In our daily lives, we find many cases of wear and tear or wear due to various causes, typically molded objects often oxidized to rust. People often use paint to prevent this.
1. Chemical corrosion
Chemical corrosion is caused by the metal reacting with water vapor or gas at high temperatures. Or I see people say more difficult as follows
Chemical corrosion is the redox process in which the electrons of the metal are passed directly to the substances in the environment.
Electrochemical corrosion is a redox process in which the metal is corroded by the action of an electrolyte solution that generates electrons that move from the anode to the cathode.
Conditions for electrochemical corrosion:
1) The electrodes must be different. For example Fe and Cu. Strong metals are cathode and are rapidly eroded
2) The electrodes must be in contact with each other (either directly or indirectly via conductor) and in contact with the electrolyte
2. Electrochemical Corrosion
Electrochemical corrosion is the destruction of metal by contact with the electrolyte solution to create an electric current.
For example: the shell of a submerged ship, the conduit placed in the ground, the metal in contact with the humid air ... Thus, electrochemical corrosion is the most common and most severe metal corrosion.
a) Electrochemical Corrosion Conditions: The necessary and sufficient conditions are:
- The electrodes must be different: they may be metal pairs, metal pairs - nonmetals (C), metal pairs - chemical compounds (xmentment). The metal that has a stronger reduction factor is the cathode. So pure metals are difficult to erode.
- The electrodes must be in contact with each other (either directly or indirectly through the conductor)
- The electrodes come in contact with an electrolyte solution
b) Electrochemical Corrosion Mechanisms: Iron or steel are Fe-C alloys, in which cathodes are Fe crystals, cathode-ray crystals. These electrodes come in direct contact with each other and with a liquid electrolyte coating. Thus, corroded objects are electrochemical:
At cathode: Fe atoms are oxidized. These ions are dissolved in an electrolyte solution which has an amount of oxygen, where they are further oxidized.
At the anode: The hydrogen ions of the electrolyte move to the positive charge, where they are reduced to free hydrogen and then released from the solution.
The Fe crystals are oxidized from the outside in turn. After a while, the iron (steel) will be eaten out.
c) The nature of electrochemical corrosion: the redox process occurs on the surface of the electrodes. At the cathode, the oxidation of the metal occurs. At the positive electrode, the reduction of the ionic (if the electrolyte solution is acid)
3. Comparison between electrochemical corrosion and chemical erosion
Often occurs in burner equipment or equipment that is frequently exposed to water vapor and oxygen
- The electrodes must be different, be it pairs of two different metals or pairs of metals - nonmetals or pairs of metals - chemical compounds (such as Fe3C). Where the metal has a smaller electrode potential, it will be cathode.
- The electrodes must be in direct contact or indirect contact with the wire, the electrodes must be in contact with the electrolyte solution.
Mechanism of erosion
Fe equipment exposed to steam, oxygen gas reaction occurs:
3Fe + 4H2O Fe3O4 + 4H2↑
3Fe + 2O2 Fe3O4
Electrochemical corrosion of a cast iron (Fe-C alloy) (or steel) in a moist air environment with CO2, SO2, O2 ... will produce a layer of electrolyte solution out of metal.
- Fe (cathode), crystal C is positive.
At the anode: the reduction reaction occurs:
2H+ + 2e → H2 ; O2 + 2H2O + 4e → 4OH-
At cathode: oxidation occurs:
Fe → Fe2+ + 2e
Fe2 + is dissolved in a solution containing oxide → Fe3 + and finally produces a rust containing Fe2O3.nH2O
The nature of corrosion
Reducing - oxidation, in which the electrons of the metal are passed directly to the substances in the environment, the corrosion occurs slowly.
Metal corrosion due to the effect of electrolyte solution and electric current.
Electrochemical wear is faster than chemical erosion.