Question

a).The cast iron is used as materials for centrifugal pump which operated every day for pumping industrial effluent. As a consequence, the surface of the inside centrifugal pump become deteriorated due to corrosion attacks as shown in Figure 1.
i) Identify the type of corrosion attacked on the surface.
ii) Explain reason(s) why resistance to stress corrosion cracking is important than resistance to uniform corrosion.

b). The corrosion of an underground steel pipeline was greatly reduced when the pipeline was connected to a buried bar of aluminium alloy. With the help of simple diagram, explain the related protection method implemented to the steel pipeline.

Subject: MATERIALS AND DESIGN OF PROCESS EQUIPMENT

Answer 1

a)

i) Corrosion is wearing of mettalic structures. Corrosion is responsible for physical, mechanical and chemical change of the composition of the structure. Galvanic corrosion is the most common type of electrochemical corrosion that is seen occuring in centrifugal pumps due to variety of materials used in the composition. Electrode potential occurs when 2 different metals or alloys are present in the same electrolytic environment. The cathode, which is more lineage than the material in the environment, acts as an anode. The metal which is an anode is corroded faster than the other material. The corroded material is anodic and the other metal has a cathodic structure.

ii) Stress corrosion cracking (SCC) is the growth of crack formation in corrosive environment under the application of some tensile stress. It is more dangerous than normal corrosion because a sudden failure of a ductile metal alloy under application of tensile stress may occur. SCC is highly chemical specific, in that a certain alloys are likely to be subjected to SCC.

The chemical environment that causes SCC for a given alloy is often one which is only mildly corrosive to the metal. Hence, metal parts with severe SCC can appear bright and shiny, while being filled with microscopic cracks. This factor makes it common for SCC to go undetected prior to failure.

b)

This method of preventing the pipeline from corrosion by using an underground burried aluminium alloy is known as cathodic protection.

In this method, the pipeline carring the effluent or fluid is made as the cathode and an aluminium alloy burried underground is made as the anode of an electrochemical cell. The anode here acts as a sacrifical material and corrodes preventing the main metal from corrosion. Both the metals are connected by a metallic connection.

For structures like long pipelines, where passive galvanic cathodic protection is not adequate, a dc power source is used to generate the electric current.