Properties Comparing: Electrolytic vs. Electroless Nickel Plating?

Compared to electrolytic nickel plating, while electroless nickel plating has a more complex bath composition, poorer bath stability, and a shorter bath life, it offers significant advantages such as a uniform and dense deposit, high hardness, good corrosion and wear resistance, excellent solderability, strong throwing and deep plating power, and independence from the workpiece’s geometric shape. Through continuous research and industrial application, electroless nickel plating has developed substantially, with various commercial baths becoming more widely used. This article compares the coating properties of two commercial electroless nickel baths with those of traditional bright electrolytic nickel and semi-bright electrolytic nickel. The superiority of the electroless nickel coating properties is analyzed based on specific data.

Experiment

The substrate material was a copper sheet with a diameter of 25mm and a thickness of 0.1mm. The treatment process was sequentially as follows: organic solvent degreasing to wiping residual solvent dry with cotton cloth to electrochemical degreasing of copper parts with low alkalinity and low current density to hot water rinsing to running water rinsing to cleaning with hydrochloric acid and corrosion inhibitor to running water rinsing to activation with 10% hot sulfuric acid (40 ℃) to hot distilled water rinsing (preheated to 50 ℃ or higher). The composition of the electrochemical degreasing solution for copper parts is: 4 to 10 g/L of NaOH, 30 to 40 g/L of Na₂CO₃, 60 to 70 g/L Na₃PO₄ · 12H₂O, and 5 to 10 g/L of Na₂SiO₃ · 9H2O.

The Electroless Nickel solutions used were commercial baths, with process specifications detailed in Table 1. The process specification for bright electrolytic nickel is shown in Table 2.

Table 1: Electroless Nickel Plating Specifications

Bath Type	Name	Specification	Content (mL/L)	pH	Temperature (°C)
EN-HP 11	Additive	EN HP 11A	55~60	4.7~5.2	85~92
High Phosphorus	Make-up Solution	EN HP 11B	120~150	4.7~5.2	85~92
Semi-Bright	Auxiliary Additive	EN HP 11H	Supplement	4.7~5.2	85~92
EN-MP08	Additive	EN MP 08A	55~60	4.3~4.9	82~90
Medium Phosphorus	Make-up Solution	EN MP 08B	145~155	4.3~4.9	82~90
Semi-Bright	Auxiliary Additive	EN MP 08C	Supplement	4.3~4.9	82~90

Table 2: Electrolytic Nickel Plating Specifications

Solution	Content (g/L)	pH	Temperature (°C)	DK (A/dm²)	Agitation (times/min)
Nickel Sulfate	240~315	4~4.8	50~60	2.0~11	20~25
Nickel Chloride	45~75	4~4.8	50~60	2.0~11	20~25
Boric Acid	37~53	4~4.8	50~60	2.0~11	20~25

Coating hardness was measured using an automatic turret digital display micro-Vickers hardness tester. Coating wear resistance was determined using a pin-on-disk friction and wear tester. The counter-body was a GCr15 ball with a diameter of 6 mm, with a load of 10 N, a wear radius of 6 mm, and a rotation speed of 318 r/min. The number of wear cycles was adjusted based on the wear condition of different coatings. Coating thickness was measured using an X-ray fluorescence thickness gauge. Electrochemical testing was performed using an electrochemical workstation in a 3.5% NaCl solution. The scan range was 0.5 V to 1.0 V, and the scan rate was 10 mV/s.

Comparison of Coating Hardness and Wear Resistance

Hardness and friction coefficient are two important metrics when selecting a wear-resistant coating. For wear-resistant coatings, good adhesion between the coating and the substrate is essential. There are also specific requirements for the substrate’s hardness, a hard substrate provides better support for the electroless nickel layer, while a hard layer on a soft substrate can be easily damaged or even penetrated by a hard and rough contacting surface. The hardness values of the copper substrates after electroless nickel and electrolytic nickel deposition, measured with the automatic turret digital display micro-Vickers hardness tester.

