Applied science Sem 1 UNIT 3

 

 UNIT 3 : Corrosion

 

Q1) Explain any five factors affecting corrosion.

1. Position in Galvanic Series:
• More active metals corrode faster when in electrical contact in a conducting medium.
2. Relative Areas of Anode and Cathode:
• Corrosion rate depends on the ratio of cathodic to anodic area.
• a larger anodic area minimizes corrosion.
3. Purity of Metal:
• Impurities create galvanic cells, leading to corrosion.
4. Temperature:
• Higher temperatures speed up both atmospheric and electrochemical corrosion.
5. Conductivity of Corroding Medium:
• The electrical conductivity of the medium affects the rate of electrochemical corrosion.
• Higher conductivity results in faster corrosion.

 

 

 

 

Q2) Define Hot dipping. Explain coating method with neat labelled diagram to protect metal from corrosion and give applications.

Hot Dipping: Hot dipping is a method that involves coating metals like steel, copper, or brass with low melting metals such as Zinc (419°C), tin (232°C), lead, etc.

1. Galvanizing:
• Method: Clean the steel article with dilute H2SO4, wash, and dry it. Dip it in molten zinc at 425-450°C with a flux like NH4Cl. Roll the article to ensure uniform coating and remove excess zinc. Cool slowly.
• Applications: Widely used for iron protection, producing items like G.I. sheets, wires, pipes, buckets, tubes, screws, nails, etc. Caution: G.I. vessels shouldn't store food due to potential formation of poisonous products.

2. Tinning:
• Method: Cleaned steel passes through a molten tin bath at about 240°C and then through palm oil to protect the surface against corrosion. Rollers remove excess tin and ensure uniform coating.
• Applications: Tin offers high corrosion resistance.

1. Tinned containers for storing foods, ghee, oils, pickles, medicines.

2.     Tinning copper wires to avoid rubber's impact.

3. Tinned copper or brass vessels/sheets for cooking utensils and refrigeration equipment.

 

 

 

 

Q3) Write a note on cementation method of applying metallic coatings on base metal.

Method: In the cementation technique, there is formation of a strong layer of alloy of coating metal and base metal, on the surface of base metal.. Here's how it works:

1. Pack the base metal articles in- coating metal powder.
2. Heat them just below the melting point of the lower melting component.
3. Diffusion of coating metal into the base metal creates an alloy.
4. The alloy composition varies, with more coating metal on the surface and a richer base metal layer adjacent to the base metal.

Applications: This method is ideal for base metals like steels, commonly used for coating small articles such as nuts, bolts, screws, spanners, screwdrivers, threaded parts, tools, furnace parts, and turbine blades. The resulting coatings are strong and offer excellent abrasion resistance.

Note:

• If zinc is the coating metal, it's called sherardizing.
• If aluminium is the coating metal, it's known as colorizing.
• If chromium is the coating metal, it's referred to as chromizing.