Chapter 5

Question 1

What is Corrosion

Answer 1

Corrosion is the deterioration of metal by chemical reaction. The tendency to corrode comes about because a metal wants to attain a more stable (less reactive) state

Question 2

Two mechanisms of Corrosion

Answer 2

1. Direct Chemical attack or oxidation
2. Electro-chemical attack or Electrolytic corrosion

Question 3

What are the basic requirements for electrolytic corrosion to take place?

Answer 3

1. An electrolyte in contact with the mental, and
2. The existence of a difference in electrode potential between the anode and the cathode.

Question 4

How does the EMF series determine the rate of corrosion between 2 metals in the series? which is corroded and which is protected?

Answer 4

The further the 2 metals are separated in the series, the greater their electric potential difference will be, so the greater rate of corrosion.

The more cathodic metal will be protected while the more anodic metal will corrode according to the series

Question 5

What is weld decay

Answer 5

Weld Decay is the intergranular corrosion of 18-8stainless steel due to welding. When the steel is heated between 500 to 800 degrees and allowed to cool slowly, precipitation of chromium carbide along the HAZ grain boundaries will result in intergranular cracking of the steel.

Question 6

How can Weld Decay be prevented

Answer 6

1) Very rapid cooling of the stainless steel in the critical temperature range (500 to 800 degrees)

2) Add strong carbide formers such as niobium and titanium

3) Lower the carbon to below 0.03%

Question 7

Compare and contract the uniform corrosion and intergranular corrosion with respect to effects on mechanical properties

Answer 7

Intergranular corrosion, the corroded zone is more like to hairline crack. The stress concentrations produced by it is much more serious. Because of the notch effect produced by corrosion, the mechanical properties, especially elongation are drastically reduced.

In Uniform corrosion, mechanical properties are not seriously affected as the corrosion occurs uniformly on a wide front.

Question 8

What are the main functions which cause steels to rust in the atmosphere?

Answer 8

Availability of moisture and Extent of air pollution

Question 9

Explain why steel structures in industrial districts tend to corrode faster?

Answer 9

In industrial districts, sulphur dioxide form the oxidation of hydrogen sulphide or combination of fuel oxidised to sulphur trioxide which forms sulphuric acid with moisture.

This in turn attacks to form ferrous sulphate. Hence, steel structures corrode faster in industrial districts.

Question 10

5 common methods to protect metals against corrosion?

Answer 10

1) Correct choice of metal
2) Design to minimise the effects of corroding agents
3) Cathodic Protection
4) Use of Protective coatings
5) Environmental controls

Question 11

Compare and contract the coating of a steel plate with tin and zinc

Answer 11

Both Tin and Zinc plating are able to protect the steel plate from corrosion provided that there is no damage to the plating.

Once there is damage to the platin which exposes the underlying steel, the zinc plate will continue to protect the steel because the zinc is more anodic than steel in the EMF series.

Steel will corrode if the tin is damaged as steel is more anodic than tin in the EMF series.

Question 12

Types of corrosion

Answer 12

1) Uniform Corrosion
2) Galvanic Corrosion
3) Crevice Corrosion
4) Pitting Corrosion
5) Intergranular Corrosion
6) Stress Corrosion
7) Biological Corrosion

Question 13

Describe Uniform Corrosion

Answer 13

Corrosion of metal commonly occurs on metal surfaces having homogeneity of chemical composition and microstructure. Access to the metal by the attacking environment is generally unrestricted and uniform.

Question 14

What are galvanic corrosion and what factors effect the severity of galvanic corrosion

Answer 14

1) The difference in electrode potential between 2 metals
2) The relative surface areas of the anode and cathode

Question 15

What is galvanic corrosion

Answer 15

This type of corrosion is the result of 2 dissimilar metals of alloys coming into contact in the presence of an electrolyte

Question 16

What is Crevice corrosion

Answer 16

Localised attach occur in a crevice (narrow opening) due to the limited access of oxygen to the area within the crevice

Question 17

What is pitting Corrosion

Answer 17

Pitting is the local corrosive that produces holes or pits in a metal. Most destructive for engineering structures it causes perforation of the metal

Question 18

What is Stress Corrosion

Answer 18

This takes place very often in cold worked materials where the distortion at the slip planes produced by the working may cause accelerated precipitation of solutes and consequent depletion (loss) of solid solution.

Question 19

How can stress corrosion be overcome

Answer 19

1) Remove stress through annealing by redesigning the area where stress concentrations are found and or
2) Remove or neutralise the chemical corroding medium.

Question 20

what is Intergranular corrosion

Answer 20

Occurs in alloys in which a potential difference exists between the grain boundaries and the center of the grain. This type of corrosion is widespread and is particularly serious in aluminum alloys containing copper and in austenitic stainless steel containing carbon.

Question 21

What are 5 corrosion controls

Answer 21

1. Correct choice of metals
2. Design to minimise the effects of corroding agents
3. Cathodic protection
4. Use of protective coatings
5. Environmental Control

Question 22

Types of protective coatings

Answer 22

1. Metallic Coatings
2. Inorganic Coatings (Ceramic and glass)