Two reactions one each at anode and cathode takes place Reaction taking place

Two reactions one each at anode and cathode takes

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Two reactions, one each at anode and cathode takes place. Reaction taking place at the anode, known as anodic reaction or oxidation reaction, destroys the anode metal. Reaction taking place at the cathode is known as cathodic reaction or reduction reaction. TYPES OF CORROSION 1. ATMOSPHERIC CORROSION Uniform attack over large areas of a metal is the most common form of atmospheric corrosion. It can be wet or dry, due to dissolved oxygen of the atmosphere. It is the formation of red-rust on the unprotected ferrous metals. This red-rust is an oxide of iron. Both oxygen and moisture are required for rusting to take place. If any of the two is not there, rusting will not take place. Once rusting starts, the action is self-generating. All traces of rust should therefore be removed before painting otherwise rusting will continue under the paint. Atmospheric pollution increases the rate of rusting of iron and steel.
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~ Page 3 of 73 ~ Waste gases discharged from chimneys normally contain CO 2 SO 2 , H 2 S and oxides of nitrogen. These gases combine with rainwater to form sulphuric acid, sulphurous acid, nitric acid, nitrous acid, carbonic acid. These are highly corrosive in the dilute state and will attack unprotected metals. Therefore atmospheric corrosion can be avoided by regular inspection and painting. 2. GALVANIC CORROSION Galvanic corrosion may occur when two different metals in contact (or connected by an electrical conductor) are exposed to a conductive solution. A difference in electrical potential exists between different metals and serves as a driving force to pass current through the corrodent. This current flow results in the corrosion of one of the metals. Fig. 1 below shows the mechanism of the Galvanic cell with iron and copper as the two electrodes. The less noble metal of the two metals, iron, will dissolve (corrode) in the electrolyte and form the anode. The more noble metal, copper, will act as the cathode. The larger the potential difference between the two metals, the greater the probability of galvanic corrosion. Table 1 gives the standard electrode potential of the metals. Gold at the top of the list is the most noble metal and will not dissolve easily. Lithium at the bottom of the list is the most active and base metal. It will readily go into solution. Galvanic corrosion can often be recognized by the increased amount of corrosion close to the junction of the two metals. It can be avoided with insulation or by restricting the use of metals together when they produce a large potential difference in the environment in question. Another method of minimizing galvanic corrosion is to avoid large noble-metal to active-metal area ratios. The relative areas of the two metals is also important. A much larger noble metal compared to the active metal will accelerate the attack, and vice-versa.
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