Metallurgical Processes

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.

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.

Anodic coating protects underlying metal due to higher oxidation potential.

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

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

The refining of a metal (such as copper) by electrolysis, in which the crude metal used as the anode goes into solution and the pure metal is deposited upon the cathode is called electrolytic refining.

Zone melting (or zone refining or floating zone process) is a group of similar methods of purifying crystals, in which a narrow region of a crystal is molten, and this molten zone is moved along the crystal (in practice, the crystal is pulled through the heater). The molten region melts impure solid at its forward edge and leaves a wake of purer material solidified behind it as it moves through the ingot. The impurities concentrate in the melt, and are moved to one end of the ingot.

 Barrier protection is the oldest and most widely used method of corrosion protection. It acts by isolating the base metal from the environment. Like paints, the hot-dip galvanized coating provides barrier protection. A barrier film is introduced between iron and atmospheric oxygen and moisture.

This process is used for refining easily fusible metals like lead and tin. The impure metal is heated on the sloppy hearth of an electric furnace. The metal melts and flows down, leaving the impurities.