The data show that the hardness of the coating is increased after plating compared to the substrate. The as-plated electroless nickel layers all have higher hardness than the electrolytic nickel layers, with the semi-bright nickel layer having the lowest hardness. Theoretically, the hardness of the Nickel-Phosphorus alloy coating increases with increasing phosphorus content. However, the hardness of the mid-phosphorus electroless nickel layer is higher than that of the high-phosphorus electroless nickel layer. This is because the high-phosphorus coating contains a larger volume of the Ni₃P phase, resulting in a minimal change in hardness after heat treatment, while the hardness of the mid-phosphorus electroless nickel layer significantly increases after heat treatment.

Table 3: Vickers Hardness of Nickel Coatings on Copper Substrate

Plating Type	EN – MP08(Electroless Nickel – Medium Phosphorus)	EN – HP 11(Electroless Nickel – High Phosphorus)	Electrolytic Bright Nickel	Electrolytic Semi-Bright Nickel	Substrate
HV 0.025	512	446	362	290	120

Figure 1 shows the wear scar morphology of the semi-bright electrolytic nickel and electroless nickel layers observed under a 50 times optical microscope, with both coating thicknesses being 8 to 10 μm. Figure 1 clearly shows that under the same wear conditions, the semi-bright (dull) nickel coating failed after 2000 cycles of wear, with a wider wear scar and complete exposure of the substrate material inside the scar. In contrast, the mid-phosphorus nickel and high-phosphorus nickel electroless nickel coatings did not fail after 16000 cycles of wear, showing a narrower wear scar and no exposed substrate material inside the scar. After 16000 cycles of wear, the mid-phosphorus nickel also showed a shallower wear depth. This experimental result indicates that the wear resistance of the mid-phosphorus electroless nickel coating is superior to that of the high-phosphorus electroless nickel coating and is far superior to that of the semi-bright electrolytic nickel coating.

Figure 1: Nickel plating wear scar (50X)

Comparison of Coating Thickness

The thickness and its uniformity are one of key indicators for evaluating coating quality. Coating thickness directly affects properties such as the workpiece’s corrosion resistance, wear resistance, porosity, and conductivity, thus significantly influencing the product’s reliability and performance. The biggest advantage of electroless nickel over electrolytic nickel is that the thickness of the deposited metal is uniform across the entire substrate surface, virtually independent of its geometry. A very uniform coating can be obtained, with no limit on coating thickness, provided the entire surface is immersed in the solution and there is a clear channel for electrolyte flow. The thickness distribution and uniformity analysis of the electrolytic nickel and electroless nickel test samples, measured using the X-ray fluorescence thickness gauge.

The data in the table show that the electroless nickel layer has excellent uniformity, with a relative standard deviation of less than 10% and an absolute deviation of less than 15%. The uniformity of the electrolytic nickel layer is relatively poor, with a relative standard deviation greater than 50% and an absolute deviation greater than 100%. The experimental results validate the preceding theoretical analysis, confirming that electroless nickel is more conducive to controlling the uniformity of the coating thickness compared to electrolytic nickel.

Table 4: Coating Thickness Distribution and Uniformity

Plating Type	Sample Point Amount	Average Thickness	Relative Deviation (%)	Absolute Deviation (%)
Electroless Nickel	6	8.42	7.48	10.77
Electrolytic Nickel	6	9.52	58.82	149.92

Comparison of Coating Corrosion Resistance

The corrosion current of the electroless nickel layer is lower than that of the electrolytic nickel layer, and the corrosion potential is higher than that of the electrolytic nickel layer. This indicates that the corrosion rate of the electroless nickel layer in saltwater is lower than that of the electrolytic nickel layer, demonstrating better corrosion resistance. Among the electroless nickel coatings, the high-phosphorus nickel coating exhibits the best corrosion resistance.

Conclusion

The hardness of the as-plated electroless nickel coatings is universally higher than that of the electrolytic nickel coatings. The mid-phosphorus electroless nickel coating exhibits the best hardness and wear resistance. The thickness uniformity of the electroless nickel layer is superior to that of the electrolytic nickel layer. In a saltwater medium, the corrosion resistance of the electroless nickel layer is superior to that of the electrolytic nickel layer, with the high-phosphorus nickel coating showing the best corrosion resistance.