This invention relates to a process for the preparation of wear and corrosion resistant coatings for electro less deposition on metal substrates.
The progressive deterioration of metallic surfaces in use, because of wear and corrosion ultimately leads to loss of plant operating efficiency and at worst a breakdown. The cost to industry of wear and corrosion in equipment is high and recognition of this fact lies behind the continuing development of technology known as surface engineering which includes the application of coatings to metal surfaces to improve their performance in specific working conditions. Conventional plating, normally designated as electrode position or electroplating is accomplished when metal ions are reduced to the metallic state and deposited as such at the cathode by use of electrical energy. Electro less plating is a chemical reduction process which depends on the catalytic reduction of a metallic ion from an aqueous solution containing a reducing agent and a subsequent deposition of metal without the use of electrical energy. Electro less Ni-P system is a well studied and discommended system. It is widely used in engineering applications for its
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superior wear and corrosion resistance. In spite of the superior qualities of the electro less Ni—P system, there is still vast scope for further improvement.
This invention has the advantage that the coatings proposed herein have an increased hardness which help in improving their wear resistance in addition to enhanced corrosion resistance when compared with conventional electro less Ni—P coatings.
The process proposed herein is also simple to carry out while being effective in its use and operation.
The process for the preparation of wear and corrosion resistant coatings for electro less deposition on metal substrates, according to this invention, comprises the steps of preparing a composition of nickel sulphate (25-30 gm./litre), sodium hypophosphite (19—18 gm./litre), sodium citrate (28-32 gm./litre), ammonium sulphate (28-32 gm./litre), sodium unstated (18-22 gm./litre); and maintaining the temperature and pH of the said composition at 83 - 87 degrees C and 5.7 -6.3 respectively.
EXAMPLE A composition of nickel sulphate (28 gm./litre) sodium

hypophosphite (16 gm./litre) sodium citrate (30gm./litre) amnioniufn sulphate (30 got./litre) sodium unstated (20 gm./litre) is prepared by dissolving the said substances in de-ionized water. Before actual deposition of the said composition on a metal substrate, the composition is heated to 85 degree C and the temperature maintained using a constant temperature oil bath. The pH of the composition after preparation is slightly acidic (6 plus or minus 0.3) and is maintained at this level during deposition. Mild reduction in the pH over long period of coating time is adjusted using ammonia solution. The plating rate is 10-14 micrometer/hour for the first hour and gradually reduces thereafter. The time of deposition is dependent on the coating thickness required. On completion of the process, the sample (substrate with coating) is removed from the bath, washed, dried completely and weighed. The coating obtained by the above Example has a composition as given below
Nil 81-83 Wi 12-14 P! 5-5.5 in weight percentage.

After electro less deposition is completed sample comprising the substrate with the costing on top will be heat treated to further increase the hardness of the coating. The samples are placed in alumina crucibles covered with lids. The crucibles are kept within a muffle furnace which is set at a desired temperature between 200 and 400 degrees C. The samples are maintained at the specified temperature for one hour each. After this time duration the samples are removed from the furnace and allowed to cool in air.
Hardness values for Ni-W-P coatings in as—plated and heat treated condition measured using a Vickers micro-hardness tester at 100g load are given below:
Condition Hardness