Video English Naration

Corrosion

Corrosion in metals occurs due to corrosive interactions between metals and the environment, the moist environment (containing moisture) and induced by the presence of O2, CO2, or H2S gases. Corrosion can also occur due to high temperatures. Corrosion in metals can also be viewed as a process of returning the metal to its original state, ie metal ore. For example, corrosion of iron to iron oxide or iron carbonate.

4Fe (s) + 3O2 (g) + 2nH2O (l)  → 2Fe2O3.nH2O (s)
Fe (s) + CO2 (g) + H2O (l)         → Fe2CO3 (s) + H2 (g)

Because corrosion can change the structure and properties of metals then corrosion is likely to be detrimental. It is estimated that about 20% of the metal is damaged due to corrosion in each year.

The corroded metal is caused because the metal is easily oxidized. According to the standard reduction potential table, in addition to the gold metal generally metals have a lower standard reduction potential of oxygen.

If half the metal reduction reaction is reversed (metal oxidation reaction) coupled with a half of the O2 gas reduction reaction it will produce a cell potential value, positive Esel. Thus, almost all metals can react with O2 gas spontaneously. Some examples of metals that can be oxidized by oxygen are shown in the following reaction equation.

4Fe (s) + O2 (g) + 2nH2O (l) → 2Fe2O3.nH2O (s) Esel = 0.95 V
Zn (s) + O2 (g) + 2H2O (l)     → Zn (OH) 4 (s)         Esel = 0.60 V

Mechanism of Corrosion in Iron

Mechanism of Corrosion in IronBecause iron is the main ingredient for various construction corrosion control becomes very important. To be able to control the corrosion would have to understand how the mechanism of corrosion in iron. Corrosion is classified as an electrochemical process, as shown in the illustration.

Iron has a non-smooth surface due to imperfect composition, also due to the difference in surface tension that poses a potential in a certain area higher than other regions.

In the anodic area (surface area in contact with water) occurs the dissolution of iron atoms with the release of electrons to form water-soluble Fe2⁺ ions.

Fe (s) → Fe2 + (aq) + 2e-

The released electrons flow through the iron, just as the electrons flow through the outer circuit of the voltaic cell to the cathodic area until there is an oxygen gas reduction from the air:

O2 (g) + 2H2O (g) + 2e- → 4OH- (aq)

The corrosion mechanism of Fe 2+ Fe2 + is dissolved in water droplets moving toward the cathodic region, as the ions pass through the salt bridge in the voltaic cell and react with the OH-ions forming Fe (OH) 2. Fe (OH) 2 formed is oxidized by oxygen to form rust.

Fe2 + (aq) + 4OH- (aq) → Fe (OH) 2 (s)
2Fe (OH) 2 (s) + O2 (g) → Fe2O3.nH2O (s)

The overall reaction to the iron corrosion is as follows (see mechanisms on the picture aside):

4Fe (s) + 3O2 (g) + n H2O (l) → 2Fe2O3.nH2O (s)                (Rust)

Due to the migration of ions and electrons, the rust often forms in the area some distance away from the corroded iron surface (holes). The color on the carat ranges from yellow to brown-red even to black. This color depends on the number of H2O molecules attached to the rust.

Corrosion can occur if there is air (especially O2 gas) and water. If there is only water or O2 gas, corrosion does not occur.

The presence of water-soluble salts will accelerate the corrosion process. This is because in salt solution there are ions that help speed up the delivery of oxidation Fe2 + ions.

Hardness of rust increases rapidly by the presence of salt because salt solubility increases the conductivity of ions by solution so as to accelerate the corrosion process. Chloride ions also form stable complex compounds with Fe3 + ions. This factor tends to increase the solubility of iron so it can accelerate corrosion.

 https://youtu.be/tlvnK_3gP9s