Stress Corrosion Cracking - Coursework Example

Analysis of Materials: Corrosion and its Associated Effects Prepared and Submitted by [Client’s Name] Submitted to [Professor’s Name] [Subject] The strength of material depends largely on its weakest attribute. In metals, their strength depends largely on how well it fends off corrosion. Corrosion is probably the nightmare of metals and its derivatives. When metals were discovered during the Bronze Age, so much of the human society has changed and so much has been achieved. The development and progress brought about by the discovery and the utilization of metals, however, is seriously impeded by the elements that tend to destabilize its strength and hence, usability – rusts or corrosions (Bertolotti & Hurst, 1978). As major industries all over the globe have to depend on metals in one way or another, the plight to increase the resistance of metals against the agents of corrosion has increasingly become important. Metals and corrosion are two different aspects of the same thing (Yan, Liang & Liu, 2007). As long as metals exist, corrosion will eventually develop to decompose metals so to speak. Even the toughest and the most reliable metals are subject to weaknesses where corrosions can easily attack. This paper will attempt to understand corrosion, its mechanisms, as well as elaborate on the most common types of corrosion known to metals. Similarly, this paper will also evaluate a case analysis where corrosion is a major problem and how were these issues resolved. In its simplest sense, corrosion is just rusting. Corrosion occurs when oxygen is introduced to a weakened part of a metal, causing the development of ferrous oxide (FeO2) (Fritz & Gerlock, 2001). In other cases, corrosion is the alteration of the chemical and physical properties of metals by the introduction of foreign chemical substance leading to the decline in the strength, hardness, malleability, or ductility of the metal. Corrosion is the most common way to explore the weakness of the metals and compensate its performance (Beech & Campbell, 2008). Understanding the mechanisms involved in corrosion is important. It gives engineers, researchers, managers, and other stakeholders a better view of what is really going on with the metals they use, what the extent of corrosion will develop in a given span of time, what will be the damage caused by the corrosion on the materials, equipments, and infrastructures using the metals, and so on and so forth. Understanding the mechanisms of corrosion will also allow scientists and chemists to devise anti-corrosive chemicals that will counter or impede the development of corrosion in metals, thus giving metals a longer lifespan. There are four major types of corrosion. These corrosions are oxygen cell corrosion, chemical corrosion, electrochemical corrosion, and microbiologically influenced corrosion (Abolikhina & Molyar, 2003). Oxygen cell corrosion is the most common type of corrosion found in metals. According to Bertolotti & Hurst (1978), metals exposed to oxygen compounds in the air are prone to corrode. This is because the exposure of these metals to the harsh external environment, constant changes in temperature and humidity, and the presence of corroding mechanisms will eventually wear out the surface of the metal and allow corrosion or rusting to occur. Because of the simple mechanisms involved in the oxygen cell corrosion, and because it is a very common problem, it is easily resolved. Scientists and engineers just need to coat the surface of the metal with a layer of oxygen-repelling substance in order to inhibit corrosion and its development (Bertolotti & Hurst, 1978). The same thing could not be said for the other types of corrosion, however. Chemical corrosion, electrochemical corrosion, and microbiologically influenced corrosion are typically complex types of corrosion involving complex mechanisms. Three of these complex corrosions are stress corrosion cracking (SCC), crevice corrosion, and accelerated low water corrosion (ALWC). Stress Corrosion Cracking The exposure of metals to oxygen does not always yield rusts. While there are metals that easily corrode, much stronger metals require more than mere presence of oxygen in order to corrode. This other factors take the form of stress and strain. Because metals are mainly used for support in various mechanisms, equipments, and other materials, they are the ones that handle much of the stress and strain of the load. This repetitive and often recurring stress and strain metals experience eventually induce cracks on the grain boundary layers whose exposure to corrosive chemical compounds jeopardizes its strength (Staehle et al, 1977). According to Fritz & Gerlock (2001) stress corrosion cracking only occurs in the areas of grain boundary layers exposed to shearing stress and strains. Materials whose grain boundary layers are worn due to the prolonged exposure to stress and strain can become the metals weak points and corrosive materials such as chemical compounds, oxygen, and hydrogen can easily find their way to these weaker areas. There are three major types of stress corrosion cracking, depending on the mechanisms that induce stress on these grain boundary layers. These are the dissolution of the active part of the metal, hydrogen (or oxygen) embrittlement, and film-induced cleaving. Dissolution of the active part refers to the cracking of the active part of the material, a microscopic area within the metallic material that experiences the most stress and strain from the loads of the metal. A crack in the active part of the metallic material does not necessarily mean a corresponding crack in the macroscopic scale. But because the integrity of the strength of the grain boundary layers are jeopardized, further stress and strain on these areas would accelerate the cracking of the active part. Since the crack is microscopic, the damage is not readily felt until everything is too late (Yang et al, 2007). In most cases, the stress alone on the active parts results to a much greater damage of the whole metal which often lead to rapid degeneration of material’s strength and malleability. The resulting damage can be disastrous as metal supports can fall down without warning. The second mechanism of SCC involves the introduction of corrosive materials and compounds on the active path. Some microscopic cracks that developed under constant and repetitive stress and strain will have a high chance to be exposed to the external environment. Exposure to the external environment entails a lot of serious issues including the exposure of the metal to the corrosive compounds like oxygen, nitrogen, and water. In most cases, the presence of oxygen and hydrogen in the air is absorbed by these microscopic cracks. When this absorption occurs, hydrogen and oxygen atoms can freely move in the crystal lattices of metals, disturbing the natural form of these lattices