PREVENTION OF CORROSION-FITTER ONLINE NOTE

Prevention of corrosion

i)- Rusting is in its simplest form, allowing it to eat away from iron at a slow pace.

ii)- Corrosion is similar to rust, but is used only to describe the corrosion of iron and its alloys.

iii)- Corrosion is a chemical process in which iron reacts with oxygen in the presence of moisture or water to produce Ferris oxide and hydroxide (called corrosion).

iv)- Rusting causes slow corrosion of iron and its alloys. This results in material weakening and eventual failure.

v)- Iron and its alloys are very widely used (some examples are pipeline structures for water and pipes such as bridges, railway tracks, ships, etc.)

vi)- Any deterioration in the quality of the metal will directly affect these structures, our economy, our health and well being. - Is going.

vii)- Thus prevention of corrosion is necessary.

viii)- There are many ways to do this, such as galvanization, paint, coating, etc.

Protective treatment of metal surface

i)- The type of protective treatment used depends upon:

ii)- The material from which the component is made 

iii)- He purpose for which it is used - the environment in which it is to operate.

Commonly used metallic corrosion-resisting coatings

a)- Hot dipping (galvanizing), b)-  Electroplating, c)- Cladding , d)- Metal spraying, e)- Cementation

Galvanizing

i)- In this process mild steel is coated with zinc.  

ii)- For hot dip galvanizing, the work-pieces are initially pickled in hot sulphuric or cold hydrochloric acid to clean the surface.

iii)- Then fluxed with zinc chloride and ammonium chloride  are dipped in molten zinc. 

iv)- Sometimes a small quantity of aluminium is added which gives a bright appearance and uniform thickness.

v)- The temperature of the zinc bath is usually maintained between 450 degree C and  465 degree C.

vi)- The hot-dipped work-pieces are then quenched  in a water bath.  

vii)- Galvanizing is done for structural work, bolts and nuts, pipes and wires which are exposed to different atmospheric conditions.  

viii)- This method is highly reliable. 

ix)- It can withstand severe working conditions and the cost is low.

Electroplating

i)- Many metals can be plated on to work-pieces electrically.In this process is called electroplating.  

ii)- In electroplating the surfaces of components are coated with another metallic coating for the purpose of obtaining decorative or protective surfaces.

iii)- In the electrolytic process the components to be plated are immersed in a solution called the electrolyte.  

iv)- The component to be plated is made as the cathode by connecting the negative pole of a low voltage, high current DC supply.   

v)- To complete the circuit, anodes connected to the positive pole of the supply are also immersed in the electrolyte. 

vi)- The electrolyte supplies the metal ions which are to be deposited on to the components (cathode).  

vii)-The anodes may be soluble and made of the same metal to be plated on the component surface i.e. nickel, copper or zinc.

viii)- Certain anodes are insoluble, for example - chromium. 

ix)- In such cases anodes are useful only to complete the circuit in the electrolytic process. 

x)- Metals like copper, chromium, cadmium, nickel, silver etc. are used for electroplating.

Cladding

i)- This is a process in which composite billets consisting of a base metal and a coating of corrosion-resistant metal are rolled or drawn.  

ii)- The thickness of the base metal and the coating reduce  proportionally. 

iii)- An application of this is cladding of steel with aluminium.

Metal Spraying

i)- Ferrous metals are sprayed with metal coatings for preventing corrosion, building up worn out shafts, providing wear-resistant surfaces etc.  

ii)- In this process molten particles of metal are sprayed on surfaces which are properly degreased and grit-blasted.  

iii)- Common metals used for metal spraying are - copper, zinc, brass, carbon steel, stainless steel etc.

Cementation

There are three  types of  cementation process for protecting metal surfaces.

a)- Sherardising (Zinc coating), b)- Calorising (Aluminium coating), c)- Chromising (Chromium coating).

Zinc

i)- A carbonate coating forms on the surface after a period of exposure, and this acts as a protective film that gradually strengthens with time.  

ii)- This coating is grey in color like the color of the parent metal itself.

iii)- This coating does not crack or peel off due to variation in temperature.  

iv)- For this reason zinc is an excellent exterior building material.  

v)- It gives excellent protection when coated on steel.

Lead

i)- Lead is one of the most corrosion-resistant of all metals. 

ii)- A large quantity of lead is used as sheathing material for underground telephones and power cables.  

iii)- The WHITE OXIDE film resulting from exposure to the atmosphere prevents further attack

Nickel

i)- Nickel is used extensively for ‘NICKEL PLATING’ as it has high resistance to chemical attack.  

ii)- When alloyed with copper in the proportion of 2:1 (Nickel two third) ‘MONEY METAL’ is produced .

iii)- Which is extremely resistant to corrosion.

iv)- Its particularly to sea water and acid

Chromium

i)- One of its most important uses is for electroplating me- tallic surfaces.  

ii)- Its do it highly resistant to the influence of corrosion .

iii)- It retains its high polish and color for a long period.

Stainless Steel

i)- It has high structural strength as well as resistance to corrosion.  

ii)- Stainless steels are not confined .

iii)- Applications requiring resistance to atmosphere corrosion. 

iv)- They are used extensively for chemical plant and food processing equipment .

v)- Where they combine corrosion resistance at elevated temperatures.