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What Alternatives Exist for Lead Anodes in Chrome Plating?


Rapid Response Question: What alternatives exist for reducing lead-bearing waste or wastewater from lead anodes used in chrome electroplating on mild steel?  

Original Request by: Oregon Department of Environmental Quality

* DISCLAIMER: PPRC does not endorse any specific products or manufacturers mentioned herein.

Background

Lead alloy anodes are commonly used for hard chrome plating. Lead alloys are commonly available in tin, and antimony and silver. Anodes are prone to decomposition over time, and/or corrosion with exposure to certain ions or reducing acids in the plating bath. Corrosion starts from the weakest surface areas, like scratches, fissures and pores in a coating.

The nature of the electroplating bath can impact the longevity of the anodes. In particular, free fluoride ions (F-), typical in many chrome plating baths, attack the metals of the anode. Fluoride is one of the leading causes of anode corrosion. The severity of the corrosion depends on the F-content, other bath constituents, anode area to cathode area ratio, or anodic current density. “Etch-free” plating baths may reduce the rate of corrosion of anodes. See this thread on finishing.com for insights on anode attack.  

Reducing acids may also be used in chrome baths, contributing to anode corrosion over time.

Lead alloy anodes are not as prone to corrosion from the fluoride electrolyte solution or acids as some other anode materials. Different fluoride-based catalysts include sulfate-fluoride, sodium fluoride used to to activate steels by polarity reversal, or fuoro-borates or silico-fluorides in self-regulating hexavalent (Cr6+) baths. The fluoride ion content of these varies but may as high as two grams per liter (g/l).

Even in optimal bath composition and conditions, anodes do decompose or corrode over time, sloughing off into the bath. When decomposition and/or corrosion occurs with lead alloys, and especially in hexavalent chromium, the decomposed material deposits in the tank as lead chromate sludge that must be removed from the tank by filtering or other wastewater treatment, or pumping and disposal as hazardous waste.

The following findings represent some alternatives that may be helpful in reducing the amount of lead-bearing waste generated in chrome plating, and occupational exposure as a result of the process.

 

Findings

Anodes:

Alternatives to lead-alloy anodes for chromium plating include platinized titanium or platinized niobium. Several international vendors were contacted in 2008. Most concurred that platinized niobium was the best recommendation as an alternative to lead anodes because it is less prone to corrosion. They suggested that platinized, non-alloyed titanium is viable but only in non-fluoride electrolyte composition, or a bath with very low concentration of fluoride ions in the electrolyte composition of the bath. One supplier recommended a fluoride ion concentration of 2 parts per million or lower for titanium anodes.

For further research on the corrosion resistance or vulnerability of titanium anodes in different plating situations or presence of various acids, this study from Titanium Materials Corporation provides useful data (1).

Titanium has a lower electrical conductivity than lead, so the anode design may be subject to changes in order to be able to pass through the required amount of current.

An advantage of platinized anodes, in addition to reducing the amount of hazardous lead sludge generation, is dimensional stability, resulting in a more stable, homogeneous, and consistent-thickness plating process. Also, platinized anodes are much more flexible in shaping and dimensioning and can be designed according to the shapes of the work pieces to be plated.

Producers often provide multi-catalyst bath mixtures for a specific plating process, but may keep the formulation proprietary, therefore users may not know exactly what is in the bath. If corrosion seems to be occurring more rapidly than it should, bath and anode pilot tests are advised to determine which type of alternative anode or bath will work best.

Notes from David Langston, EPA on actual experience with titanium and niobium anodes (Interviewed in 2008):  

Practical experience shows a platinized titanium anode (PTA) with 5 microns works nearly one year at 50 mg F- / l, nevertheless we recommend platinized niobium anode (PNA). Niobium (Nb) withstands a F-content until 600 mg / l, a PNA is used from 50 to 500 mg F- / l up to currents of 20 Amps / sqdm. Mit4ch

Also fluoride is added to decorative black Cr3+-baths. For these baths mixed oxides activated titanium is applied. In the version of IrTa-oxide users have added F- up to 50-100 g F-/ l. The coating withstands up to 600 g F- / l, but scratches and pores in the coating limits use. The whole anode immerged inclusive the splash zone needs to be covered with the MMOA-layer. PTA with 5 microns Pt-layer failed after 11 months in a bath of 90 mg F- / l. MMOA Ti-anodes were used for years in a trivalent chrome bath at 70 mg F- / l.

PTA can only be used at low F- content of ~ 10 mg F -/ l. For F- content up to 600 mg/l PNA should be used as an alternatives to lead. Any higher F- content requires tungsten as the base.

Processes:

If a process can be changed from use of a hexavalent chromium bath to a trivalent chromium bath, trivalent produces approximately thirty times less sludge (by volume) than hexavelant chromium baths, which can significantly reduce hazardous material handling and disposal costs (2). A major hurdle has been the ability to increase the plating thickness while maintaining the bath longevity. A few companies have developed proprietary solutions with the incorporation of ligands and complexants to increase bath longevity (3).

High Velocity Oxygen Fuel (HVOF) is a newer “thermal spray” technology that can replace hard chrome plating, and has environmental, safety, and productivity benefits over conventional plating (4). The HVOF process utilizes a material in powder form injected into a flame of supersonic gas. The material softens in the flame and forms a dense coating on the substrate. The fuel for the flame is a gas such as hydrogen, acetylene, propylene, or a liquid such as kerosene, the coating material is usually a metal alloy such as chrome carbide, and the typical deposit coating layers range from 0.003-0.015” in thickness. More information on an actual application and comparison of other chrome plating alternatives is available here. A cost comparison was not found at this time, but suppliers claim it is affordable.

The aircraft industry has worked for years testing whether HVOF is a suitable replacement process for chrome plating.

Oerlikon Metco reports in a 2014 case study: “In the past, the hydraulic actuators of aircraft landing gear were almost exclusively hard chromium plated. Now, new aircraft from Boeing and Airbus are nearly all equipped with HVOF coated landing gear. Beside the main shaft, all surfaces previously coated with hard chromium plate can now be coated with HVOF. As an added bonus, the HVOF process can also be used to repair bearing seats locally without destroying the paint or any of the labels.” (5)

In this case study (5), Oerlikon Metco also provides information on additional potential technologies that may be viable replacements hard chrome plating in various circumstances or conditions. They discuss physical vapor deposition, and a patented process that combines thermochemical processing — gas nitrocarburizing, plasma activation (plasma nitrocarburizing), and oxidation.

Other process options may include chemical vapor deposition (CVD), and electroless nickel plating.

 

Conclusions

There are alternative anode materials to replace lead, but corrosion may occur depending on the bath conditions. Users are advised to test the suitability of different anode substrates under the actual plating conditions, prior to full changeout.

Alternative processes, such as HVOF or PVD may be viable for some applications.

 

Sources

  1. Titanium Materials Corporation. Undated. Corrosion Resistance of Titanium.
  2. Nov 2003. NEWMOA. Pollution Prevention Technology Profile: Trivalent Chromium Replacements for Hexavalent Chromium Plating
  3. Johnson, M. 2010. “Greening” the Chrome Plating Industry. Presentation.
  4. Undated. Are HVOF coatings an alternative to hard chrome plating?
  5. Oerlikon Metco. 2014. Thermal Spray, PVD and IONIT OX Processes are Excellent Alternatives to Hard Chromium Plating. (See Case Study).
  6. MAGNETO Special Anodes B.V. http://www.magneto.nl/en/products/verschillende-soorten-coatings
  7. David Langston, SR. Engineer, RCRA Programs. david@epa.gov. 404-562-8478

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