Optimizing Trivalent Chromium Plating for Color Match and Corrosion Resistance

I learned this adage earlier in my greenhorn career while servicing copper, nickel, and hex chrome electroplating shops: “You can’t make a silk purse from a sow’s ear.”

Paul Cook, CEFThis was at a time when thick copper deposits were electroplated, buffed to a smooth finish, and then activated and finished with nickel and Cr⁶⁺. Automotive and motorcycle parts were primarily made of zinc die-cast or cold-rolled steel. Hand jacks with buffing and polishing were employed using cut-and-color compound in liquid and bar compositions. The quality of these articles was the norm during the heyday of decorative finishing.

Gradually, much of the copper and copper buffing was replaced by semi-bright nickel containing coumarin, index (SAS)- based bright nickel, and Cr⁶⁺. The idea was to level in a laminar, semi-bright manner, increase leveling and brightness, and incorporate bright nickel and Cr⁶⁺. Buffing and polishing of the substrate were new practices, and, depending on the skill of the buffing department, part quality was acceptable. Cr⁺⁶ was bright blue, and although the throw was low, the overall process remained.

The Era of Chrome Deposits From a Cr⁺³ Electrolyte

Fast-forward to the last 50 years, to the era of chrome deposits from a Cr³⁺ electrolyte. One of, if not the, processes was developed in England by Albright and Wilson. This is the basis for the chloride-type process that utilizes graphite anodes. This electrolyte was sensitive to copper and nickel contamination from drag-in from the preceding processes. The deposit would gradually darken and appear more smoky rather than approaching a bright blue. 

Chemical compounds of a rather nasty nature were used to precipitate the metal impurities. This gave Cr⁺³ a poor start, and little interest remained among the old-school players. The discovery of Ion-Exchange resins capable of removing impurities kept the dream alive. The development of a high-performance chelating resin solved the problem of copper and nickel contamination. Additional refinements to the proprietary wetting and surface-tension additives result in improved deposition brightness, color, and throw. Electroplating from a Cr³⁺ electrolyte is a far more complex chemistry than hexavalent chrome with a catalyst.

The advantages are also greater, including improved throwing power, tolerance of current interruption, and no whitewash. With proper training, if you can perform a bright nickel process, you can learn trivalent electroplating.

Throw is Dough

In the world of electroplating, throw is “dough”. Please read the best article ever published on throwing and covering power -- Throwing Power and Covering Power on Electroplating Solutions by W. de Bruijn -- a paper presented at a Joint Meeting of the Electro Depositors’ Technical Society and the Studiekring Galvanotechniek, held in Holland on September 28, 1950.

Achieving a trivalent chromium finish over nickel that closely resembles traditional hexavalent chrome is a known challenge in the plating industry. The argument has focused on the color difference between the two deposits. Industry resists change, and the inability to match a part plated in hexavalent next to a part plated in trivalent was a mismatch that trivalent could not overcome. Gradually, the OEMs are slowly changing course. 

The key differences between hexavalent vs. trivalent:

Factors That Influence Color

Factors that influence color are related to the underlying nickel deposits. Ask any old-time electroplater to define how they determine chrome miss or nickel show. Yellow nickel contrasts with the bright blue chrome deposit. Satin nickel under trivalent deposits is evidence of the color spectrum. Bright nickel formulations are available that deposit a white nickel with fine micro-leveling at low current density.

The trivalent chrome will throw better due to the throw and covering power of the underlying nickel in the LCD. Bare metal = no chrome deposit. This will change the B-value most favorably. The use of the premier ion-exchange resin is essential. High bed volume during production and low bed volume during off-hours will maximize the removal of metal impurities.

The use of post-treatment processes, such as electro-passivation, will clean the microporosity, remove trapped salts and metal ions, and improve overall reflectivity and corrosion resistance. Consider the nature of residual chlorides on parts and their effect on corrosion resistance. Innovative post-sealing of the microporous chrome layer further enhances the performance of this deposit.

I have advocated for the use of trivalent chromium to replace hexavalent chromium since the 1990s. I was servicing an account that was a firm adopter of Murphy’s Law and saw just about anything that could go wrong did, and we always kept trying to make the silk purse. They are still running strong in 2025.

Lessons learned, and I have high hopes for the future of this environmentally sound process for a better tomorrow and a thriving electroplating industry.

Paul Cook CEF is with Procom in Ann Arbor, Michigan. Please visit www.procom-us.us.