What to do about Hard Chromium

Hard chromium effectively combats corrosion while providing lubricity, as well as resistance to wear, fatigue and impact. The problem with hard chrome is not with the plated metal, but with the environmental problems associated with the plating process. Hard chromium plating uses chromic acid, which releases fumes into the air during the plating process. This mist contains chromium +6 ions that are carcinogenic and can cause other medical problems, such as perforated nasal passages and skin rashes.

Because of this, the EPA has promulgated rules to limit chromium releases to the environment. The EPA air emission limit for hexavalent chromium is 0.015 mg/dscm. Also, OSHA has enacted a new regulation that severely limits the amount of chromium mist that workers can be exposed to in the plating shop (1 ug/m3). The cost of these regulations is expected to be high. The National Defense Center for Environmental Excellence estimates it will cost $22 million for the first year and $46 million for each subsequent year for the Navy alone.

Other issues affecting the hard chromium plating industry include the End-of-Life Vehicle Directive of the European Union.

So what are platers and end users supposed to do? There is no "drop-in" replacement for hard chromium plating. To determine what process is right for your applications requires you to evaluate the application and match the requirements to the available technologies.

Some items to consider when evaluating alternatives are to ensure that the material and substrate are compatible, to avoid corrosion, poor adhesion or coating stress. You also want to be sure the process and substrate are compatible to prevent degrading the properties of heat-sensitive substrates and substrate etching, corrosion or embrittlement. Pre- and post-treatments must be considered, as well as any capital equipment, training and labor costs you may incur. Certain alternatives may also be regulated, which would require additional investigation on the plater‘s part.1

Part Modification

One alternative is to re-engineer or redesign the part or use a different substrate, such as improved wear and/or corrosion resistant steels designed for specific applications. This option is not often feasible, since it can take years to approve part redesign in industries such as automotive and aerospace.

Surface modification can also be considered. Heat can diffuse elements into the near surface region of a substrate to form an alloy or compound layer with improved properties. Nitrocarburizing, nitriding, plasma treatments and carburizing are among the methods that can be considered. Plasma surface ion implantation of coatings is also used commercially.

Other surface modification techniques include laser alloying, whereby a precursor mixture is applied to the part. This mixture is then heated to fuse the coating. Another type of surface modification is electrolytic diffusion, which includes plasma/arc processing.

Dry Plating Alternatives

Research and development continues on a number of alternative dry plating processes that do not contain chromium. Some of these coatings are designed to replace decorative chromium plating, while some have commercial applications.

Micro-welding is a dry alternative process that is good for plating on conductive substrates. A power source is used to produce arcs, which in turn form droplets that are transferred to the substrate, forming a metallurgical bond.

Micro-welding does not hurt the environment and uses materials efficiently. Minimal substrate preparation is needed, and thick coatings are produced; however, it is a slow process that produces a rough finish.

Chemical vapor deposition (CVD) is another alternative to chromium plating. Parts are placed in a reaction chamber. During the process, a gas reacts chemically at low temperatures with the parts’ surfaces to form a smooth, dense, conformal coating. CVD has mainly been used with electronic and optical parts; however it can be tailored for finishing high-strength steels. But the coatings are thin and deposition rate is slow.

Physical vapor deposition (PVD) or magnetron sputtering can be used as an alternative. PVD finishing uses a vacuum in which atoms are dislodged from the target, which is made of the coating material (e.g. aluminum) and accelerated toward the substrate where they are deposited. Magnetron sputtering is a similar, yet more efficient process.

The PVD process, as with CVD, produces a smooth, dense, conformal coating. Various targets can be used to produce multi-layer coatings that can be applied to a variety of substrates; however, it is a line-of-sight process with limited applications.

Thermal Spray Technology

Thermal spray is also an alternative process that can be used with both chromium and non-chromium finishes. The Hard Chrome Alternatives Team (HCAT), a joint effort of the defense department of the United States and Canada, aerospace companies and military overhaul depots, has done extensive research on high-velocity oxy-fuel (HVOF) thermal spray coatings for finishing landing gear, turbine engines, propeller hubs, hydraulic parts and helicopter rotary heat components.

During the HVOF process, the coating material is fed into a combustion chamber of a gun where a fuel is burned with oxygen. The heated and softened powder leaves the gun as a spray.

The HCAT team realizes that there is not a single replacement technology for chromium, but it has found that the cobalt-cemented tungsten carbides are some of the easiest materials to spray and provide the largest range of applications.

Because the HVOF process is in development for aerospace parts, surface finish is critical. A WC-Co (tungsten-carbide-cobalt) finish needs to be six microns Ra or better. This ensures a better seal life than that of a seal running against hard chromium plating.

