Application of pH Meter in Improving Electroplating Process

Electroplating is a common metal finishing process with a variety of industrial applications ranging from purely cosmetic to the application of protective coatings. The process, which uses an electric current to drive ions to a receptive metal surface, is sensitive to several key parameters, the importance of careful control to achieve the desired plating effect cannot be overemphasized. Several components including ion concentration in the plating bath, bath temperature and current density can be adjusted to affect the final result. The plating process yields many rules of thumb for proper operation, but by replacing conventional rules with pH measurements, quantitative feedback can be used to guide plating process control for improved product consistency and efficient waste disposal.

Prepare Plating Chemicals

The importance of the preparation of the electroplating bath to the success of the electroplating process cannot be overemphasized. Plating chemistries involve dissolving metal salts in solvents. The resulting plating solution needs to have a sufficiently high concentration of metal ions to facilitate plating of parts. Plating occurs at the cathode when hydrogen ions (H ⁺ ) are reduced to hydrogen gas (H _{2 ).} As these hydrogen ions are lost from the cathode membrane, electroplating occurs when metal ions, such as nickel ions, "step in" to replace the hydrogen ions.

Occasionally, the electroplating process produces dark gray or black flake buildup on the cathode. This is often called "burning". If the plating solution is not prepared properly, it will burn during the plating process. If the concentration of metal ions is too low, the electric current will hydrolyze the water to produce hydrogen ions (H ⁺ ) and hydroxide ions (OH ^- ). The generation of hydroxide ions can lead to the formation of metal hydroxides, such as nickel II hydroxide (Ni(OH)2), instead of metal ions being deposited on the surface of the part. Measuring the pH allows the operator to detect this effect as the production of hydroxide ions causes the pH to increase.

Some strategies to prevent burning during plating include:

• Increase the concentration of metal ions in the plating solution

• Increased agitation of the solution near the cathode

• Raise the working temperature

Controlling pH During Electroplating

Additionally, some plating processes may benefit from the use of buffers such as boric acid. The buffer ensures that the pH near the cathode is stable at the proper level for metal plating. The proper pH for the electroplating process depends on the metal to be plated as well as the solvent and any other additives. The appropriate pH can be acidic, neutral or basic, depending on the solubility of the metal in the solvent. Nickel and copper plating in acid baths are better at 3.8 to 4.2, with difficulties occurring at pH values ​​above 5. Alkaline baths can be used to more quickly plate materials such as gold, zinc and chrome, and have an optimum pH of 9.0 to 13.0.

Monitoring pH during the initial make-up process and throughout the plating process can help catch procedural errors. Even if the process is properly configured before starting, pH can be used to monitor ion depletion over time and inform when and how to replenish the plating solution. The effective current is also a function of concentration, and since the melt pool acts as a resistance between the two electrodes, pH monitoring can be used to ensure a constant current is supplied, resulting in a consistent deposited layer.

Electroplating wastewater treatment

Plating wastewater needs to be treated to remove toxic chemicals before being discharged into municipal sewers. Just as pH needs to be controlled during electroplating to keep metal ions in solution, pH can also be used in wastewater treatment to precipitate ions for removal by sedimentation and filtration. Hexavalent chromium is highly toxic to the human body and can be removed by a two-step process. Hexavalent chromium is first reduced to trivalent chromium by lowering the pH and increasing the redox potential. Then, the pH is raised above 8.5 to precipitate the chromium as chromium hydroxide.

Zinc and copper can also be removed by high pH adjustment, which causes the metals to precipitate as hydroxide complexes. However, when a cyanide bath is used as the solvent, the metal complexes with the cyanide and cannot be removed by adjusting the pH. Zinc, copper, gold and nickel are all commonly plated with cyanide baths. To remove cyanide, the redox potential is increased to reduce cyanide to cyanate, thereby releasing metal ions and causing them to complex with hydroxide and form a precipitate.

Select pH Electrode

Metal finishing involves several industrial water and wastewater treatment processes, which can be achieved using in-line sensors and instruments. However, due to the high concentration of metal ions in plating baths and wastewater, combination pH electrodes may require frequent replacement. Combination pH electrodes rely on a silver and silver chloride reference solution within the probe, which can be contaminated with heavy metal ions such as chromium, lead, and cyanide. The new probe design protects the reference solution by preventing contact with the sensor through the junction or prolonging the path for ions to travel within the probe. Some probes even use other reference solutions. Regular cleaning to remove buildup and sediment will maximize the life of your pH sensor.