Corrosion

Question 1

Corrosion

Answer 1

destruction of a substance due to reaction with environment

Question 2

Rusting

Answer 2

corrosion of iron

Question 3

examples of corrosion

Answer 3

1) Rusting of iron
2) Formation of basic CuCO3 on the surface of Cu
3) Reaction of Cl gas with Mg or Sn

Question 4

2 types of corrosion reactions

Answer 4

1) Direct chemical corrosion / dry corrosion

2) Electrochemical corrosion / wet corrosion

Question 5

When does wet corrosion take place?

Answer 5

1) when a metal is in contact with a conducting liquid

2) 2 dissimilar metals or alloys are immersed partially in a conducting solution

Question 6

conduction current

Answer 6

electrons formed in anodic area and moving through the conducting medium

Question 7

Why does corrosion take place at anode and rust is deposited at cathode?

Answer 7

the result of diffusion of Fe2+ ions and OH- ions the product Fe(OH)2 is formed more commonly at the cathode because Fe2+ ions diffuse more readily than the larger OH- ions

Question 8

cathodic protection

Answer 8

method of preventing a metallic object which is either buried in the ground or immersed in water, from corrosion. It is an electrochemical process in which the metal is to be protected is forced to act as a cathode

Question 9

2 main methods of cathodic protection

Answer 9

1) sacrificial anode protection

2) impressed current cathodic protection

Question 10

Sacrificial anode protection

Answer 10

the metallic structure which is to be protected is connected to a more anodic metal through a wire, so the entire corrosion is concentrated on the anodic metal and the metallic structure is protected sacrificially.
eg. Mg, Zn, Al and its alloys

Question 11

Sacrificial anode protection consist of mainly:

Answer 11

1) Anode usually a solid block of Mg, Zn or Al
2) Cable connection to the structure
3) test facility to monitor anode current output
4) Isolation of material between above or below ground structure

Question 12

Applications of sacrificial anode protection

Answer 12

protection of buried pipelines, underground cables, marine structure, ship body, open water tank coolers, condensers etc.

Question 13

impressed current system consist of:

Answer 13

1) a power source, usually a rectifier power or any other DC power source
2) Anodes like graphite or silica ( within a ground bed or distributed around the structure)
3) cabling and isolated material between above/below ground structures

Question 14

Applications of impressed current cathodic protection

Answer 14

1) open water box, coolers, water tanks etc
2) concrete buildings, bridges and similar structures
3) vessels, pipelines, tanks used to store or transport liquids
4) marine cathodic protection covers many areas, jetties, harbours, offshore structures etc

Question 15

galvanic series

Answer 15

series which shows the corrosion tendencies of both metals and non metals in actual environments

Question 16

factors which determine rate of corrosion

Answer 16

1) distance separating the metals on the galvanic series chart
2) temp and conc of electrolyte
3) relative size of metal pieces

Question 17

electroless plating

Answer 17

chemical, autocatalytic, metal coating process without the use of electricity

Question 18

basic principle of electroless copper plating

Answer 18

oxidation and reduction reactions are performed on the activated solid surface in solution, the reduced copper ion, in the form of film is deposited on the surface of the substance

Question 19

advantages of electroless plating

Answer 19

1) even coating, does not use electric power
2) flexibility in plating volume and thickness
3) Complex filtration method is not required
4) Matte, semi bright or bright finishes available
5) improved hardness, strength, ductility and resistance to corrosion

Question 20

applications of electroless plating

Answer 20

1) plating of printed electronic circuits
2) oilfield valves, rotors, fuel rails, door knobs etc
3) numerous consumer goods, to create durable and attractive surfaces