Corrosion - full details ~ MONU TUTORIALS ACADEMY

Introduction

Definitions : corrosion is the process of slow eating up away of the metal surface by the action of surroundings. Rusting of iron, furnishing of silver jewellery formation of green field on the surface of copper etc. Are some of the common example of corrosion which we come across in daily life. all metal except noble metals get corroded to some extent due to attack by natural agency light atmospheric gases and moisture.

Types of corrosion

The corrosion of metal is of two types

Chemical or dry corrosion
Electrochemical or wet corrosion.

1. Chemical or dry corrosion

this type of corrosion occurs due to direct attack by the atmospheric gases like oxygen carbon dioxide commercial for hydrogen sulphide etc. on the surface of metal when metal is kept export in the atmosphere for a long time.the surface of the metal get eaten up due to formation of compounds like oxide, hydroxide chloride basic carbon nets. On the surface of metal usually in the absence of moisture. oxygen present in the air is mainly responsible for this type of corrosion besides other gases.

According to pilling bedworth rule : an oxide will be protective or non porous oxide, if the volume of oxide formed is at least as great as the volume of metal on which it is formed. while the oxide will be porous or non protective if the volume of oxide is less than the volume of metal on which it is formed.

2. Electrochemical corrosion or wet corrosion

This type of corrosion occur when the metal is in contact with air or is kept dipped in some aquas medium. Tiny voltaic sell or gal when Excel or set up between two dissimilar metal or between dissimilar matter parts of the same metal. The metal more active metal surrounded by an AC was medium or moisture has a tendency to pass into the solution as metal ions.

an equal number of electrons are produced at the surface of metal. The positively charged metal ion combined with negative electrons at the same rate at which they are formed. And equilibrium is thus reached and father reaction stops.

Corrosion due to differential aeration or concentration cell action

this type of corrosion occurs due to difference in concentration at two points on the metal surface. Hair coma the galvanic cell are set up due to differences in concentration and are called concentration cells full stop one part of the metal is exposed to different concentration of oxygen for example than the other path which leads to difference in potential between two differential aerated areas. This causes the flow of current called differential current.

Example :-

an iron ladder when kept immersed in water for a long time, the lower part of it gets corroded to a larger extent full stop the part of ladder just below the surface of water bill become the cathode due to high concentration of dissolved oxygen whereas the path and greater that behaves as a no due to lessen

concentration of Oxygen and gets corroded first.allowed to stand on the surface of metal. The concentration of oxygen is low at the middle of the drop as compared to outer part which are more accessible to oxygen.

anode will be formed at the centre of the drawer as part of the metal touching the outer portion of the drop will be here as cathode. The flow of current take place from and not to cathode and anode dick area gets corroded.

Factor affecting corrosion

The rate and extent of corrosion depend upon the following factors:

Purity of metal :- pure metal do not undergo covid Zen common lesser is the percentage purity of the metal, faster will be the rate of corrosion. Due to the presence of impurity in the metal komatineni galvanic cells are set up. The more active metal become the anode and gets corroded.
Physical state of matter :- the rate of corrosion depend upon the physical state of the metal such as grain size,orientation of crystal. The similar is the grain size of the metal, greater will be the corrosion. Also, the area under stress suffer more corrosion.
Position of metal in the electrochemical series :- the metal higher up in the electrochemical series tend to be more anodic and hence suffer mor corrosion. Moreover, greater is the difference in the position of two matter in the electrochemical series, faster in the rate of corrosion.
Relative ratio of anodic and cathodic areas :- this rate of corrosion of a metal is more, if the inorder carier issmaller than the cathodic area. This is due to much greater current density at the smaller anodic area.
Nature of corrosion products :- the rate of corrosion is more, If the corrosion products are soluble in the recording medium. Rapid and continuous corrosion of metal takes place if the corrosion product are volatile as in case of molybdenum oxide.
Temperature :- the rate of corrosion increases with increase in temperature. Therefore caustic embrittlement takes place at high temperature in high pressure boilers.
Presence of moisture :- metal do not undergo corrosion when kept in air. The greater is the humidity the faster is the rate of corrosion. This is because moisture act as solvent for gases like oxygen carbon dioxide full stop to furnish the electrolytic essential for the setup of electrochemical cells.
pH of the medium :- the rate of corrosion depend upon the pH of the medium fullstop acidic medium is generally more than alkaline or neutral medium. The rate of corrosion of metal which are rapidly attacked by acid can be reduced by increasing the pH of the medium.

