Corrosion and Its Prevention - 2

In electrochemical corrosion, corrosion depends on the size of the anodic part of the metal. Smaller area of anode as compared to cathode will lead to faster corrosion.

In galvanic series, oxidation potential of metals is arranged in decreasing order of activity of a series of metals.
Metals, which come first in galvanic series corrode easily. Hence, Zn will corrode at the fastest rate.

Anodizing is an electrolytic passivation process, used to increase the thickness of the natural oxide layer on the surface of metal parts. While electroplating, galvanizing and metal spraying are methods of metallic coating for protection against corrosion.

Crevice corrosion occurs due to concentration difference between two regions of the same metal component. Corrosion takes place at the part of lower concentration.

Anodic coating is done by a metal, which is situated prior to the base metal in the electrochemical series.

Desiccant method is a temporary measure of corrosion protection. Desiccant bags are intended to protect the package contents from humidity during transport and storage in order to prevent corrosion, mould growth, etc.
Galvanization, tin plating and copper plating are permanent corrosion protection methods.

All the given factors affect soil corrosivity.
With the increase in acidity of soil, corrosion rate increases.
Since corrosivity is associated with electrochemical reactions in soil, high soil resistivity slows down the corrosion reactions.
Concentration of chlorides and sulphides in soil plays an important role in soil corrosivity.

Magnesium is one of the most common sacrificial anodic metals.
Magnesium is more positive than iron, and hence in electrochemical cell it acts as anode and iron acts as cathode.

If a metal rod is dipped in electrolyte, the portion dipped in water is poor in oxygen concentration and works as anode and gets corroded and the portion above the water acts as cathode, which gets protected. The system will act as a concentration cell. This type of corrosion is called differential aeration corrosion.

Drying oil absorbs oxygen and forms peroxides, hyper oxides, etc and forms tough, coherent, insoluble and highly cross-linked structure on the surface.

Chemicals, which reduce the rate of corrosion are called corrosion inhibitors. They are of two types: cathodic and anodic.
Phosphate, chromate and tungstate protect anode by forming sparingly soluble products, which are adsorbed on the surface of the metal and hence check the corrosion.
Cathodic protection is done by organic amines, mercaptans, thiourea and substituted urea. These chemicals retard reduction reaction, taking place at cathode.

Anodic coating protects underlying metal due to higher oxidation potential.

Calomel electrode is used as a reference electrode for devising galvanic series.

Sulphur reducing bacteria are anaerobic bacteria, mainly responsible for the most instances of accelerated corrosion damages to ships and offshore steel structures.

Development of corrosion-resistant alloys and the addition of inhibitors to the aggressive medium are examples of active corrosion protection.