Corrosion of metals
|Electrochemical corrosion||Types of corrosion|
Corrosion of metals
Corrosion of metals - (from the Latin corrodere - is chew) the processes, that occur as a result of chemical exposure of environment, and result is destruction of metal.
Electrochemical corrosion- the most common type of destruction of metals. An example of electrochemical corrosion is the destruction of parts of machines, devices and various metal structures in soil, groundwater, river water and sea water, in the atmosphere under the film of moisture, in the technical solutions under the action of lubricant-coolant, etc. As already mentioned, electrochemical corrosion flows on the surface of metals under the action of electric currents, that is to happen a redox chemical reaction, characterized by the faile of electrons and transfer them as generate cathodic and anodic areas. The formation of cathodes and anodes contribute to the chemical heterogeneity of metals (impurities and inclusions), the presence of areas of residual deformation, the heterogeneity of the covering metal protective films, etc. Most often in the formation of type of destruction of the metal involved is not one factor, but several. When the metal begins to destruct, it turns into a many-electron galvanic element.
For example, consider what happens if copper (Cu) is in contact with iron (Fe) in the electrolyte. This system is a galvanic element, where iron is the anode ( " + " ) and copper - cathode. Iron gives electrons to copper and goes into solution as ions. Hydrogen ions move to copper, where discharged. The cathode is gradually becoming more negative, in the end, becomes equal to the potential of the anode and corrosion slows down.
Just on this topic you can spend the experience at home. We will need three cups with a solution of sodium chloride (table salt), 3 iron nail, a piece of zinc and copper wire (without insulation). So, let's start. The first nail drop into the glass with the salt solution. The second nail with screw copper wire drop into the next glass with the salt solution, and a piece of zinc drop into the last glass. Then dip each nail into your glass with salt solution (there were 3) and leave them for 2-3 days.
What happens: all of our nails will have traces of rust (corrosion). In the worst condition will be the nail that was in the solution together with copper wire, and the least corroded one that is bound to the zinc! Explanation: all metals have different ability to give up electrons. To compare them in this property by reading number of stresses metals:
Li←K←Rb←Cs←Ba←Ca←Na←Mg←Al←Mn←Cr←Zn←Fe←Cd←Co←Ni←Sn←Pb←H2←Cu←Ag←Hg←Pt←AuThose metals that are in the row voltages are to the left (e.g. Zn - zinc is to the left of the Fe - iron), it is easier to give their electrons than the metal to the right (for example Cu - copper to the right of the Fe - iron). So, as soon as both of the metal into the electrolyte (conductive path - salt solution), then immediately form a galvanic couple. The more active metal (standing left) is charged positively, while the less active - negative.
Back to our experience: the same thing happened in our glasses with solutions. Iron (Fe) is to the left than copper (Cu), so it is charged positively, rapidly oxidized. In a glass with zinc - zinc (Zn) is more active than iron. Therefore, until all the zinc will not rust, the iron will not be destroyed (which is often used for technical purposes).
Types of corrosionThe main types of corrosion include:
Electrochemical corrosion is more intensive if the cathode is impregnated metal, less active than corrosive. For example, if corrode steel (steel is an alloy of iron and carbon in which the partially formed iron carbide) role of such sites is iron carbide(FeC).
Protection against corrosion, electrochemical protection from corrosion