Switching from Decorative Hexavalent Chromium to Trivalent

Q: My company is considering switching from decorative hexavalent chromium to trivalent. Can you explain what that would entail and the advantages and disadvantages of making the switch?

Today, many plating operations are finding themselves considering a safer and more environmentally friendly alternative to Cr (VI). There are two basic chemistries for Cr (III) systems so we will review both to help your company choose which option will best fit your application.

Chloride:

Chloride-based systems refer to a mixed salt system that uses chloride salts as the primary source of conductivity. This system requires less heating due to an operating temperature of 85 – 100°F compared 110 – 115°F for Cr (VI). Having pH control is extremely important, as fluctuations in the pH can affect the plating speed, thickness distribution and low current density coverage. The anodes are made of graphite, instead of the lead alloy anodes in Cr (VI). This will eliminate the handling of hazardous lead sludge and create a much safer environment for your employees. The chloride system is also more conductive and more efficient than Cr (VI). This means that more surface area can be processed per rack and you will have lower rectification costs due to less voltage needed to maintain the optimum current density. Air agitation will be needed for this system to eliminate the risk of hydrogen gas streaks during deposition, and filtration with the ability to add carbon is also necessary to remove organic contamination. All trivalent chromium systems are less tolerant to metallic contamination, so ion exchange is necessary to maintain a consistent color. Since Cr (VI) in concentrations as low as 10 ppm can severely impact the Cr (III) bath, we recommend the Cr (VI) tank be removed and replaced with one made of polypropylene. The chloride systems maintain the highest corrosion resistance, as the deposit is microporous as plated. The color of the deposit is not quite as “blue” as hexavalent chromium, so if two parts are next to each other, you can see a slight difference. As the distance between the two parts is increased, it becomes more and more difficult to distinguish between the two.

Sulfate:

The largest advantage the sulfate system has over the chloride-based is the color. If the additives and ion exchange resin are properly maintained, the color is extremely close to Cr (VI). This system operates at a temperature of 120-130°F. As with the chloride-based system, pH control is critical so an accurate pH meter will be necessary. The anodes used in sulfate-based baths are MMO-coated titanium. These are expensive compared to Cr (VI) and chloride-based Cr (III) anodes. For agitation, we recommend cathode rod, but many customers use mild air or no agitation at all. We recommend having air agitation available for proper mixing during the initial makeup and for maintenance additions. Just like the chloride system, the sulfate chemistry must have adequate filtration with the ability to add carbon for organic contamination removal. Also like the chloride system, more surface area can be processed per rack than Cr (VI) bath, resulting in higher production levels. The sulfate system is not quite as corrosion resistant but can almost match the chloride system if microporous nickel is deposited under the chromium layer. Again, we recommend removing the Cr (VI) tank and replacing with polypropylene to eliminate the possibility of cross-contamination. Lastly, another difference is that the sulfate-based system can be used as a barrel process. The equipment needed is basically the same as for the rack sulfate application, with some chemical adjustments.

Even though the chemical costs of producing a Cr (III) deposit are higher than Cr (VI), after you consider all the advantages of Cr (III), the overall cost difference is not as significant as you might think. Those advantages include:

• Less rejects due to whitewash, chrome burn and current interruption.
• Higher production capacity.
• Less regulatory paperwork.
• Less drag out of solution due to a lower viscosity.
• Less expensive WWT process due to less chromium in the waste stream and no need to use bisulfite in the treatment.

Also, a popular topic in the media today is the use of PFAS in our industry and the emergent need to eliminate it from our environment. Cr (VI) systems employ the use of PFAS in the mist suppression products. As more and more regulations are implemented, it will become very difficult and very expensive to operate a hexavalent chromium process and continue to meet these regulations. Cr (III) systems do not contain PFAS, which has recently initiated many conversations about making the switch from Cr (VI) to Cr (III).  To stay updated on regulations toward these two substances, and to find more information on Cr (III) plating, please visit nomorehex.org