- 09 June 2006 -
Electroplated Zinc Nickel
By Marc Mertens, Research Manager; Enthone D.V., The Netherlands

Zinc nickel is widely plated from both alkaline and acid electrolytes. Alkaline zinc nickel electroplating processes can be generally described as having uniform brightness, excellent alloy distribution versus current density, and very good plated thickness distribution on complex geometries. The attributes make alkaline zinc processes commercially viable and robust.

Cathode efficiencies of alkaline processes typically range from about 40 to 60% for new solutions and will decrease as the solution is utilized due to accumulation of organic breakdown products as well as the build-up of sodium carbonate. Nickel is typically introduced to the bath and replenished by use of proprietary nickel salt-complexor additives. The low plating efficiency, coupled with nickel replenishment from proprietary additives, results in a relatively high operating cost for the process.

Acid zinc nickel systems, on the other hand, have cathode efficiencies of about 95%. Nickel for solution make-up comes from commodity nickel salts. Replenishment of nickel can be done with either soluble nickel anodes or commodity nickel salts. The result is a zinc nickel process that is much less expensive than alkaline zinc nickel from a chemical consumption standpoint and allows greater throughput due to the higher cathode efficiency. In addition, acid zinc nickel electrolytes are useful for the direct plating of cast iron, as in the plating of brake calipers.

Acid zinc nickel processes are not without difficulties that make the process less robust for commercial applications, however. Zinc anodes will galvanically dissolve in acid chloride electrolytes, causing zinc metal control problems. Dual rectification with separate bussing to nickel and zinc anodes has been used to allow use of soluble nickel anodes. Recently, proprietary insoluble anodes have been used to eliminate the need for dual rectification. Proprietary salts are used to replenish both zinc and nickel. Operating cost will increase versus the soluble anode system, but operational robustness of the process is generally enhanced and total operating cost is still approximately half of alkaline processes.

Alloy distribution versus current density in acid zinc nickel processes is dependent on the type of conducting salts used and presence of a mild complexor. In order to obtain alloy composition required by the automotive industry for optimum corrosion resistance, 12 to 15% nickel content needs to be uniformly obtained on plated articles. Baldwin et al.¹ have reported that zinc nickel alloys containing greater than 21% nickel no longer provide cathodic protection to steel. From a cosmetic perspective, zinc nickel alloys greater than 21% produce black deposits from electrolytes described in this article.

Table 1: Study of Three Acid Zinc Nickel Electrolytes

Figure 1: Electrolyte 1 alloy distribution versus current density.

Figure 2: Electrolyte 2 alloy distribution.

Experimental and Discussion
Three different acid zinc nickel electrolytes, as outlined in Table I, were studied and used in commercial applications. Electrolyte 1 is based on ammonium chloride. Electrolyte 2 is based on potassium chloride and does not contain a complexor for the nickel. Electrolyte 3 is based on potassium chloride but does contain a mild complexor for the nickel.

Mild steel cathodes, 20 x 8 cm, were plated in a 500-ml Tosei plating cell (also known as a Long Hull Cell) with magnetic spin bar agitation for 10 minutes at two amperes. Alloy content was measured by X-ray fluorescence on a Seiko Instruments Model SEA 5120 spectrometer. Data were collected at two-cm intervals from the high current density edge of the panel.

Results from Electrolyte 1, ammonium chloride-based electrolyte, are shown in Figure 1. Figure 1 shows typical anomalous deposition behavior of zinc with iron group elements. As current density decreases, the amount of co-deposited nickel decreases. Increasing solution temperature raises the amount of co-deposited nickel, but does not change the anomalous deposition behavior.

From a practical perspective, the areas of the cathode from the high current density edge to about 10 cm from the edge represent current densities on most production plated articles. This allows alloy ranges from 10 to 15% to be plated that provide desired levels of corrosion protection and are sacrificial to steel.

Results from Electrolyte 2 are shown in Figure 2. Electrolyte 2 displays alloy deposition behavior which differs from Electrolyte 1. Electrolyte 2 is characterized by anomalous deposition over most current density ranges, but with a trend to approach normal deposition at extremely low current densities. The trend to approach normal deposition becomes more apparent as temperature increases.

