Q.4 Explain briefly the following with suitable examples :
(i) Electroplating (ii) Galvanizing (AKTU. 2008-09)
Related Questions -
Q. Explain th purpose of galvanizing. (AKTU. 2012 - 13)
Ans. (i) Electroplating: -
Electroplating is a very popular process of providing metallic coatings by means of electrolysis. The article to be coated may be metallic or non-metallic (plastics). If metallic, the base metal and the coated metal may be same or different. Coatings of several different metals and alloys can be provided through this technique. Some common metals and alloys, of which coatings can be provided on metallic and non-metallic surfaces, include chromium, zinc, tin, nickel, copper, cadmium, silver, gold, platinum, rhodium, brass, bronze, etc.
The basic principle involved in electroplating of all metals is the same. However, some small variations are there in different cases. The main elements involved in this process are an electrolyte, a tank containing this electrolyte, an anode (+ve terminal), a cathode (-ve terminal) and a low voltage direct current (d.c.) supply. The electrolyte used is nothing but a metallic salt solution i.e., dissolved salts of the plating metal. The work pieces to be plated are suspended and immersed in the electrolyte filled tank. Electrically they are connected to the negative terminal so as to act as cathode in the circuit. The material to be deposited to provide the coating is connected to the positive terminal, so as to act as anode. When plastics are to be plated, a primary coat of an electrically conductive material is first provided over them and then they are electroplated.
As stated above, a wide variety of metals are used as plating materials. But, out of the these, the most commonly used plating material is chromium. Tin, however, is an exclusive plating material is so far as the making of tin plates for container manufacture is concerned. In majority of the cases, a thin primary layer of copper and nickel is provided over the base metal before chromium plating. While the electrolyte mainly consists of dissolved salts of the plating metal, other materials may be added to it to improve its conductivity.
(ii) Galvanizing: -
Galvanizing can be described as that process which employs an electrochemical action for providing a coating of a highly corrosion resistant material on the surface of another metal. This process (galvanizing) is largely used for providing a coating of zinc on iron and steel. It is a fairly low-cost process and is widely used for corrosion resistant coatings on sheet metal, house hold items of daily used made from iron and steel-like buckets, tubs and others containers, and machine parts, tools, ships, tanks, wires, different threaded fasteners, etc. Galvanized metal sheets, pipes and wires are probably the most popular galvanized items which find application both in industrial use as in articles made for domestic use.
Q.5. In which process coating of highly corrosion resistant material is done by means of electrochemical reaction? (AKTU. 2011 - 12)
Ans. Galvanizing
Q.6. Explain the purpose of galvanizing. Discuss the hot dip galvanizing process. (AKTU. 2012 - 13)
Ans. Hot - dipping Galvanizing:- It is a very ancient and easy technique and is probably the cheapest of all the galvanizing techniques employed in mass production.
The first step in applying this process is through cleaning of the parts or items to be galvanized. This may involve degreasing, acid rinsing in cold water, ect. depending upon the condition of the work-surfaces. The basic idea is to obtain an absolutely clean surface. If the item to be galvanized is sheetmetal, it is annealed after surface cleaning and then cooled in an oxide free atmosphere. This cooling continues until the sheet metal temperature comes down to the level of molten zinc bath temperature. The sheet is then dipped in the bath. If perfect and uniform thickness is an important consideration the sheets are drawn through a set of rollers, immediately following the coating process. This process drives away the excess zinc and allows only the required amount of zinc to remain on the surface, which is just enough to provide the coating of desired thickness. Without this process the coating thickness can not be controlled. This is followed by drying, and then the sheets are ready for shipment. However, this process does not suit well for galvanizing very delicate and complex shaped parts with deep and complex interior designs.
