Avoiding Roughness and Pitting in Decorative Bright Nickel

Doug Lay is director of technical marketing for Coventya Inc. Visit coventya.com

Q: We are experiencing frequent issues with roughness and pitting in the deposit from our decorative bright nickel process. Is there information that can assist in understanding the difference of each and in finding the root cause?

A: Roughness is usually caused by the incorporation of insoluble particles in the deposit and can come from many sources. Insoluble particles may enter the solution from many sources from the entire plating process. These examples include: incomplete polishing of the basis metal so that slivers of metal protrude from the surface; incomplete cleaning of the surface so that soil particles remain on the surface; detached flakes of deposit from improperly cleaned racks; dust carried into the tank from metal polishing operations and other activities near the plating line; insoluble salts and metallic residues from the anode, parts on the bottom of the plating tank, among others.

Roughness from incomplete polishing, cleaning and inadequate rack maintenance is avoided with good housekeeping practices, regular inspection and control.

Roughness caused by dust or air particulate can be controlled by isolating surface preparation and metal polishing operations away from the plating area; or by providing a supply of clean air and by removing dirt from areas near and above the tanks.

Continuous filtration, through a 1-micron retention media, of the plating solution turning over the full volume of solution at least once an hour is important for minimizing the potential for roughness problems. Anode residues must be contained within properly maintained anode bags. Care should be taken not to damage the bags or allow the plating solution level to rise above the tops of the bags.

Deposit pitting is caused by many factors including but not limited to the entrapment of air or hydrogen bubbles to the parts being plated. Air should be expelled uniformly throughout the bulk of the solution as already mentioned. Pitting from adherent hydrogen bubbles can result from a solution that is chemically out of balance, has too low a pH, or is inadequately agitated across part surfaces. Other sources to investigate include incorrect racking of complicated components; too low a concentration of wetting or anti-pitting agents; or the use of incompatible wetting agents. Additionally, the presence of organic contamination, the presence of copper ions and other inorganic impurities, incomplete surface cleaning of the basis material, or incomplete dissolution of organic additives that may form oily globules can all result in pitting. Pitting is, therefore, avoided by maintaining the composition of the plating solution within specified limits, by controlling the operating pH and temperature, and by preventing impurities of all types from entering the solution.

The following abbreviated troubleshooting guide is a good start and narrows the scope of the search for a solution to help identify the source of your problems.

Problem	Possible Cause(s)	Corrective Step(s)
Roughness	Particulate in the bath	Filter the bath thru 1-micron filters. Identify and eliminate the source (torn anode bags, solution level above anode bags, burned areas on parts or bare rack tips, dust in the air, etc.).
	Low nickel chloride concentration (chloride controls anode corrosion and improves the conductivity of the solution (less tendency for burning)	Analyze and adjust the nickel chloride as necessary.
	Current density too high (especially for parts that have protrusions)	Reduce current density or position parts on racks to reduce excessively high current density areas.
	Anodes bags breached	Reduce solution level below the top of the bag or replace bags with holes.
Pitting	Organic contamination (either due to drag-in or of breakdown of process addition agents)	Carbon treat solution. Follow the procedure provided by the chemistry supplier.
	Supersaturated air (micro-bubbles)	Repair pump seals, intake leaks and/or filtering intake location.
	Poor cleaning	Inspect cleaning steps and improve as necessary (including rinsing prior to the Nickel solution).
	Poor agitation	Correct agitation pattern and/or agitation rate. (New parts and new configurations can lead to pitting).
	Imbalance in proprietary addition agents.	Analyze and adjust as necessary.
	High solution surface tension	Check and adjust accordingly with wetter package.

In conclusion, as with any plating operation, decorative bright nickel processes will periodically experience problems/defects for many reasons. Many times the root cause may be the nickel plating electrolyte due to additive, salt and operating parameters imbalance or it may be the result of external influences such as pre-treatment, racking and equipment problems. It is critical that skills are developed to narrow the scope to take the appropriate corrective steps to solve the problem.