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. Read more about the process of electroless nickel plating here.
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.
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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.
Despite its name, stainless steel is not really fully stain proof. It doesn’t corrode, rust or stain easily with water as ordinary steel would do. But under conditions where there are low levels of oxygen, high levels of salt or poor circulation, stainless steel will stain and even corrode.
There are many advantages to using electroless nickel plating as an alternative to stainless steel. Stainless steel can be used in environments where corrosion resistance is important and where the levels of hardness of the metal are also important. However, electroless nickel plating mild steel can achieve the same effect, with properties of hardness and corrosion resistance exceeding the benefits offered by stainless steel.
Stainless steel is a lot more expensive than mild steel or cast iron that has been electroless nickel plated. It is far more cost effective to electroless nickel plate parts made of mild steel or cast iron for the ultimate levels of corrosion resistance.
Electroless nickel plating is an auto-catalytic reaction and is used to deposit a nickel coating onto a substrate. To form this deposit, no electric current is needed to pass through the solution, so unlike electroplating, electroless nickel plating has the advantage of being free from problems with power supply.
Electroless nickel plating is also an excellent preventative of corrosion and wear. An electroless nickel coating provides an even deposit whatever the shape of the component. It is also possible to deposit electroless nickel on non-conductive surfaces when the right catalyst is used before plating.
Other advantages of electroless nickel plating include:
The electroless nickel plating process is a high quality, cost effective solution for coating metals such as mild or stainless steel.
The simple process of diffused nickel plating is the most effective coating to ensure the highest levels of corrosion resistance.