Question

UNIT EIGHT - ANSWER EACH QUESTION

1.  Explain the difference between coating and cladding

2. Describe the two main methods of painting: dipping and spraying (manual, automatic or electrostatic).
a) State the characteristic of each method.
b) Identify materials suitable for each method.

3. Describe the electroplating process.
a) List typical plating materials and give reasons for their use.

4. Explain the process of anodizing and state typical applications.

5. Classify vapour deposition processes into physical vapour deposition (PVD) and chemical vapour deposition (CVD).
a) Describe the principle of each process.
b) Identify applications of each process.

6. Recognize the importance of thermal spraying.
a) Explain the principle of thermal spraying.
b) Identify typical thermal spraying processes according to the technology used to melt the coating material.:- oxyfuel flame, electric arc and plasma arc.

Answer 1

1) Cladding:-

• It is the process of sandwiching the base metal between two thin layers of coating metal by hot-rolling the composite to produce a firm bonding.

The coat metals are usually metals of least reactivity (Cu, Ni, Ag, Pt, Ti) The cladding layer should be very thin and its thickness is only 5% of the total composite metal.

Example :- Duraluminium sandwiched between Al sheets and hot rolled to produce Alkad composite which is free from stress corrosion.

Coating:-

It is a method of deposition of one material on other material in order to secure the inner material for various reasons like increasing strength or to protect it from corrosion etc. It is mostly done for metals.

There are two types of metal coating:

(i)Anodic coatings:

• If the coating metal is higher placed in galvanic series than the base metal, then the coating is called as anodic coating. e.g. Zn, Al, Cr coated on steel.

(ii)Cathodic coatings:

• If the coating metal is lower placed in galvanic series than the base metal, then the coating is called as cathodic coating. e.g. coating of tin on steel, silver or gold on brass, etc.  

2)

Dipping:-

a) Dipping method completely submerges a component in a tank. This method is effective for fully coating aerospace parts, which makes it a good choice for protective coatings. The full immersion technique ensures that all areas are coated; however, this technique does not typically provide a high quality finish as drips are possible when the parts are hung to dry.

The dipping method requires a large tank to fully immerse parts, which makes it a poor choice for finishing larger aircraft components as the size of the tank becomes cost prohibitive. Dipping is most often used in factories and at large repair stations.

b) In general, dipping is best suited for small components and protective coatings.

Spraying:-

a) Spray applications use an atomized cloud, or spray, of the finishing material and compressed air to evenly coat surfaces. The spray application method has the highest quality finish of the three options discussed in this article. However, to maintain the high quality finish, spray applications have more stringent requirements. Water-based paints cannot be contaminated by oil or by other debris and filters are required to collect any debris within the painting environment. To adhere to these requirements, spraying is often completed in a spray paint booth where the environment can be controlled for the proper humidity and air cleanliness levels.

b) As a general rule, spray application is recommended for High quality finishes and Large components.