pH Control to improve the electroplating process
Electroplating is a common metal finishing process with several industrial applications, ranging from the purely cosmetic to the application of protective coatings. The process, which uses electric current to drive ions towards receptive metal surfaces, is sensitive to multiple key parameters and careful control is crucial to achieving desired plating effects.The process of electroplating has produced many rules of thumb for proper operation but by replacing conventional rules with pH measurement, quantitative feedback can be used to guide electroplating process control for improved product consistency and efficient waste treatment.
All aqueous electroplating solutions contain hydrogen ions, in addition to those metal ions from which deposition takes place. The pH is an important tool for proper quality control in electroplating and metal finishing operations. The pH must be held within well-defined limits to maintain optimum deposition speed, mainly governed by cathode current efficiency (CE). The cathode current efficiency depends primarily upon the ratio in which metal and hydro-gen are deposited. The extent of hydrogen evolution in any given electroplating bath is contingent on the pH and the hydrogen over-voltage on the cathode. The primary purpose of pH measurements in electroplating is to define and control the acidity (or alkalinity) of a given bath within certain limits that produce the desired performance of the bath and optimum quality of deposits.
Preparing Electroplating Chemicals
Preparation of the electroplating solution is critical to the success of the electroplating process. Electroplating chemical preparation consists of dissolving metal salts into a solvent. The resulting electroplating solution must have a high enough concentration of metal ions to promote plating of a part.Sometimes, an electroplating process produces a dark grey or black, flaky build up at the cathode. This commonly referred to as “burning.” Burning in an electroplating process can occur if the electroplating solution is not properly prepared.Some strategies to prevent burning in the electroplating process include:
- Increasing the concentration of metal ions in electroplating solution
- Increasing agitation of the solution near the cathode
- Increasing operating temperature
Controlling pH in the Electroplating Process
Additionally, some electroplating processes may benefit from the use of a buffer, such as boric acid. The buffer ensures that pH near the cathode remains stable at the optimum level for metal plating. The optimum pH for an electroplating process depends on the metal to be plated as well as the solvent and any other additives. Proper pH can be anywhere from acidic to neutral to alkaline, depending on the solubility of the metal in the solvent. Nickel and copper plating in acid baths are optimized around 3.8 to 4.2 and difficulties occur at pH above 5. Alkaline baths can be used for faster plating of materials such as gold, zinc, and chrome, and will have an optimum pH of 9.0 to 13.0.
Electroplating Wastewater Treatment
Electroplating wastewater must be treated for toxic chemical removal before discharge to municipal sewers. Just as pH needs to be controlled in the electroplating process to keep metal ions in solution, pH can also be used in wastewater treatment to precipitate ions for removal via sedimentation and filtration.
Choosing a pH Electrode
Metal finishing involves several industrial water and wastewater treatment processes that can be achieved using online sensors and instrumentation. However, due to the high concentration of metal ions in electroplating baths and wastewater, combination pH electrodes may require frequent replacements. Combination pH electrodes rely on a silver and silver chloride reference solution within the probe that can become contaminated by heavy metal ions such as chrome, lead, and cyanide. New probe designs protect the reference solution by either preventing contact with the sensor with junctions or lengthening the path that ions need to travel within the probe. Some probes even use alternative reference solutions. Regular cleaning to remove build-up and deposits will maximize the life of the pH sensor.
