How Duplex Electroless Nickel Coatings Can Be Advantageous

Q: I own a plating shop in the Northeast, we have been plating electroless nickel (EN) for over 30 years. It has been brought to my attention that the use of duplex EN coatings can be advantageous for different applications. Can you provide information on this subject and what could I achieve?

A: That is an excellent question, and yes, a significant amount of attention has been placed on this subject in recent years. As applications for electroless nickel coatings continue to expand, it has become obvious that one type of EN (for example HP, MP, LP, Composite EN and more) cannot possibly meet all the physical/mechanical properties that a specific application may require. Duplex coatings have provided that possibility.

Double-layer (or duplex) EN coatings typically involve the combination of two types of EN deposits to create a better finish of either single layer EN. There are numerous examples of these coatings already in use in the metal finishing industry. Some of the properties that can be improved for specific applications are as follows:

• Improved corrosion resistance
• Better wear resistance
• Better lubricity and release properties
• Longer bath life on aluminum
• More reflective deposits
• Blackening of the deposit

The advantage of duplex coatings is that they can achieve several of the properties outlined above whereby a single layer EN deposit might only achieve one. Here are samples of current duplex coatings:

High Phos EN over Low Phos EN: When high hardness and wear resistance may be required of an EN deposit, low phosphorus EN is typically recommended. However, as a single layer, the corrosion resistance of low phosphorus EN is generally poor. If an underlayer of high phosphorus EN is plated first, the duplex coating can provide both wear resistance and corrosion protection.

Composite EN over High Phos EN: When an application requires corrosion resistance along with good lubricity and release properties, a single layer high phosphorus EN coating may not be capable of achieving this. However, utilizing a top layer of EN/PTFE or EN/BN, one can provide a deposit with a lower coefficient of friction and/or better wear resistance. However, since composite coatings are generally very porous, they provide little corrosion protection. The combination, however, of utilizing both the high phosphorus EN as an underlayer and a topcoat of composite EN, properties of both chemistries can be achieved on the same part.

Bright Mid-Phos EN over High Phos EN: In certain applications the appearance of the EN deposit is critical. Replacing bright electrolytic EN with midphosphorus EN has quite often been employed. Where midphosphorus EN deposits will provide the finish required for most applications, it may not provide the additional corrosion resistance that some applications may also require. Plating a bright midphosphorus layer over a high phosphorus EN layer can provide both a bright deposit with corrosion resistance.

Mid- or High-Phos EN over an Alkaline EN Strike: The bath lives of most acid electroless nickel chemistries are shorter when plating aluminum substrates than steel substrates. However, recent advancements have demonstrated that introducing an alkaline EN strike prior to the acid EN deposit not only extends the bath life of the acid EN chemistries but also improves adhesion in some cases. Reduced plating costs have also been realized as a result.

Matte Low Phos EN over High Phos EN: Recent developments have also found plating certain low phosphorus EN deposits over a high phosphorus EN layer can provide an excellent coating for black finishes. By applying an easy-to-oxidize low phosphorus EN coating over a corrosion-resistant high phosphorus EN layer, a shiny or matte black finish can be created for a multitude of applications.

In summary, electroless nickel duplex coatings offer a range of properties that can create many new applications now and in the future. By combining the different EN coatings available today, more versatility can be achieved than any single layer coatings of the past.

 

Mike Aleksinas is the president of MetalChem. He can be reached at mike@metalchem-inc.com or at 864-877-6175.