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According to electrochemical mechlanism for corrosion, the metal undergoing corrosion acts as |
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Answer» cathode `Fe(s) rarr Fe^(2+)(aq.)+2e^(-)` We can predict that active metals, which are low oin the `emf` series, should be most susceptible to corrosion. Cesium `(Cs)` and rubidium `(Rb)`, whicha re toward the bottom of the `emf` series, corrode very quickly in MOIST air. Other metals near the bottom of the `emf` seriesusually are STORED away from air, to prevent the swift corrosion that occurs when they come in contact with the atmosphere for any period of time. However, there are exceptions to this role as some active metals like `Al` corrode quite slowly because they form protective oxide conatings. Given an oxidation, there must be a reduction to consume the ELECTRONS released in the anode process. Some of the importment cathode, or reduction, half-reaction in corrosion are `2H^(+)(aq.)+2e^(-) rarr H_(2)(g) ["acid solution"]` `O_(2)(g)+4H^(+)(aq.)+4e^(-) rarr 2H_(2)O(l) ["acid solution"]` `O_(2)(g)+2H_(2)O(l)+4e^(-) rarr 4OH^(-)(aq.) ["neutral or alkaline solution"]` The standard potential of the half-reations in which dissolved `O_(2)` is reeduced are positive,so these reduction are more favorable than those involving `H^(+)` or `H_(2)O` alone. Metals corrode faster aat higher partial pressures of oxygen. because more `O_(2)` then dissolves into the layer of moisture that is in contact with the metal. There is an interesting aspect of corrosion called the principle of differential aeration: If a part of a metal surface is exposed to a relatively high concentration of `O_(2)`, corrosion occurs in anothr region of the metal. The potential DIFFERENCE that leads to corrosion requires the physical separation of the oxidation and reduction process in corrosion, which is the reduction of `O_(2)`, occurs in the region where the concentration of `O_(2)` is highest. Therefore the oxidation half-reaction which does the real damage, takes place elsewhere. In case of iron, the `Fe^(2+)` cations tht form migrate toward the cathodic regions of the metal surface. There they react with water or `OH^(-)` to form `Fe(OH)_(2)`, which undergoes further oxidation to form `Fe(OH)_(3)`, the familiar reddis material called rust. Menshile, the anodic region of the metal surface undergoes the real damage: Holes appear, the surface is eaten away, and the metal's structural strength is wakened. The worst damage seems to occur when the reduction process liberates `H_(2)(g)`, apparently because this gas penetrates below the surface and further weakens the metal. The corrosion of an automoblie a familiar demonstration of the principle of differential aeration. When some of the paint that protects the metal of the automobile chips off, corrosion does not occur at the site of the chipping, Rust does form at this spot, becuase it is the place where the reduction half-reaction occurs. The real damage is done at the anode region, which is a site near the exposed area. We can also demonstrate corrosion by differential aeration by embeddingmetal rods in sand, leaving the upper part of the rods in water. It is the segment of the rod in the sand that corrodes, not the segment in the water. The sand prevents corrodes, not the segment in the water. The sand prevents `O_(2)` from REACHING the metal. The reducing occurs in the oxygen-rich water while the damaging oxidation process occurs in the sand. A metal can corrode if the potential of the half-reaction for its oxidation is relatively more positive than any other oxidation that can occur when it is exposed is the basis of several methods for preventing corrosion. |
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