Chapter 13, Problem 1CO

Interpretation Introduction

Interpretation: types of corrosion and its reaction mechanism

Conceptual introduction: When metal is exposed to certain environment it get deteriorated because of the reaction occurring between environment and metal surface. The surface becomes uneven and rough and weak.

Example of corrosion: loss of silver shine, green layer formed on copper and bronze.

Answer to Problem 1CO

Solution:

There are various type of corrosion pit formation, crevice corrosion and galvanic corrosion. Reaction occurring there is

$2Fe(s)+O_2(g)+4H^{+}(aq)\to 2F{e}^{2+}(aq)+2H_{2}O(l);E_{cell}^0=1.67V$

Explanation of Solution

There are different types of corrosion:

Pitting corrosion: is the most destructive type of corrosion. It can deteriorate the metal to level where holes and pits are formed on the surface of metal. It is the localised type of corrosion. Pits penetrate vertically. It can deteriorate the complete structure. Main reason for this type of corrosion is the uneven surface if metal.

These are of following types:

a) Narrow deep cuts goes deeply in the metal structure.

b) Shallow and wide: upper area suffers the destruction

c) Undercutting: upper layer remain intact but layer below it de grades.

d) Subsurface: where on surface shows a small hole but deep inside the metal a large area is been rusted.

Crevice corrosion: is localised form of corrosion. It occurs on the metal surface which do not have equal concentration of metal ions at all the places. It occurs at bolt heads, gaskets, etcpalces where metal surface is covered with another type of surface which promotes rusting. At the point of contact the pH shifts from neutral. Since it occurs at the point of joining the proper construction of joints can reduce this type of corrosion. This type of corrosion occurs at very low temperature.

Galvanic Corrosion: is degradation of metal surface where two electrochemically dissimilar metals are joined and are in electrical contact in electrolytic environment. Joining metals which are close on galvanic series help in reducing the risk of galvanic corrosion.

Mechanism of rusting is similar in all kind of corrosion form just surface appearance is different.

According to electrochemical theory chemically non-uniform parts of iron surface acts as miniature galvanic cell in the presence of water containing the dissolved oxygen and carbon dioxide. One part act as cathode and another as anode. Electrons flow from anodic area to cathodic.

Anodic area: $\text{ Fe (s)}\to {\text{ Fe}}^{\text{2+}}{\text{(aq)+2e}}^{\text{-}}{\text{;E}}_{F{e}^{2+}/Fe}^o=-0.44V$

Oxidation occurs and iron atoms get oxidised to Fe²+ ions.

The electrons released moves through the metal to the cathode area. In cathode area the electrons reduces the oxygen in the presence of H+ ions. H+ions are formed in the water due to the dissociation of ${\text{H}}_{\text{2}}{\text{CO}}_{\text{3}}$ which is formed due to dissolution of carbon dioxide in water.

In water film reaction:

${\text{H}}_{\text{2}}{\text{O(l)+CO}}_{\text{2}}\text{(g)}\to {\text{H}}_{\text{2}}{\text{CO}}_{\text{3}}\text{(aq)}$

${\text{H}}_{\text{2}}{\text{CO}}_{\text{3}}\text{(aq) }\to {\text{ H}}^{+}(aq)+HC{O}_3^{-}(aq)$

In the cathode area:

${\text{O}}_2(g)+4H^{+}(aq)+4e^{-}\to 2H_{2}O(l);E^0=1.23V$

Overall reaction is: $2Fe(s)+O_2(g)+4H^{+}(aq)\to 2F{e}^{2+}(aq)+2H_{2}O(l);E_{cell}^0=1.67V$

Conclusion

There are various types of corrosion pit formation, crevice corrosion and galvanic corrosion. Reaction occurring there is

$2Fe(s)+O_2(g)+4H^{+}(aq)\to 2F{e}^{2+}(aq)+2H_{2}O(l);E_{cell}^0=1.67V$