What is Electroless Nickel Plating？

Electroless nickel plating is a deposition process that uses hypophosphite reduction to form a nickel-phosphorus alloy coating for enhanced functional performance. This article provides a comprehensive technical overview of this surface treatment.

Overview of Electroless Plating

Electroless plating is an advanced metal surface treatment process. Unlike electroplating, which relies on an external power source to drive the reduction and deposition of metal ions at the cathode (workpiece), electroless plating utilizes a self-sustaining chemical reduction reaction within the plating bath to provide the necessary electrons for metal ion reduction.

Electroless Nickel Plating Process

Electroless nickel plating involves the reduction and deposition of nickel ions onto a catalytically active substrate surface using a reducing agent. The most common reducing agent employed is the hypophosphite ion. The reducing agent reduces the nickel salt to metallic nickel while simultaneously incorporating a specific amount of phosphorus into the deposited metal layer. The precipitated nickel film possesses autocatalytic properties, allowing the reaction to continue spontaneously.

Numerous theories exist regarding the reaction mechanism of electroless plating, including the atomic hydrogen theory, the hydride transfer theory, the electrochemical theory, and the hydroxyl-nickel ion coordination theory. The most widely accepted framework, the atomic hydrogen theory, is used to illustrate the fundamental principle:

#1. Hydrogen generation. Upon heating, the hypophosphite ion (the reducing agent) undergoes catalytic dehydrogenation in the aqueous solution, forming phosphite ions and releasing atomic hydrogen.

H₂PO₂^– + H₂O → HPO₃²⁺ + H⁺ +2[H]

#2. Nickel reduction. The atomic hydrogen is adsorbed onto the catalytically active metal surface, activating it. This activated surface then facilitates the reduction of nickel cations (Ni²⁺) from the electrolyte, depositing metallic nickel.

Ni²⁺ + 2[H] → Ni + 2H⁺

#3. Phosphorus co-deposition. As the phosphite ions are further decomposed, they are reduced to phosphorus.

H₂PO₂^– + [H] → H₂O + OH^– + P

#4. Alloy formation. Nickel atoms and phosphorus atoms are co-deposited, forming a Nickel-Phosphorus (Ni-P) alloy.

The fundamental mechanism of electroless nickel plating, therefore, is the co-deposition of nickel through ionic reduction and the incorporation of phosphorus atoms resulting from the decomposition of the hypophosphite. The resulting deposit is a supersaturated solid solution of nickel and phosphorus, often referred to simply as Nickel-Phosphorus (Ni-P) plating.

Material Used in Electroless Nickel Plating

The core components that constitute a typical electroless nickel plating bath include:

• Main salt (Nickel source). Primarily a nickel salt (e.g., nickel sulfate) to supply the nickel ions for the plating process.
• Reducing agent. A salt containing two or more active hydrogen atoms (most commonly hypophosphite) that drives the reduction reaction through catalytic dehydrogenation.
• Complexing agents: Usually hydroxy-carboxylic acids (e.g., lactic acid, malic acid, amino acids). These agents improve bath stability, increase the deposition rate, and enhance the quality of the deposit.
• Stabilizers. Components added to inhibit the spontaneous, bulk decomposition of the plating bath. They ensure the reaction proceeds in a controlled, orderly manner, preventing uncontrolled or excessive breakdown that would compromise bath life and film quality.
• Accelerators. Used to increase the rate of chemical deposition.
• Buffers: Employed to stabilize the deposition rate over time, ensuring consistent film quality.
• Surfactants (Wetting agents). Small additions aid in the release of hydrogen gas generated during the reaction, thereby reducing film porosity and improving overall deposit quality.

Properties of Electroless Nickel Plating

Appearance

Electroless nickel films typically have a bright, semi-bright, or slightly yellowish appearance. The phosphorus content influences the final color and luster, the initial roughness of the substrate, film thickness, deposition rate, and the specific plating parameters employed.

