Corrosion And Rusting Essays (2147 words) - Corrosion, Iron, Rust

Corrosion and Rusting Introduction Some people may be annoyed by their car "wearing out". Kids may have trouble with rust forming on their bicycles. One may think how to prevent rusting, but do one knows what is happening when a metal corrode? "Corrosion is defined as the involuntary destruction of substances such as metals and mineral building material by surrounding media, which are usually liquid (i.e. corrosive agents)." Most metals corrode. During corrosion, they change into metallic ions. In some cases, the product of corrosion itself forms a protective coating. "For example, aluminium forms a thin protective oxide layer which is impervious to air and water. In other cases (e.g. iron), however, the coating either flakes off or is pervious to both air and water. So the whole piece of metal can corrode right through." The most common forms of metallic corrosion are caused by electrochemical reactions, wherein two metallic phases (e.g., iron oxide and iron) react in the presence of electrolytic solution. Another mechanism of metallic corrosion is caused by chemical reaction, which explains how the protective layer of the metal is formed. Rusting is the corrosion of iron which is the most widely used structural metal. Most of it is used in making steel. The wide range of products made from steel includes all types of vehicles, machinery, pipelines, bridges, and reinforcing rods and girders for construction purposes. Therefore, rusting causes enormous economic problem and is the reason why extensive measures of corrosion protection have had to be developed. The economic importance of corrosion and corrosion protection can be shown by the following example: "It is estimated that roughly 3% of the annual production of steel is lost by corrosion. In 1974, 140 millions tons of steel were produced in the United States at a cost of approximately $400 per tons. So this gives a monetary loss of about 1.7 billion dollars." It is clearly of the utmost importance to reduce as far as possible the financial loss by corrosion, which not only affects steel but to the extent all other building metal as well. It is obvious that corrosion and rusting affect significantly the life of the society, so it is worthy to investigate this topic. In this essay, the cause of the corrosion and rusting and consequently the protection of the corrosion will be explored. Electrochemical corrosion reactions This type of corrosion takes place when two metallic phases with different electrochemical potentials are connected to each other by means of an electric conductor. Electrolytes such as acids, alkalis, salt solutions, or even milder media (e.g., rainwater, river water, groundwater, or tap water) also need to be present. "Metallic phases with different electrochemical potentials exhibit electric potential differences. Potential differences may also arise because of impurities, internal stresses, corrosion products, damaged protective coatings, etc. They also occur when different metals are used. The larger the potential difference, the faster the rate of corrosion." The electrochemical EMF series (Table 1) gives the electrochemical potential of metals under normal conditions with respect to hydrogen (hydrogen is 0). The farther two metals in electrochemical series are apart, the larger the potential difference between them. A metal is said to be less noble than those which stand to its right in the electrochemical series. In the case of electrochemical corrosion it is always the less noble metal which is removed. Table 1. Electrochemical Potential Series, Volts. K Ca Mg Al Zn Cr Fe Ni Sn Pb H Cu Ag Au -2.92 -2.84 -2.38 -1.66 -0.76 -0.71 -0.44 -0.24 -0.14 -0.13 0.00 0.34 0.80 1.42 not noble -----------------------------------------------------------------> noble Likelihood of passing into solution decreases from left to right. The potential difference does not, however, always fully correspond with the corrosion phenomena experienced in practice. The reason is that oxide and other metal compounds have differing electrochemical potentials. Chemical corrosion reactions Metals have a tendency to combine with oxygen to form oxides and this is one of the chemical reactions. This tendency is the stronger the less noble the metal. The layers of oxide on the metal surface which are formed even in dry air may be insoluble and stable against an aqueous medium in contact with them. Therefore, if the oxide layers are dense and adhere well to the metal, they prevent further attack and act as a corrosion prevention layer.