and possibly inducing internal stress that tends to collapse the material with slight pressure. Moreover, oxygen and hydrogen can easily combine with the metallic compounds to form rust and other compounds, deteriorating the strength of the material over a long period as these combinations results to an internal rusting and eventually caving in (Frankel 1998). The third mechanism of stress corrosion cracking is the film-induced cleavage. The manner with which this mechanism propagates is very similar to cleavage corrosion but the initial stress is still covered by the SCC mechanism. In film-induced cleavage, corrosion occurs on the thin film metal coating that is used to protect the external surface of the metal against corrosive agents. When the integrity of this thin film coating is breached, corrosion easily occurs and when corrosion occurs along the cracks of these films, it easily propagates throughout the metal. The principle is similar to dissolution of the active part corrosion but with the active part of the thin film coating rather than that of the metal itself (Fritz & Gerlock, 2001). This suggests that stress-related corrosion is dependent of the path of the propagation which is most likely in the weakest area of the metal. Crevice Corrosion As the term implies, crevice corrosions are corrosions that occur in crevices (Frankel, 1998). Crevice corrosion occurs in crevices or in weak areas of the metal where stress and chemical compounds can easily eat away. Usually, these weak areas are found in the junctions (or crevice) of adjoining metal sheets, strips, bars, or plates. Because these junctions are often unseen and untended, cracks can easily occur and stagnant water can easily penetrate the area. With stagnant water, water molecules and ions can easily penetrate the materials and combine with the metallic elements in it, resulting in an altered crystal lattice of the affected area in the metal and hence the risk on the strength of the material (Mueller, 1980). The presence of the exposed area in the metal as well as the chemical compounds that initiates corrosion prompts the corrosion along the crevices. And because crevices are the areas in most metals that receives too little attention, the risk of developing crevice corrosion on the metals on the security of the infrastructure, materials, or equipments are high. Accelerated low water corrosion (ALWC) Although there is not much science known about accelerated low water corrosion, its implication to modern infrastructure is immense, considering that almost all ports around the world are using metals and most boats and ships floating in the sea are made of metals and metal sheets. Accelerated low water corrosion is a type of microbiologically enhanced corrosion through the presence of SCC and cleavage corrosion. The metals commonly affected by this type of corrosion are those that are submerged in water or in liquids that allows microbial growth (Beech & Campbell, 2008). Metals submerged in water have a higher tendency to develop minute cracks and roughness due to its exposure to microbes in the liquid. When metal sheets develop cracks along the surface or have exposed areas where the microbes can cling to and grow, the integrity of the metal is jeopardized. This is because as the microbes lodges themselves in these microscopic cracks, it will take root and the roots will probe deeper into the metal, creating a crack where hydrogen, oxygen, and other elements in the water can easily pass through. Because of this, corrosions can speed up inside the metals and the metals will be rendered useless in the short run. Case Analysis As the world gradually accepts the viability of nuclear energy to meet the energy requirement of the growing number of population, scientists and engineers are becoming more concerned on the infrastructural issues of their nuclear plants. The fear of committing the same mistakes as what occurred in Chernobyl sent scientists, engineers, and maintenance managers to look for ways to protect the metals enclosing the radioactive compounds as well as the whole infrastructure from any possible issues of corrosions. In fact, scientists and engineers in various nuclear plants across the world are making great progress in their research on how to protect the metal supports, equipments, and materials in nuclear power plants from corrosion issues (Clanfield et al, 2008). But instead of developing high-technology machinery that would coat the metals sheets and plates from corrosion, the scientists and engineers devised a process that ensures material safety and integrity from rust from a suspended period of time. Scientists and engineers in various nuclear power plants in Europe and USA are working on the present technology allowing typical galvanized iron sheets to be coated with a thick film of titanium carbide to reinforce its strength and durability and resistance against nuclear exposure. The engineers have also revisited the design of the nuclear power plants and coating the metal sheets with various layers of nuclear-resistant substances in order to reinforce its strength and durability against corrosion. They also developed a system that would eliminate potential exposure of metals to external corrosive environment by lowering the levels of oxygen and hydrogen in the internal atmosphere of the plant. By doing this, the engineers were not only able to protect the metal plates from nuclear meltdown but also the community from unwanted radiation energy to seep through these plants. References Abolikhina, E & Molyar, A. (2003). Corrosion of Aircraft Structures made of Aluminium Alloy. 39(6). 889-894 Beech, I & Campbell, S. (2008). Accelerated low water corrosion of carbon steel in the presence of a biofilm harbouring sulphate-reducing and sulphur-oxidising bacteria recovered from a marine sediment. Electrochemica Acta. 54(1). 14-21. Bertolotti, R & Hurst, V. (1978). Inhibition of Corrosion during Autoclave Sterilization of Carbon Steel Dental Instruments. Journal of American Dental Association. 97(4). 628-32 Clanfield, A. Et al (2008). Corrosion Issues in Nuclear Industry Today. Retrieved online http://www.materialstoday.com/view/1611/corrosion-issues-in-nuclear-industry-today-/ Frankel, G. (1998). “Pitting Corrosion of Metals, A Review of the Critical Factors.” Journal of the Electrochemical Society, 145(6),186-198 Fritz, J & Gerlock, R. (2001). Chloride stress corrosion cracking resistance of 6% Mo stainless steel alloy. Desalination. 135(1). 93-97 Mueller, R. (1980). Pitting and crevice corrosion in ERW carbon steel heat exchanger tubes. Journal of Materials for Energy Systems. 2(2). Pp 60-64 Staehle, RW et al (ed). 1977. Stress–Corrosion Cracking and Hydrogen Embrittlement of Iron Base Alloys, NACE Yang, C., Liang, C. & Liu, X. (2007). Tarnishing of silver in environments with sulphur contamination. Anti-Corrosion Methods and Materials. 57(1). 232 -254 Read More