Corrosion Mechanisms and Processes - Term Paper Example

Analysis of Materials: Corrosion Mechanisms Submitted by: [Client’s Name] Submitted to: [Professor’s Name] [Subject] [Date] Mankind has dealt with corrosion since they began using metals. During the early times where indirect methods of controlling corrosion is yet to be discovered, mankind has struggled to overcome the limitations of their most astounding discovery, the metals, knowing that all the efforts they have done to extract and process these metals from their ores will be in vain if the rate of corrosion is not reduced. Corrosion is the archenemy of metals (Kruger, 1958). Most metals corrode when submerged under water, exposed in water vapour, salts, acids, bases, oils, and other solid and liquid chemicals. When metals are exposed to these elements, the probability of the metal getting corroded is high. In most cases, the application modern society has found for metals make corrosion a very valid concern that needs immediate attention. The structural and engineering improvement experienced by modern society relies too much on the modernity of the applications and uses of metals. Metals serve as structural supports for buildings, foundations for large stadiums, the core element used in various types of machines and equipments, and are among the main ingredients of the electronic devices that are rapidly becoming a part of our daily existence. Despite this massive development in engineering and research on the applications and uses of metals, eradicating the very thing that nullifies the strength and performance of metals is yet to be achieved (Cotton & Potter, 1993). Corrosion in itself requires its own science. Corrosion is commonly known as rusting but the processes involved in corrosion are more complex than what is seen by the naked eyes. When metals are corroded, it is easy to see that the main components of the metal have reacted with the corrosive agents to form other chemical compounds which has a totally different property to the metal. In the case of iron metal and water vapour, the oxygen component of water vapour reacts with iron and forms In the simplest sense, corrosion is the process that results in the weakening of the metals as its primary compositions are being eaten by corrosive agents like oxygen, water (or water vapour) and other chemicals (Fritz & Gerlock, 2001). Corrosion can take place anywhere and its effects on the materials it has affected can be adverse, ranging from a simple chaffing off of layers to something that totally stops the performance of the equipments that has metals in it. This is because the change in the composition of the metals changes the properties of metals as well, making strong, ductile, and malleable metals more brittle and weaker in the presence of rust. Because most structures, equipments, and materials of today’s modern world rely heavily on metals, the need to reinforce metals and maintain its strength is becomes a major priority. In other words, understanding the very mechanisms that results to rusting of metals as well as the mechanisms that causes rust to propagate across the material allows engineers and researchers to look for ways to nullify the effects of corrosion or reinforce the composition of metals to resist rusting. Moreover, this will make structures, equipments, and machineries with metals on it to become more durable and have longer useful life spans. Generally speaking, there are four major types of corrosions - oxygen cell corrosion, chemical corrosion, electrochemical corrosion, and microbiologically influenced corrosion (Abolikhina & Molyar, 2003).each of these types of corrosion have different mechanisms and although they are distinctly identified from each other, their occurrence in metal structures and supports are so entwined that it makes to differentiate one from the other. Three of the most common kinds of corrosions involving the four types of corrosions mentioned is stress corrosion cracking (SCC), crevice corrosion, and accelerated low water corrosion (ALWC). Stress Corrosion Cracking (SCC) One of the many reasons why corrosion occurs in metals is when the metals are subjected to various stress and strains that these exposed areas are weakened considerably in time. Metals exposed to stress and strain as required by their application will eventually have microscopic areas that are no longer reliable and may have the tendency to break under prolonged stress. The weakened state of these areas in metals makes it more apt for corrosion to occur. More specifically, these areas house the type of corrosion known as stress corrosion cracking. Stress and strain in metals will eventually destroy the grain boundary layers of metals, a microscopic boundary layer that signals a change in orientation of the crystals of the metal. Metals are composed of miniscule striations called grains and these grains are group together and are separated by boundaries. The properties of these boundaries are important in the definition of the properties of metals as these microscopic boundaries are technically the weakest part of the metals (Staehle et al, 1977). These grain boundaries of metals receive the blunt edge of the repetitive stress and strains applied to the metals, particularly since it is already the metals’ weakness. In stress corrosion cracking, the grain boundary layers of metals are not only exposed to repetitive stress and strain. They are also exposed to various corroding agents like water vapours and chemical compounds that eat up the boundary layer and weakens the integrity of the metal furthermore. As the metals are exposed to corroding agents, the main composition of the metals are changed due to the potential chemical reaction of the metal with the agents, or the physical features will experience deformities due to the alteration of the composition of the metals causing stress corrosion cracking to occur (Fritz & Gerlock, 2001). Research suggests three main types of SCC based on the propagation or creation of the cracks. These are the dissolution of the active part of the metal, hydrogen (or oxygen) embrittlement, and film-induced cleaving. These three types of SCC works more like a process where one directly follows when the other ends. Dissolution of the active parts of the metal’s grain boundaries refers to the weakening of the active parts of the metals