Total drag and its variation with altitude Essay Example

Total drag and its variation with altitude Essay Example Total drag and its variation with altitude Paper Total drag and its variation with altitude Paper The equation for total drag is: D = CD x S x ? rV2 (Preston, R) where, CD is the coefficient of drag. It must be subdivided into two parts, the Cdi (Coefficient of induced drag) and CDp (Coefficient of parasite drag. ). Therefore it can be written as: D = (Cdi + Cdp) x S x ? rV2 (Preston, R) The airplanes total drag determines the amount of thrust required at a given airspeed. Thrust must equal drag in steady flight. Lift and drag vary directly with the density of the air. As air density increases, lift and drag increase and as air density decreases, lift and drag decrease. Thus, both lift and drag will decrease at higher altitudes. Fig 1 shows the total drag curve which represents drag against velocity of the object. The fuel-flow versus velocity graph for an air graph is derived from this graph, and generally looks as shown in Fig 2 From the above drag it is seen that the total drag is minimum at a certain velocity. This occurs when the parasitic drag is equal to the induced drag. Below this speed induced drag dominates, and above this speed parasite drag dominates. Design engineers are interested in minimizing the total drag. Unfortunately many factors may conflict. For example, longer wing span reduces induced drag, but the larger frontal area usually means a higher coefficient of parasite drag. Conversely, a high wing loading (i. e. a small wing) with a small aspect ratio produces the lowest possible parasite drag but unfortunately is the produces for a lot of induced drag. In recent time it is seen that jet airliners have longer wings, to reduce induced drag, and then fly at higher altitudes to reduce the parasite drag. This causes no improvement in aerodynamic efficiency, but the higher altitudes do result in more efficient engine operation. (Preston, R) Angle of Attack (AOA), is the angle between the wing and the relative wind. Everything else being costant, an increase in AOA results in an increase in lift. This increase continues until the stall AOA is reached then the trend reverses itself and an increase in AOA results in decreased lift. The pilot uses the elevators to change the angle of attack until the wings produce the lift necessary for the desired maneuver. Besides AOA other factors also contribute to the production of lift, like relative wind velocity and air density i. e. temperature and altitude. Changing the size or shape of the wing (lowering the flaps) will also change the production of lift. Airspeed is absolutely necessary to produce lift. If there is no airflow past the wing, no air can be diverted downward. At low airspeed, the wing must fly at a high AOA to divert enough air downward to produce adequate lift. As airspeed increases, the wing can fly at lower AOAs to produce the needed lift. This is why airplanes flying relatively slow must be nose high (like an airliner just before landing or just as it takes off) but at high airspeeds fly with the fuselage fairly level. The key is that the wings dont have to divert fast moving air down nearly as much as they do to slow moving air. Air density also contributes to the wings ability to produce lift. This is manifested primarily in an increase in altitude, which decreases air density. As the density decreases, the wing must push a greater volume of air downward by flying faster or push it down harder by increasing the angle of attack. This is why aircraft that fly very high must either go very fast e. g. Mach 3, or must have a very large wing for its weight. This is why the large passenger airplanes cruise at higher altitude to reduce drag, and hence save on the furl costs. (â€Å"Aircraft for Amateurs†, 1999) Small sized aircrafts have lower than normal Reynolds number. The drag coefficient attributable to skin friction is hence higher for the small aircraft. For this reason, the maximum lift-drag ratios characteristic of business jet aircraft tend to be lower than those of the large transports. Hence, the smaller flights can fly at relatively lower altitudes. References Books John A. Roberson Clayton T. Crowe, 1997, Engineering fluid Mechanics, 6th ed. , John Weily Sons Inc., ISBN 0-471-14735-4. Clement Klienstreuer, 1997, Engineering Fluid Dynamics, Cambridge University Press, ISBN 0-521-49670-5 Websites â€Å"Aircraft for Amateurs†, 11th Jan. 1999 fas. org/man/dod-101/sys/ac/intro. htm Benson, T. , â€Å"The Beginner’s guide to Aeronautics†. , 14th March 2006 grc. nasa. gov/WWW/K-12/////airplane/ Johnston, D. , â€Å"Drag†, centennialofflight. gov/essay/Theories_of_Flight/drag/TH4. htm â€Å"Parasitic Drag†, http://adg. stanford. edu/aa241/drag/parasitedrag. html Preston, R. , â€Å"Total Drag† and â€Å"Flight Controls†, http://selair. selkirk. bc. ca/aerodynamics1/