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Plating Specialists
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Technical Information
As the deposit formed by electroless nickel plating is based on a chemical reaction, the action to form the deposit is equal over all the part, regardless of its shape. The deposit formed is therefore of equal thickness over all the part. This is unlike electrolytic plating which relies on an electrical current passing from an anode to the component, the cathode, which is then subject to changes in current density, applicable to its shape. Variations in current density result in variations in deposit thickness.
The uniformity of the coating obtained from electroless nickel plating, makes it highly suitable for use on engineering components as the machined tolerances of a component are not deformed by the plating process. The uniformity of the coating and the fact that it is a chemical reaction, give the electroless nickel deposit a number of properties which are advantageous to many applications.
Electroless nickel deposits are excellent for corrosion prevention. The deposit exhibits electro-negative or 'noble' behaviour with respect to the corrosion protection mechanism and therefore relies on total encapsulation of the substrate to be effective. As the deposit is a uniform coating, provided the substrate is of good condition and has been properly prepared, long term corrosion resistance can be achieved in many different environments. The chemical reaction that forms the deposit however, means the deposit is not pure nickel, but an alloy of nickel and phosphorus.
The phosphorus is present in small quantities and can vary in content from 1 - >12% by weight. The variation can be controlled and therefore results in a number of deposit types.
The choice of deposit type depends on the environment to which the component will be exposed. Most conditions in the highly corrosive oil, gas and coal industry require deposits with a high phosphorus content (>10%). Conventional medium phosphorus systems (6-11%) offer excellent corrosion protection for a large majority of applications, while low phosphorus systems (2-5%) are best suited to the chloro-alkali industry, such as the transportation of liquid soda.
The chemical formation of the deposit and the alloying element, also affect the deposit structure, further altering the deposit properties. The 'as plated' deposit has an amorphous structure, but if high temperatures are applied, the deposit becomes crystalline. This can alter the deposit hardness.
As plated deposits of electroless nickel plating can have hardness values in the range of 450 - 750 VHN, depending on the phosphorus content in the deposit. Generally, the higher the phosphorus content, the lower the as deposited hardness.
Wear resistance is usually a secondary requirement, after corrosion prevention. A common assumption is that hardness is synonymous with wear resistance. It is actually dependant on the wear mechanism involved, but in most cases true.
Hard chrome is frequently used where there is a requirement for hardness and wear resistance. It can offer as deposited hardness figures in the region of 800 - 1100 VHN. The deposit however, is applied electrolytically, giving an uneven coating which may require post plating machining. Electroless nickel deposits therefore have the advantage of uniformity, but as plated, do not achieve the hardness values of chrome. This can be overcome with heat treatment.
Unlike hard chrome, which tends to soften with heating, all types of electroless nickel plating can be heat treated to increase the hardness. With the correct heat treatment, some electroless nickel deposits can achieve in excess of 1000 VHN, making the deposit hardness comparable to that of hard chrome.
