PLATING OF CHROMIUM
Chromium coating is done on metal sheets. It may be done by two methods as chromium cementation and electro deposition of chromium.
The chromium cementation is also known as chromizing process. The chromizing process is of three types as powder process, vapors process and fused salt bath.
Chromium cementation processThe chromium cementation process provides corrosion resistance to chromium steel and carbon steel. It also provides resistance to oxidation.
In this process the temperature condition is high. The design of the furnace is specific in nature.
Powder processIn this process the metals are packed in specially designed cell. The heating atmosphere is created by 3 to 4 hours.
The operating temperature is between 1300'C to 1400'C. the powder mixture is 55% chromium powder and 45% alumina powder.
Chromium is directly defused on the metals and equipments.
Gaseous processThe powdered mixture contains chromium, ferro chrome, alumina and kaolin. The atmosphere is charged with hydrogen gas.
The operation process is between 4 to12 hours. The operation temperature is 1100'C. The coating may vary from 0.102mm to 0.204mm.
At elevated temperature gaseous chromium halide is formed and deposited on the surface of the metal.
Salt bath process The metal parts are immersed in fused chromium salt bath. The fused salt bath contains chromium chloride,
barium chloride sodium chloride and chromium flakes. The operating atmosphere is charged with argon gas.
The rate of deposition of chromium is dependent upon the bath temperature.
The chromium salt decomposes on steel surface and defuses. The chromium is deposited on the metal surface.
Electroplating of chromium : Electroplating of chromium is produced with the solution of chromic acid and small amount of sulphuric acid.
The anode is an alloy of lead and tin. The cathode is the metal sheet or equipment on which chromium is deposited. The electrolytic tank is made of lead lined steel.
The electrolytic process is started. The composition of bath is checked at regular intervals. Addition of chromic acid and sulphuric acid are made to maintain the composition
of the electrolytic bath. Care is to be taken to prevent the contamination of impurities like copper, zinc and iron. The optimum temperature is between 40'C and 50'C. the solution
is to be concentrated in nature to avoid concentration and to catalyze the decomposition. The bath may have high conductivity and less voltage is required. The chromium
plating may be decorative with thin layer of chromium deposited on the metal surface. But in industrial application, the plating is hard and thick. In such case, chromium
is directly deposited on the metal surface without any intermediate coating. The chromium electroplating has a number of applications. These are printing plates, wrist pins,
steering knuckles, tube drawings, bearings, cylinder liners, piston rings, cutting tools and files.
Due to presence of impurities in the electrolyte the deposit may be rough. So impurities are to be removed before the start of the electroplating work. If the temperature is high then there is possibility of soft deposits. Even the deposits may look dull or milky. Embrittlement occurs due to presence of hydrogen. Pits are produced due to presence of gas bubbles. No deposition or slow deposition may take place due to bad electrical contact, low current density and low cathode efficiency. So,there must be sufficient care to tackle all these problems during electroplating of metals and equipments with chromium.