Corrosion

1.       Principle of corrosion

2.       Electrolytic Corrosion

3.       Cathodic Protection, Sacrificial Anodes

Corrosion:

Corrosion can be defined as any reaction between a metal and its environment due to which a chemical reaction takes place and metal returns to its original form of metal oxide.

Factors that causes corrosion are moisture in air, acids, salts and temperature

Principle of Corrosion

Principle are listed as

1.       During the corrosion process polarization occurs. Polarization is the formation of protective oxide film which prevent further corrosion

2.       Ferrous metals such as iron and steel when exposed to air will resist corrosion if kept dry but when in a moist or humid atmosphere the corrosion rates become rapid. If these metals are immersed in fresh water having no dissolved oxygen present in water corrosion will be active.

3.       Corrosion can be taken place under different electrical potential (voltage) when immersed in  fresh water and forming part of complete circuit.

4.       Corrosion due to acid takes place due to the formation of sulphuric acid. Vapours and water vapours in the funnel gases if the temperature is droped to new their dew point.

5.       Corrosion is the results of two actions and corrosion thus takes place is called electrochemical.

a.       Electrical

b.      Chemical

6.       The surfaces of metal forming corrosion forming cell are known as anode (corrosion area) and protected surfaces as cathode. Protection area is established by a layer of hydrogen atoms on its surface.

7.       If the water contains dissolved CO₂ gas corrosion is step up which will be continuous if O2 is present in the water the corrosion will not only be continuous but due to the regeneration of CO2 gas it will be out of all proportions to the intial contents of CO2.

Cathodic Protection

Sea water is circulated, heated and stored on ship for various purposes. It is strongly corrosive medium because it is good electrolyte. If dissimilar metals are in sea water, galvanic action results and the more anodic metal corrodes.

The metal which has lower potential becomes anode while the other with high potential becomes the cathode. Steel is anodic to bronze in sea water. Therefore it will corrodes in other word we may say that steel has given cathodic proctection.

Sacrificial Anodes

Sacrificial anodes are used to give cathodic protection to more expensive metals eg Iron anodes (Iron small plates) give protection to the brass tube and plates in condenser, magnesium, aluminium or zinc anodes gives protection to steel plates in tank and ship steel hull. Following is the table from the galvanic series of materials. In sea water any material in the table is anodic to those above it.

Titanium

Monel Metal

Stainless steel

Nickel

Gun Metal

Bronze

Copper

Brass

Cast Iron

Mild Steel

Zinc

Aluminum

Magnesium                        Base end of table

The metal with high potential generally know as metal with good resistance and called nobel metal. Metal with low negative potential are called metal.

Electrolytic Corrosion

When the different metals of potential are joined togather in an electrolyte such as sea water, an electrical current flow between them due to the potential difference (Voltage) present. Each metal has certain electrical potential which is determined by their individuals atomic structure. An atom is electrically neutral. If an atom will loose or gain electron know as Ion and becomes electrically unbalanced and due to this action electrical current flow in the circuit.

When two dissimilar metal are in contact, the on which has lower potential becomes anode while the metal with high potential becomes the cathode. Thus if these two metals are in contact in the presense of water (Electrolyte) a corrosion cell is established. The metal (with low potential) will remain protected.