Monitoring Nickel-Plating Baths in Surface Engineering

Pure nickel is very hard, ductile, shiny, resistant to corrosion, and has a silvery-white color. Thanks to these extraordinary properties, nickel is often utilized in surface engineering and coating applications.

Nickel coatings have exceptional polishing characteristics, which make them suitable for aesthetic and optical applications, such as furniture fittings and decorative items. Additionally, superior optical and mechanical properties make nickel an attractive option for the electronics and machine industries. Nickel coatings can also be soldered.

During the nickel-plating process, nickel’s elemental characteristics are transferred to the workpiece surface. Color, ductility, corrosion resistance, hardness, and other functional properties can be controlled through the process parameters and with the addition of chemical additives.

For instance, extremely shiny nickel layers can be obtained by adding the ostensible brighteners. Also, other metals can be applied above or below the nickel layer to obtain other properties like color, or to achieve an additional increase in corrosion resistance. For example, this process is performed in alloy wheel rims that include a chromium layer above the adhesion layer of nickel (bottom layer). The nickel-plating process is used in almost all industries.

ProcessLab from Metrohm for Bath Analysis

A company or a specialized service provider can perform the coating process. In either case, the nickel-plating process must be carefully tracked and recorded for complete traceability. This way, a reliable quality can be ensured and the needs of the contractual partner can be fulfilled.

Metrohm’s ProcessLab analysis system (Figure 1) provides a host of analysis methods to track the whole plating process.

Figure 1. With the Metrohm ProcessLab analysis system, the entire process from preparatory baths to the nickel- plating baths and rinsing baths can be analytically monitored.

The ProcessLab analysis system can be used to monitor the following standard process steps and parameters:

• Contamination of cleaning and rinsing baths
• Acidity and alkalinity in degreasing baths
• Content of components in preparatory baths
• Content of main components in nickel-plating baths
• Physical properties, like density etc. (through the integration of extra instruments)

Figure 2. ProcessLab system for the analysis of nickel-plating baths. A completely automated system with sample changer is recommended for the fast and convenient analysis of samples from several production lines.

The customized ProcessLab is installed on site very close to the process. The integrated database saves all of the measurement and traceability data and offers data regarding the direction of the process at any given time. A control chart is used to analyze measurements and they are exported for additional processing.

The ProcessLab provides an easy and intuitive operation through touchscreen. Based on Metrohm’s industry proven components from the lab sector, the ProcessLab is housed in a rugged and splash-proof container. The system provides the following benefits:

• Reliable traceability and documentation
• Accurate control ensures more narrow process window
• Independence from the laboratory
• Minimal response times during bath-related issues
• Extended bath life, leading to lower disposal costs

Main Process: Nickel Electroplating and Electroless Nickel Plating

During the nickel electroplating process, nickel is added to the workpiece surface by applying electricity (Figures 3 and 4). However, during the electroless nickel-plating process a reducing agent is utilized for the same purpose. (Figure 5) However, both processes considerably vary in terms of bath composition, the uniformity of the deposit, the speed of deposition, and the surface characteristics of the finished workpieces. The selection of the plating process is mainly based on the application aspect of the manufactured workpiece.

Figure 3. The aluminum cooling fins, copper heat pipes and the copper base of this CPU cooler are completely nickel-plated.

Figure 4. Nickel electroplating is used for connections, plugs, contacts, etc. A highly uniform coating thickness is not essential in these areas.

Figure 5. Transmission, hydraulic and pump parts, valve housings, pressure cylinders, etc., are typically finished with electroless nickel plating because of the more precise coatings obtained.

Nickel Electroplating

During the electroplating process, electrical current is applied to electrolytically deposit nickel from the nickel bath onto the workpiece. It is easy to control the thickness and quality of the deposited layer via current density. The nickel bath’s electrolyte solution can be formulated either from nickel-containing concentrates or nickel salts.

A standard bath comprises nickel sulfamate, boric acid, nickel chloride or nickel sulfate, and other chemical additives. Extremely high workpiece throughputs can be obtained in nickel electroplating, but it is important to do a regular analytic monitoring of the baths.

The ProcessLab can be used to measure the following parameters:

• Chloride content (precipitation titration with AgNO₃)
• Nickel content (EDTA titration)
• pH value (direct measurement)
• Boric acid content (acid/base titration)

Electroless Nickel Plating

In electroless nickel plating, nickel is deposited without applying any current. Through a chemical reaction, the electrons required to reduce the nickel ions can be directly produced in the electrolyte solution by utilizing sodium hypophosphite, which is a type of reducing agent (see equations 1 and 2).

Equation 1

Equation 2

Equations 1 and 2 demonstrate that the bath’s chemical composition changes over a period of time. The hypophosphite content and the nickel ion decrease, and orthophosphite and sulfate concentrations (NaH₂PO₃), which are products of increasing salinity, increase. Normally, baths contain nickel salt in the form of nickel sulfate, and nickel is autocatalytically deposited.

As phosphorus is also deposited, it results in a nickel-phosphorus alloy. Workpieces can be extremely resistant to corrosion (high phosphorus content), or wear resistant and extremely hard (low phosphorus content) based on the phosphorus content present in the deposited layer (usually 1...14%).

The deposition of more nickel than phosphorus leads to the formation of more sulfuric acid than sodium hydroxide, and as a result the pH reduces continually. If the bath is used for a longer period of time, the concentration of impurities will also increase, which would negatively impact layer thickness and surface quality. To meet the process parameters, the bath composition should remain within certain limits of concentration (process window). This can be achieved by ensuring accurate control.

If the actual concentrations in the nickel bath are known, then appropriate measures, such as addition of auxiliary agents, replenishment of consumed bath components, or isolation of contaminants formed, can be undertaken to improve the quality of the end products and prolong the life of the bath.

The ProcessLab can be used to monitor the parameters of electroless nickel baths in an easy and efficient way:

• Alkalinity (acid/base titration)
• Nickel content (titration with EDTA)
• Sulfate content (precipitation as barium sulfate with ensuing EDTA titration)
• Hypophosphite concentration (indirect titration with sodium thiosulfate) (Figure 6)

Figure 6. Hypophosphite determination – titration curve of the back titration of excessive iodine with thiosulfate solution.

Conclusion

Metrohm’s ProcessLab analysis system helps to determine the condition or status of the baths. Users will always know the accurate concentrations of all substances that are used in the entire process, whether they opt to coat their workpieces with electroplating or electroless nickel plating. This will allow users to adopt early measures to improve the quality of their end products and prolong the life of the bath.

The ProcessLab analysis system produces less waste, saving a considerable amount of money and protecting the environment. The system can be used easily and can examine a host of different baths in the plating industry.

This information has been sourced, reviewed and adapted from materials provided by Metrohm AG.

For more information on this source, please visit Metrohm AG.