Prevention of corrosion:-

corrosion can be stopped only under ideal conditions, which are impossible to attend. However the rate and extent of corrosion can be minimised by taking suitable measures as described below,

1. Purification of metal

Pure metal do not undergo corrosion as equilibrium state is reached between metal and its ions.

the impurities present in metal are responsible for their corrosion due to formation of galvanic cells full stop the greater is the amount of impurities, the more will be the corrosion. show the impure metal are purified to control their corrosion.

2. Alloying of metal

the corrosion of metal can be decreased by making its alloy with suitable elements. For example alloying effect of chromium with iron increase its corrosion resistance due to passivation of metal surface.

3. By improving design feature

By avoiding the close contact of two dissimilar metals.
By using two dissimilar metal as close as possible in the electrochemical series.
Avoid crevices aur crack on the surface of the metal which act as moisture traps.
The area of a nordic matter should be kept large as compared to cathodic metal.
Replace riveted joint with welded.

4. Modifying the corrosive environment

defect of corrosive environment in the contact with metal can be neutralized to some extent either by removing constituent responsible for corrosion or by the addition of substances which reduce the effect of this constitutes.

Metallic coating

Usually, Alice noble metal having more reactive is used as coat metal.the coat metal having higher oxidation potential behaves as anode and protects the base metal which become the cathode. The surface of the metal to be quoted is first cleaned by mechanical rubbing and then by dipping in hot alkalis to remove oil and grease from the surface. The various method of providing protective metallic coating are discussed below,,

1. Electroplating :-

it is the process of depositing a superior metal coat on the surface of an inferior metal by the passage of electric current.

2. Hot dipping method :-

This method is used when a coating of metal having low melting point such as zinc ok and lad is desire on iron and steel or copper. The surface of the metal to be coated is first cleaned properly and dipped into a bath of molten metal whose coating is to be applied.

the process of coatings Inc on the surface of iron and steel is called galvanization and the process of coating 3 by this method is known as tinning.

3. Metal cladding :-

it is the process of bonding are comparatively thick lining of protective metal on the surface of the base metal by hot rolling. In this process the two metal sheet are rolled between heavy rollers under the conditions of high temperature and pressure and the two are bonded together. At thick, h********** layer of coating metal is bonded friendly and permanently to the base metal.a sheet of low carbon steel is covered with the thin sheet of stainless steel by this method.

4. Metal spring:-

this process involves spraying of coating metal in the molten state on the surface of base metal with the help of spray gun. A protecting coating of desired thickness can be obtained and irregular surface can also be coated well by this method.

Non metallic coating :

metal can also be protected from corrosion by applying coats of certain substances other than metals on their surface. The non metallic coating may be organic or inorganic in nature.

1. In organic coating:-

the corrosion of metal can be reduced by forming a thin film of oxide sulphide phosphorus on the metal surface.

a). Oxide coating

this coating are produced by testing the base metal with alkaline oxidizing solution or gases. The oxide film formed on the surface of metal is quite stable. When iron metal is dipped in concentrated nitric acid and oxide layer is formed on its surface which prevent further corrosion fullstop metal like aluminium chromium zinc and nickel from a protective oxide film on their surface when exposed in the air and this film protects the metal from further corrosion.

b). Phosphate coating :-

Metal like Karen commerce zinc aluminium and steel etc.can be protected by forming phosphate coating on their surface. This coating are produced by reacting the base metal with an aqua solution of phosphoric acid and phosphates of of zinc iron or manganese. This coating do not offer complete resistance to corrosion.

2. Organic coating :-

Are thin film of certain organic compounds like paint on the surface of metal may protect the underlying metal from corrosion. It can be done by the following methods

a). Buy paints and varnishes:-

area of paint of furnace applied on the metal surface protect the base metal from corrosion by Oxygen and moisture this layer tend to crack after a certain period of time. So they protect the matter for a small period of time only.

b). Plastic coating :-

the plastic coating are particularly effective in protecting the metal from corrosion when it is to be used underwater example shipping or stored in tropical climates. The surface of metal is cleaned 3D and dipped in molten plastic when a thick, adherent coating informed on cooling.

Very short answer questions

1. What is meant by corrosion?

Ans. The process of slow eating up away of the metal surface by the action of surrounding. For example rusting of iron, turnishing of silver.

2. Name three things which are mainly responsible for causing corrosion.

Ans. Oxygen, carbon dioxide and moisture.

3. Why do metal undergo corrosion?

Ans. the metal undergo corrosion due to formation of compound like oxide, hydroxide, carbonates and chlorides on the surface of metal

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