003) 4. Modern Electrochemistry 1 Second Edition by J. O’M Bockris, Amulya K. N. Reddy, Mario Gamboa-Aldeco (2003) 5. Wikipedia – Online Encyclopaedia 6. ReefEd – Online Encyclopaedia 7. 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