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Common Types of Metal Corrosion <br />Fiber reinforced composites do not match the <br />characteristically high elastic modulus and ductility of <br />steel and other metals, yet they display lower density, <br />this often translates to favorable strength/ weight ratio <br />which, in turn, leads to favor in transportation and <br />various industrial and architectural applications. <br />Composites can present other advantages over <br />steel, such as low thermal conductivity and good <br />dielectric or electrical insulating properties. However, <br />an overwhelming advantage to composites rests with <br />corrosion resistance. <br />When the cost and benefits of FRP and special resins <br />are considered for particular environments, it is useful to <br />understand the common mechanisms by which metals <br />are oxidized or corroded. FRP is immune or otherwise <br />quite resistive to many of these influences, at least <br />within the range of practical limits of temperature and <br />stress. <br />Oxygen Cell -Galvanic Corrosion <br />The most commonly observed instances of corrosion <br />to carbon steel involve oxidation-reduction galvanic <br />couplings in the presence of molecular oxygen and <br />hydrogen ion associated with acids. <br />Oxidation (anode) <br />Fe – 2e Fee+ <br />Reduction (cathode) <br />02 + 21-12O + 4e- , 40H - <br />2H+ + 2e — H2 <br />1 "Now„ e- <br />W <br />Most forms of steel corrosion relate to some variation <br />of these mechanisms, as hereby the steel effectively <br />functions as an anode and becomes oxidized. Dissolved <br />salts and ionic components can accelerate this type <br />of corrosion by increasing electrical conductivity. It <br />can also occur in the presence of stray leaks of direct <br />current, such as in the vicinity of mass transit systems. <br />Galvanic corrosion of steel is accelerated in the vicinity <br />of metals such as copper which are cathodic to steel. <br />Due to impurities, as well as various metallurgical or <br />geometric factors, steel substrates are not always <br />uniform. There can be numerous microscopic anode - <br />cathode couplings along the surface or cross-sectional <br />gradients of the steel, and each can effectively function <br />as a galvanic oxidation cell. <br />Apart from paints and other protective or dielectric <br />coatings, various forms of cathodic protection are often <br />employed with steel. For small structures, sacrificial <br />anodes may be located near to the steel, so that <br />these anodes corrode selectively, or preferentially, <br />to the steel. Sacrificial anodes employ metals which <br />are more electronegative than iron within the galvanic <br />series. Examples include zinc, magnesium, or various <br />aluminum alloys. For larger structures, such as tanks, <br />impressed current methods are frequently used. This <br />involves use of separate anodes and DC current to <br />reverse or alter polarity, allowing the steel to function as <br />a cathode rather than as an anode, which is where the <br />oxidation occurs. <br />Galvanic corrosion is exceptionally severe in wet <br />acidic environments where free oxygen is present. <br />Flue gas desulfurization is a good example of where <br />the conditions strongly favor this type of corrosion. This <br />is due to the presence of sulfuric acid in combination <br />with oxygen associated with the excess air ordinarily <br />employed in coal combustion. Polyesters and vinyl <br />esters display excellent acid resistance and common <br />galvanic corrosion mechanisms do not influence <br />properly designed FRP. <br />El <br />