Mechanical Properties

The electroless nickel plating deposit is generally classified as a brittle coating. While offering high tensile strength, it has low elastic modulus and minimal ductility. This mechanical behavior is attributed to the deposit’s amorphous or microcrystalline structure, which effectively hinders plastic deformation. Tensile strength increases with higher phosphorus content; for instance, a Ni-P coating with 1% to 3% phosphorus may have a tensile strength of 150 to 200 MPa, whereas a high-phosphorus coating with 10% to 12% can reach 650 to 900 MPa. However, the corresponding elongation typically remains below 1%.

Corrosion Resistance

The excellent corrosion resistance of electroless nickel is a primary reason for its widespread industrial adoption. The key factors ensuring resistance are film thickness and integrity. Studies suggest that a Ni-P layer with approximately 11% phosphorus achieves optimal corrosion resistance at a thickness of 30μm. Research indicates that higher phosphorus content generally correlates with better acid resistance, as the passivation film formed in acidic media is a tenacious metal phosphide layer.

Crucially, only a complete, intact coating ensures superior corrosion resistance. If the coating is locally damaged or breached, the corrosion of the substrate is accelerated. This occurs because the Ni-P coating is cathodic (more noble) relative to the steel substrate. Once the coating is compromised, an electrochemical cell forms, and the base metal (the anode) is preferentially and rapidly corroded.

Hardness

The electroless nickel plating deposit is a supersaturated solid solution characterized by a high degree of lattice distortion. Increasing phosphorus content promotes grain refinement, which contributes to the film’s initial high hardness. Subsequent heat treatment induces the diffusion and precipitation of phosphide phases within the coating. This phase change can raise the coating hardness significantly, with precipitation-hardened films often reaching up to 1100 HV.

Wear Resistance

Given its high achievable hardness, the Ni-P coating has superior wear resistance compared to pure electrodeposited nickel. This performance profile drives the widespread use of the process in aerospace, chemical processing, oil and gas, mining, and military engineering. For applications requiring both wear and corrosion resistance, such as pumps and impellers, coatings greater than 75μm may be specified. High-phosphorus coatings in the 25 to 75μm range are commonly used on components like slurry pumps and oil well pumps to address combined wear and corrosion challenges.

Applications of Electroless Nickel Plating

Aerospace and Aviation

The aerospace sector is one of the highest-frequency users. A notable application involves the repair and overhaul of jet engine impellers. For example, high-phosphorus electroless nickel plating technology is used to restore impellers in certain jet engine models (e.g., the U.S. 778D series) to ensure their safe reuse in flight operations.

To minimize weight, the aerospace industry heavily uses aluminum alloys. After electroless nickel plating and subsequent surface hardening, these aluminum components gain enhanced corrosion and wear resistance and maintain weldability. Furthermore, electroless nickel plating is used in space systems for precision metal mirrors. Here, high-strength, lightweight aluminum is used as the substrate, and a specially developed high-phosphorus electroless nickel coating (12.2% to 12.7% P) is applied and polished to achieve a mirror surface finish meeting 9A level.

Automotive

The automotive sector utilizes electroless nickel plating to protect critical components, often replacing older processes. A standard industry practice involves using electroless nickel plating to protect zinc die casting, such as carburetors, from corrosion. Electroless nickel plating is a standard surface protection method for fuel delivery systems, including carburetors and fuel pumps. The inherent uniformity of the electroless nickel plating layer is particularly advantageous for complex geometries like gears and fuel injectors, where traditional electroplating struggles to achieve consistent thickness.

Chemical

In the chemical industry, electroless nickel plating is often used as a cost-effective alternative to expensive corrosion-resistant alloys. This application improves the purity of chemical products, reduces environmental impact, and enhances operational safety and transport reliability, providing a competitive economic advantage. Electroless nickel plating is widely applied to protect the internal surfaces of large-volume reactor vessels against aggressive chemical attack.

About Getzshape

At Getzshape, we are an experienced manufacturer of CNC machined metal and plastic parts. Our surface finishing options include electroless nickel plating, chrome plating, zinc plating and more. If you are looking for a manufacturer offering both machining and post processing, feel free to contact us.