Atmospheric corrosion

CORROSION AND ITS INHIBITION
1.What is corrosion?
Corrosion is defined as the gradual destruction or deterioration of metals or alloys by the chemical or electrochemical reaction with its environment.

2.What is meant by rusting of iron?
The atmospheric corrosion of iron and steel leading to the formation of a layer of reddish scale and powder of ironoxide (Fe3O4) on the surface is known as rusting.

3. Distinguish between dry corrosion and wet corrosion.

S.NO.	DRY OR CHEMICAL CORROSION	WET OR ELECTROCHEMICAL CORROSION
1.	It occurs in dry state.	It occurs in presence of moisture or electrolyte.
2.	It follows adsorption mechanism.	It follows the mechanism of electro chemical reaction.
3.	Corrosion product accumulate on the same spot, where corrosion occurs.	Corrosion occurs at anode while products gather at cathode.

4. Explain the consequences of corrosion.
1.Plant shut down due to failure
2. The product gets contaminated due to corrosion.
3. Corrosion releases toxic products, health hazards
4. Loss of efficiency of equipment.

 

5. What is Pilling- Bedworth ratio (or) Pilling – Bed worth rule? Explain the nature of Aluminium oxide and Magnesium oxide.
The ratio of the volume of the oxide formed to the volume of the metal consumed is called “Pilling – bedworth rule.
Aluminium oxide – Stable oxide layer (non-porous and protective)
Magnesium oxide-Unstable layer (porous and non-protective)

6. What is meant by hydrogen embrittlement?
Loss in ductility of a material in the presence of  hydrogen is known
as hydrogen embrittlement.

7. What is decarburisation?
The process of decrease in carbon content in steel is termed as
“decarburisation” of steel.

8. What are the conditions of electrochemical corrosion?
Electrochemical corrosion is the one which occur,
i) When two dissimilar metals or alloys are in contact with each other
in presence of an aqueous solution or moisture.
ii) When a metal is exposed to varying concentration of oxygen or any
electrolyte .

9. What is galvanic cell corrosion. Give examples.

           When two different metals are in contact with each other in presence of an aqueous solution (or) moisture, galvanic corrosion occurs.

Here the more active metal acts as anode and the less active metal acts as cathode.
Example:
In Zn – Fe couple, Zn acts as anode and undergoes corrosion. But in     Fe-Cu couple, Fe acts as anode and undergoes corrosion.

10. Steel screw in a brass marine hardware corrodes- Explain.
This is due to galvanic corrosion. Iron (higher position in electrochemical series) becomes anodic and is attacked and corroded, while brass (lower in electrochemical series) acts as cathodic and is not attacked.

11.Name the factors which affect corrosion.
i) Air and moisture
ii) Electrolyte in water
iii)Presence of impurities in a metal
iv)Presence of gases like SO2 and CO2 in its vicinity
v) Presence of strains in the metal.
vi) Differential aeration

12. Bolt and nut made of the same metal is preferred in practice. Why?
It is preferred in practice, because galvanic corrosion is avoided due          to homogeneous metals (no anodic and cathodic part).

13. Iron corrodes under drops of salt solution.Explain.
This is due to differential aeration.Areas of iron covered by drops,having poor access to oxygen,become anodic with respect to other areas which are freely exposed to air.Due to electrochemical corrosion ,the areas under drops (anodic) undergo corrosion,while the freely exposed parts remain unaffected.

14.Corrosion of a metal is the highest at the metal junction in a galvanic couple. Account the reason.
Metal junction,where air cannot diffuse easily, is less aerated and becomes anodic, the remaining parts are more aerated and becomes cathodic. So, the corrosion concentrates at the metal junction in a galvanic couple.

15.Iron is corroded faster than aluminium, even though iron is placed below aluminium in the EMF series. Give reason.
Al forms a thin, non-porous, tightly adhering protective film of        Al2O3 on its surface and this film does not allow Al to undergo further corrosion.

16.Impure metal corrodes faster than pure metal under identical 
conditions. Give reason.
Impurities in a metal generally cause heterogeneity and form minute electrochemical cells at the exposed parts.The anodic parts get easily corroded.

17. An iron pipe does not rust when connected to zinc but rusts rapidly when
it is in contact with copper.Explain.
In the former case, zinc acts as anode and iron acts as cathode.But   in the latter case, iron acts as anode (higher in electrochemical series) and copper acts as cathode. Hence, iron rusts rapidly when it is in contact with copper.

18. What is a sacrificial anode? How does it protect a submerged pipeline?
A sacrificial anode is more active metal than the anodic part of the corrosion cell. The sacrificial anode is connected to the submerged pipeline. So the submerged pipeline will be converted to cathode and corrosion concentrates over the sacrificial anode.

19.What is cathodic protection?
It is the protection of the parent metal from corrosion by connecting it with a more active or anodic metal like Al,Mg.The connected anode  metal undergoes corrosion thereby protecting the parent metal  from corrosion.

20.What are corrosion inhibitors? Give examples.
Inhibitors are organic or inorganic chemicals which when added in small quantity to the corrosive environment effectively decrease the corrosion rate.
Anodic inhibitors-phosphates,chromates.
Cathodic inhibitors-mercaptans,aniline and its derivatives.

21.What are vapour phase inhibitors(VPI)?
Vapour phase inhibitors (VPI) are organic inhibitors, which readily vapourise and form a protective layer on the metal surface. VPI are used in the protection of storage containers, packing materials, sophisticated equipments, etc.

22.Which of the following metals could provide cathodic protection to iron:
Al, Zn, Cu, Ni.
Al and Zn

23. What are the advantages of electroless plating over electroplating?
i) No electricity is required
ii) Complicated parts can also be plated uniformly
iii) Electroless plating can be carried out on non-metals like 
Glass, plastics and semiconductors.

 24.Why is moderate temperature used during electroplating?
High temperature leads to spongy and loosely held deposit.
Low temperature favours burnt deposit (dark and powdery deposit).
Hence a moderate temperature is used.       

 

25.How electroplating differs from electro less plating?

 

S.No	Electroplating	Electroless plating
1.	It is carried out by passing current.	It is carried out by autocatalytic redox reaction.
2.	Separate anode is employed.	Catalytic surface of the substrate acts as an anode.
3.	Object to be coated is cathode.	Object to be coated after making its surface catalytically active.
4.	It is carried out on conducting materials.	It is carried out on conducting,semiconducting (plastics) material.