due to prolonged or recurring exposure to stress and strain (Yang et al, 2007). The stress experienced by the metal will eventually find its way towards the weakest area and then causes overload to that area, causing the area to break or weaken even further. When the active parts are hit by this stress, it all becomes a matter of time before the first signs of breaking come into play (Abolikhina & Molyar, 2003). Because grain boundaries serve as the weakest link to the strength of the metal, anything that hastens this weakness becomes a serious stress to the metal and its intended function. Once the integrity of the active parts of the metal is lost, the microscopic properties of metals become more receptive to corrosion than when its main properties of strength, durability, and flexibility are not jeopardized. Hydrogen or oxygen embrittlement typically occurs in metals when the active parts of the metals are considerably weakened. With stress and strain creating an opening in grain boundaries, molecules smaller than the metal molecules can easily go through the crystals of the metals, resulting to reactions with the metal and hence corrosion (Frankel 1998). For example, when water vapour enters iron, the smaller water molecules reacts with iron to form ferrous oxide (FeO2) which is just rust. Similar scenario occurs in other metals whose active parts are dissolute and are exposed to various corrosive agents. Most find it important to coat the metals with a thin protective film (like paint or coating) in order to lessen the exposure of the metal to corrosive environments. While the protection is sometimes effective, it is not absolutely free from rusting. Thin coats on metal are prone to film-induced cleaving where the film protecting the metal is corroded and the corrosion works its way towards the metal (Fritz & Gerlock, 2001). When corrosion eats up the film, it will eventually find its way towards the dissolute active parts of the metals. Crevice Corrosion Areas in the metals where there is a gap for corrosive agents to lodge themselves typically house crevice corrosions. Crevice corrosions occur in crevices or in junctions. Crevices and junctions offer a unique environment for the corrosive agents. Chemicals, water vapours, and other physical corrosive agents eat their way into the crevices and thus destroying the property and the integrity of the metal (Frankel, 1998). Because these areas are typically unnoticed, maintenance is hardly done and preventive measures are typically not planned. In some cases, crevice corrosion is more like a follow-up process of the initial corrosion done by SCC. Crevice corrosions can also occur when the SCC creates a crevice on the metals where larger volume of corroding agents can enter the crevice and thus create and propagate even serious corrosion problems. Crevice corrosions deepen the cracks initiated by stresses and strains experienced by metals (Mueller, 1980). It can be said that crevice corrosions are more destructive than SCC because the effects of crevice corrosions are typically noticed when a larger, more visible portion of the metal is noticed. Accelerated low water corrosion (ALWC) Accelerated low water corrosion is easy to identify among the kinds of corrosions because this corrosion occurs in metals submerged in water. Apart from the water, the main corrosive agent in ALWC is the bacteria that thrive in the water (Beech & Campbell, 2008). In other words, AWLC is a type of corrosion that is prompted by microbes living in water. Bacteria colonies in water find its way to minute cracks in metals or its film coatings. As bacteria lodge themselves in small cracks, it take roots and emits (or injects) corrosive agents into the metal themselves. As a result, ALWC This type of corrosion typically occurs in metals that are submerged in water for a long time like ships, metal beams found in ports, and other metals submerged in water. To some degree, ALWC requires immediate attention particularly for those structures, equipments, and machineries that were submerged in water for a long time like ports and ships. Corrosion of these metals is inevitable and the best thing to do to lessen the impact of the corrosion is to replace the older submerged metals with new ones since technology is still to develop microbe-resistant metals. Case Analysis Companies dealing with various corrosive agents face the issue of controlling corrosion or stopping corrosion from interfering with its operations. One of these industries is the brewing industry where all kinds of corrosive agents – chemicals, stress and strain, and microbes are always present within its premise. According to Palmer (n.d), beer is corrosive since it contains the fauna that has the ability to destroy the integrity of metals and it is naturally acidic. Breweries are in constant struggle to minimize or eliminate the presence of corrosion in their plants because (a) the corrosion caused by beer on equipments and machineries adds to cost, (b) if such chemical reactions causes the breweries to cease its operation, it will become a major inconvenience, and (c) such corrosive reactions can produce unwanted flavours on the brewed beer. The brewing industry deals with corrosion and its adverse effects on an annual basis. Because any changes in the equipments result to significant additions to the expenses incurred by them, the brewing industry resolve this problem using carefully controlled materials planning and integration. The industry has created the maintenance standard that generally (a) does not allow stainless steel to stay exposed to the fauna and the chemicals for extended period of time, (b) use alloys specifically designed to inhibit rapid corrosion in its materials, structures, and equipments, and (c) remove the possibility of deoxidation in the solutions prepared in the production lines. 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 Cotton, J. & Potter, E. (1993). Impressions of Russian Corrosion Research. Anti-corrosion Methods and Materials. 6(4). 117-118 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 Kruger, J. (1958). Corrosion: Some Fundamental Corrosion Research at the National Bureau of Standards. 50(3); 55A-56A. Palmer, J. (n.d). Corrosion Problems in Brewing. Retrieved online http://www.realbeer.com/jjpalmer/brewcorr.txt 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