Electroless nickel plating is commonly applied successfully to all aluminum alloys. Its uniform thickness, corrosion resistance and high hardness are an efficient barrier against corrosive agents and situations where wear plays an important role.
Of the metals used in precision mechanics, aluminium is the one that best lends itself to the concept of functional and energy improvement of machinery due to the excellent mechanical qualities of some alloys, comparable to those of steel, the possibility of extrusion, gravity or pressure casting and the invaluable lightness with a specific weight of around 2.7 g/cm3 against 7.9 g/cm3 of steel. This benefits the moving parts where the forces in play and the resulting friction and energy consumption are reduced.
Aluminium alloys, however, have some weaknesses with regard to surface characteristics:
- Low corrosion resistance: aluminium alloys are particularly prone to galvanic corrosion and have no chemical resistance in contact with acidic or alkaline substances;
- Low wear resistance: the low surface hardness and susceptibility to galling make them unsuitable for sliding contact with other components;
- Tendency to galling: during sliding, aluminium parts easily tend to create galling.
Two coatings are most commonly used to improve the surface characteristics and overcome the flaws of the base metal: Anodizing and Electroless Nickel Plating.
Leaving aside anodising in its technical and decorative forms, which deserve separate books, let us focus on the characteristics and application methods of electroless nickel plating.
All aluminium alloys can be electroless nickel plated
The first great property of electroless nickel plating is its applicability to all aluminium alloys. The resulting technical surface characteristics are therefore the same for all alloys. Anodizing, on the other hand, does not give satisfactory results on certain alloys, either because of unattractive aesthetics or because of poor corrosion or wear resistance. Die-casting alloys with a high Silicon and Copper content suffer the most.
To nickel plate aluminium, it is necessary to activate the surface of the parts with a treatment called zincate, which deposits a light layer a few nanometres of zinc on the part before the nickel plating step. Each Aluminium alloy, depending on the metals in the alloy, Cu rather than Si, Mg, Zn, requires appropriate and differentiated procedures to create the zinc layer. This step is very important because the adhesion of the electroless nickel coating and its consequent protective functions are strongly influenced by the quality of the zinc layer, its thickness, distribution and uniformity.
Corrosion resistance of electroless nickel plating on aluminium alloys
Electroless nickel protects the surfaces it covers because it is a noble metal, not very sensitive to aggression by saline solutions and industrial pollutants. Its ability to protect is given by the degree of covering and impermeability that can be achieved towards the base metal.
However, one must take into account the role of Aluminium as a sacrificial metal compared to the electroless nickel in corrosive initiation situations. In the presence of wet corrosion, due to the difference in electrochemical potential between Al and Ni, the so-called ‘battery effect’ occurs, which accelerates the corrosion of the base metal and tends to make it progress along the Ni-Al interface by undermining the Ni coating.
To remedy this situation, thicknesses of electroless nickel appropriate to the aggressiveness of the usage environment are established and modified electroless nickel electrolytes are used in the alloy components to reduce the inherent porosity of the coating layer.
Niplate® eXtreme is an electroless nickel plating designed for aluminium alloys, giving maximum corrosion resistance and avoiding the phenomenon of peeling in the event of corrosive initiation. For the same thickness, Niplate® eXtreme, compared to other electroless nickels, gives greater protection as it has almost zero residual porosity. In addition, with Niplate® eXtreme, corrosion remains localised to the point of initiation, progressing only near the point itself.
When choosing the best solution to a corrosion problem, in addition to the most suitable type of electroless nickel with the correct thickness, the following factors must be taken into account that affect the end result:
- the intrinsic porosity of Al alloy, of which die-cast porosity is a common example,
- the possible inert inclusions in the Al alloy that create discontinuities in the nickel coating,
- the more or less fine machining with material cut by the tool rather than torn off,
- well rouded edges rather than sharp edges,
- the handling of the workpieces after treatment which can cause denting of the workpiece with triggering cracking of the hard nickel layer. By adopting the correct nickel plating preparation cycles and the appropriate electrolytes, good corrosion resistance results are obtained on compact and finely machined alloys in excess of 100 hours with the ISO 9227 salt spray test at thicknesses of 10-15µ and up to 720 hours of sealing with thicknesses of 30-35 µm.
Wear resistance of nickel-plated aluminium
As mentioned above, the surface characteristics of electroless nickel plated aluminium are those of the electroless nickel layer. The hardness of the coating is between 550 and 650 HV depending on the type of electroless nickel chosen, with wear resistance indicated in the data sheets of the Niplate® 500 e Niplate® 600 to which reference is made for more details.
The further hardening of electroless nickel, with a consequent increase in wear resistance, passes through heat treatments. The temperatures at which electroless nickel normally hardens, i.e. 280°C and 340°C for a few hours, allow the following to be reached 800±50 HV and 1000±50 HV respectively but may, for some aluminium alloys, influence the mechanical properties.
When evaluating and designing a electroless nickel plated aluminium part subject to wear, the localised loads that the part has to withstand when sliding on its mechanical counterpart have to be taken into account in order to check whether the thickness and hardness of the nickel are adequate.
In practice, many articles are produced in almost all aluminium alloys and electroless nickel plated for wear resistance requirements, due to the excellent sliding capacity it possesses, both on the metal counterpart and on rubber seals or gaskets.