From a practical perspective, Electrolyte 2 has limited commercial value. The alloy obtained over typical current densities will range from 6 to 15%.

While still providing enhanced corrosion protection and remaining sacrificial to steel, this electrolyte would make it difficult to meet many alloy specifications required by automotive OEMs. In addition, the operating temperature must be controlled to 33 ±2°C to avoid alloy containing greater than 20% nickel, which will produce cosmetically unacceptable deposits, as well as deposits that are no longer sacrificial to steel.

Figure 3: Electrolyte 3 alloy distribution.

Results from Electrolyte 3 are shown in Figure 3. Electrolyte 3 shows alloy distribution data similar to Electrolyte 2, but the trend to normal deposition can be suppressed by increasing complexor concentration. The concentration of nickel and the complexor must be controlled at a given temperature to produce alloys with compositions defined for automotive applications without the black, non-sacrificial alloys in the extremely low current density areas. In practice, it has been demonstrated that Electrolyte 3 is capable of producing alloys in the 12 to 14% nickel range in rack plating applications. The ability to produce alloys in the 12 to 14% nickel range without the black high alloy in low current densities in barrel applications is dependent on article configuration, available current, and agitation.

Table 2: Alloy Content Obtained Versus Current Density

Figure 4: 2Θ scan of zinc nickel alloy obtained from Electrolyte 1.

Figure 5: 2Θ scan of zinc nickel alloy obtained from Electrolyte 2.

For X-ray diffraction studies, mild steel panels were plated in a conventional hull cell with spin bar agitation at two amperes for 10 minutes. Alloy content obtained versus current density is summarized in Table II. Alloy content and thickness were determined by XRF. XRD data were obtained on a D8 Discover diffractometer with GADDS detector from Bruker Analytical X-Ray Systems, Inc.

Figure 4 is a 2q scan of zinc nickel alloy obtained from Electrolyte 1. Only Υ phase Ni₅Zn₂₁ is present at all current densities. The phase (Ni₅Zn₂₁) does not appear to change with current density, while the texture varies slightly with current density. Qualitative analysis of the scan indicates the (330) orientation is the only orientation seen at 4 ASD. As current density increases, (600) orientations start to appear and increase as current density decreases.

Figure 5 is a 2q scan of zinc nickel alloy obtained from Electrolyte 2. Only Υ phase Ni₅Zn₂₁ is present at all current densities. However, the texture changes significantly with current density. Qualitative analysis of the scan indicates the (600) orientation is the dominating orientation seen at 4 ASD. As current density decreases, (330) orientation increases. At 0.2 ASD, the (330) orientation appears to be more dominant than the (600).

Deposits from the potassium electrolyte appear to have opposite texture characteristics of deposits obtained from the ammonium chloride electrolyte.

Conclusion
Zinc nickel deposits produced from acid chloride electrolytes contain Υ phase Ni₅Zn₂₁ when nickel content is 12 to 15% by weight. Ammonium chloride electrolytes produce deposits with crystal orientations versus current density that are opposite of that seen with a potassium chloride electrolyte. The significance of this on deposit properties, such as stress and elongation, and the ability to post-plate form articles need further investigation.

Ammonium chloride electrolytes will produce alloys with nickel content versus current density distribution that is more favorable for commercial applications. In fact, ammonium chloride zinc nickel baths are in commercial operation in both rack and barrel applications. However, where there are restrictions on the use of ammonium chloride, commercially viable potassium-chloride-based electrolytes are available.

In order to control the alloy content in extremely low current density areas, a mild complexor is preferred to control the alloy deposition. While non-complexed potassium chloride processes have been made commercially available, the high nickel content alloy obtained in extremely low current density areas and the narrow temperature operating range required, do not make these electrolytes attractive for commercial operation.

References

1. Baldwin, K.R.; Smith, C.J.E.; Robinson, M.J., Corrosion, pp 932-940; Dec. 1995.

For more information, contact John Commander at (e-mail) jcommander@cooksonelectronics.com; Chen Xu at (e-mail) chenxu@cooksonelectronics.com; or Marc Mertens at (e-mail) mmertens@cooksonelectronics.com.