Electroless nickel plating on aluminum

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.

Among the metals used in precision engineering, aluminum is particularly well suited to improving machinery performance and energy efficiency. Some alloys offer excellent mechanical properties comparable to steel, can be extruded or gravity- and pressure-cast, and have a density of about 2.7 g/cm³, compared with 7.9 g/cm³ for steel. Moving parts benefit from this lower mass through reduced forces, friction and energy consumption.

Aluminum alloys, however, have some weaknesses with regard to surface characteristics:

• Low corrosion resistance: aluminum 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, aluminum 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.

Technical and decorative anodizing warrant a separate discussion; here, the focus is on the characteristics and application of electroless nickel plating.

All aluminum alloys can be electroless nickel plated

The first great property of electroless nickel plating is its applicability to all aluminum 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.

Before electroless nickel plating, the aluminum surface must be activated with a zincate treatment, which deposits a zinc layer a few nanometers thick. Each aluminum alloy requires a specific procedure according to its alloying elements, such as Cu, Si, Mg or Zn. This step is critical because coating adhesion and the resulting protective performance depend strongly on the quality, thickness, distribution and uniformity of the zinc layer.

Corrosion resistance of electroless nickel plating on aluminum 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.

The sacrificial behavior of aluminum relative to electroless nickel must, however, be considered wherever corrosion can initiate. In wet conditions, the difference in electrochemical potential between Al and Ni creates a galvanic cell. This accelerates corrosion of the substrate and can cause it to progress along the Ni-Al interface, undermining the nickel coating.

To remedy this situation, thicknesses of electroless nickel appropriate to the aggressiveness of the usage environment are established and electroless nickel electrolytes modified in their alloying components are used to reduce the inherent porosity of the coating layer.

Niplate® eXtreme is an electroless nickel coating developed for aluminum alloys to provide maximum corrosion resistance and prevent peeling where corrosion initiates. At the same thickness, Niplate® eXtreme provides greater protection than other electroless nickel coatings because its residual porosity is almost zero. Corrosion also remains localized near the initiation point.

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-rounded edges rather than sharp edges,
• post-treatment handling, which can dent the part and initiate cracks in the hard electroless 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 resistance with thicknesses of 30-35 µm.

Wear resistance of nickel-plated aluminum

As mentioned above, the surface characteristics of electroless nickel plated aluminum 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 and Niplate® 600 to which reference is made for more details.

Electroless nickel can be hardened further by heat treatment, with a corresponding increase in wear resistance. Treatments at 280°C and 340°C for several hours produce hardness values of 800±50 HV and 1000±50 HV, respectively, but may affect the mechanical properties of some aluminum alloys.

When evaluating and designing an electroless nickel plated aluminum part subject to wear, the localized 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 aluminum alloys and electroless nickel plated for wear resistance requirements, due to its excellent sliding properties, both on the metal counterpart and on rubber seals or gaskets.