The structure of electroless nickel is responsible for the unique properties it possesses, making it such a useful metal coating. Electrodeposited nickel typically has a crystalline structure, whereas an electroless nickel deposit is normally described as having an amorphous structure or one consisting of ultra-fine crystallites. The amorphous nature of electroless nickel deposits become more dominant as the phosphorus content increases, and phosphorus contents above 10.5% are typically considered truly amorphous.
The absence of a well-defined crystal structure eliminates the possibility of inter-granular corrosion that can be a problem with crystalline deposits, such as electrolytic nickel. Electroless nickel, therefore, provides a barrier that is more effective at protecting a substrate from corrosion.
Heat Treatment and Electroless Nickel Plating
Heat treatment can also be used to change the properties and structure of nickel phosphorus deposits. High temperature heat treatment solutions for sustained periods can be used to diffuse the electroless nickel layer into the steel, giving a degree of flexibility to the coating. This process does not affect the increased corrosion protection offered by this type of deposit.
The Composition of Electroless Nickel Coatings
The phosphorus content of an electroless nickel coating has a great effect on the properties of the deposit, and can be varied over a wide range.
High Phosphorus Electroless Nickel Coatings
For example, high phosphorus electroless nickel typically contains a phosphorus content of between 10-12%, making it ideal for applications where a uniform, hard and highly corrosion resistant coating is required. Other advantages include low porosity, pit-free deposits and a very stable and robust coating.
Medium Phosphorus Electroless Nickel Coatings
Medium phosphorus solutions contain between 6 and 10% phosphorus and produce a coating which effectively combines intermediate hardness with good corrosion resistance. Medium phosphorus coatings deposit rapidly and produce a bright or semi-bright coating.
Low Phosphorus Electroless Nickel Coatings
A low phosphorus electroless nickel coating typically contains 2.5% phosphorus. This produces a very uniform porous free coating which uniformly covers the complete surface of the component, including inside recesses. It has excellent corrosion resistance in alkaline conditions, but the coating is relatively soft, below 235VPN, and is therefore not particularly wear resistant.