Corrosion is the deterioration or degradation of metals due to a redox reaction with a metal with a variety of substances in the environment that produce compounds that are not wanted. Corrosion is an electrochemical process or a natural reaction that goes by itself, therefore corrosion can not be controlled or its speed is slowed, thus slowing down the process of corrosion is most prevalent damaging. The chemical formula of iron rust is Fe2O3.xH2O.
Corrosion process is the transformation of metal into their compounds, especially so in an environment that contains water, or events of a metal by oxidation of gaseous oxygen in the air. A metal will experience corrosion if there is a layer on its surface that acts as the anode and the other layer as a cathode.
Several types of corrosion:
1. Uniform corrosion. corrosion that occurs on metal surfaces in the form of erosion of the metal surface evenly so that the metal thickness is reduced due to surface rust is converted by the products that typically occurs in open equipment.
2. Pitting corrosion. shaped corrosion pits on the metal surface protection films because of the destruction caused by metal corrosion rate of speed which is different from one place to another place on the metal surface.
3. Stress corrosion cracking. retak0retak shaped corrosion that are not easily seen, is formed on the surface of metal and company creep into. This occurs in many metals are a lot of pressure. This is due to a combination of tensile stress and corrosive environments that weaken the metal structure.
4. Erossion corrosion. corrosion that occurs because prevented protective film formation caused by high fluid flow rate, such as sand abrasion.
5. Galvanic corrosion. corrosion that occurs because there is a relationship between the two metals which are connected and there is a difference of potential between the two.
6. Crevice corrosion. corrosion that occurs on the sidelines continued superimposed gaskets, screws or kelingan formed by sediment or impurities arising from the corrosion products.
7. Selective corrosion. corrosion associated with removing one element from a mixture of metals.
Factors Affecting the Corrosion
Corrosion on a metal surface can be accelerated by several factors, among others:
1. Direct contact of metal with H2O and O2
Corrosion on metal surfaces is a process that contains a redox reaction. The reaction is a mini Volta cell. for example, iron corrosion occurs when there is oxygen (O2) and water (H2O). Ferrous metals are not pure, but contains a mixture of carbon that is not evenly spread in the metal. Consequently lead to differences in electrical potential between the metal atoms with carbon atoms (C). Metal atoms of iron (Fe) atoms act as anode and C as a cathode. Oxygen from the air dissolved in the water would be reduced, while the water itself functions as a media venue for the redox reaction in the event of corrosion. The more the amount of O2 and H2O which are in contact primarily to the metal surface, the faster the course of corrosion on the metal surface. Note the animation. the following: animation corrosion of iron.
2. The presence of Substance Impurity
Substance Impurities in the metal surface can cause an additional reduction reaction so that more metal atoms are oxidized. For example, the presence of carbon dust from the burning fuel on the surface of a metal capable of accelerating the oxygen reduction reaction on metal surfaces. Thus the events of accelerated corrosion.
3. Contact with Electrolyte
The presence of electrolytes, like salt in seawater can accelerate the corrosion rate by increasing the occurrence of additional reactions. While a large concentration of electrolyte can conduct electron flow so that the corrosion rate increases.
Temperature affects the rate of redox reactions on corrosion events. In general, the higher the temperature the faster the occurrence of corrosion. This is due to the increasing temperature also increases the kinetic energy of the particles so that the possibility of collisions effective in the greater redox reactions. Thus the rate of corrosion of metals is increasing. Corrosion effects caused by the influence of temperature can be seen in the instruments or machines are in use generate heat due to friction (such as cutting tools) or subjected to direct heat (such as motor vehicle engines).
• The surface of the metal
More rugged metal surface will cause the potential difference and have a tendency to be a corroded anode.
• Effects of Galvanic Coupling
Low metal purity indicates the number of atoms of other elements contained in the metal thus causing Galvanic Coupling effects, namely the emergence of a potential difference at metal surfaces due to differences in E ° between the metal atoms of different elements and found on the surface of the metal with lower purity . These effects lead to corrosion of metal surfaces by increasing the oxidation reaction at the anode.
Presence of microbial colonies on metal surfaces can lead to increased corrosion of metals. This is because the microbes are able to degrade metals through redox reactions to obtain energy for survival. Microbes are capable of causing corrosion, among others: protozoa, bacteria, iron manganese oxide, bacterial sulfate reduction, and bacterial oxidation of sulfur-sulfide. Thiobacillus Thiobacillus thiooxidans ferroxidans. Bacteria, certain types of bacteria can accelerate corrosion, because they will produce carbon dioxide (CO2) and hydrogen sulfide (H2S), during the round of his life. CO2 will lower pH significantly thereby increasing the speed of corrosion. H2S and iron sulfide, Fe2S2, the reduction of sulfate (SO42-) by sulfate reducing bacteria in anaerobic conditions, can accelerate corrosion when sulfate is in the water. These substances can increase the speed of corrosion. If there is corrosion of metallic iron then this may encourage bacterial iron (iron bacteria) to thrive, because they are happy with water containing iron.