Finish fatigue is also a critical issue. HVOF coatings were found to cause little or no fatigue debit when applied to 4340 high-strength steel, which is in strong contrast with chromium plating, which causes a significant debit.

When WC-17Co is applied to aluminum 7075-T73 alloy using HVOF, it provides little or no difference in fatigue from hard chromium plating. Fatigue data show that WC-Co cannot be used as a universal chromium replacement, however.

The HCAT testing has demonstrated that tungsten carbide coatings applied using HVOF are equal in performance to hard chromium in all measured properties and much better than hard chromium finishes in fatigue and wear.

Nickel-Based Alternatives

Many nickel-based coatings can be used as alternatives to hexavalent chromium finishes. Nickel with molybdenum added provides a slightly better hardness than just nickel; however it cannot measure up to the hardness of hard chromium. Nickel with tungsten added provides hardness up to 800 VHN and up to 1,000 VHN after heat-treating. Adhesion was also found to be first-rate.

Nickel cobalt alloy deposits also have been tested, with as-deposited hardness values from 820-900 VHN. With heat treatment, a precipitation hardening process happens and microhardness values of 1,000–1,150 have been achieved.

Other nickel-based coatings include nickel-boron, nickel-cobalt-phosphorus, nickel-phosphorus, nickel-phosphorus-tungsten and nickel-molybdenum.

Electroless nickel and electroless nickel alloy coatings also have been tested as potential alternatives to hard chromium plating. The most common are nickel-boron and nickel-phosphorus. The hardness of nickel-alloy coatings depends on the composition, reducing agent concentration and type of heat-treating. Testing has shown that there is no correlation between hardness and composition with heat-treated finishes; however ductility increases when the phosphorus content is increased.

Electroless nickel composite coatings infiltrated with boron nitride, diamonds, molybdenum disulfide or silicone carbide particles have hardness values between 900 and 1,200 VHN. Coating hardness depends on composition, concentration of the reducing agent, heat treatment and particulate additions. Particle size also affects hardness; larger particles result in greater hardness.

Trivalent Chromium

Trivalent chromium can be used in some instances as a replacement coating for hexavalent chromium in decorative applications. Trivalent chromium has few applications as a replacement for hexavalent chromium in functional applications, however. Trivalent chromium plates better in low-current-density areas and produces a more even coating than hard chromium. It is also a faster plating process and needs less than half the current density of hard chromium plating.

Two major concerns of platers who are considering a conversion to trivalent chromium plating processes from a hexavalent chromium process are color and corrosion resistance. The color issue has nearly been resolved, with few able to discern the difference between a trivalent chromium plated part and one that has been plated with hexavalent chromium.

Trivalent chromium deposits have two properties not exhibited by hexavalent chromium deposits. To obtain corrosion resistance, hexavalent chromium-plated parts must have a pre- or post-operation that provide the benefits of microporous chromium. Trivalent chromium deposits as plated, without additional steps, have the same or better corrosion resistance than hexavalent deposits that have been rendered microporous. However, without a nickel deposit or some form of post-treatment, thick trivalent chromium deposits do not impede corrosion as well as some types of hexavalent coatings.

Conclusion

Hard chromium plating has been around for decades. The finish reduces wear and corrosion. The problem is not with the finish but with the plating process. Chromium metal is inert and is used on everyday items such as prosthetic implants. However, the EPA and OSHA have decided to limit and heavily regulate its use, forcing platers to look for alternatives to the process. There is no drop-in solution, and there is no solution that can completely replace a hard chromium finish; however, the industry continues its research and development on alternatives.

Endnotes

1. Alternative to Chromium Coatings: Wet Deposition Technologies; T.A. Naguy Air Force Research Laboratory, Wright Patterson Air force Base, Ohio; M.L. Klingenberg, M.A.B. Pavlik and M.J. Beebe, Concurrent Technologies Corp., Johnstown, PA; presented at Sur/Fin®2004, Chicago, IL.
2. Wear Behavior of Environmentally Acceptable Alternatives to Chromium Coatings: Nickel -Based Candidates; Eric W. Brooman, Ph.D.; M&P Professional Services, Dublin, OH; Metal Finishing, September 2004, 75-82.
3. Alternatives to Chromium Coatings: Dry Deposition Technologies; Dr. E.W. Brooman, Air Force Research Laboratory, Wright Patterson Air Force Base, OH; presented at Sur/Fin 2004, Chicago, IL.
4. Hard Chromium Alternatives Team Update: Improving Performance While Reducing Cost, Keith Legg, Rowan Technology Group, Chicago, IL, and Bruce Sartwell, Naval Research Laboratory, Washington, DC.
5. Decorative and Functional Trivalent Chromium: Alternative to Hexavalent Chromium Coatings, Donald L. Snyder, Ph.D., Atotech GMF, Rock Hill, SC; presented at Sur/Fin 2004, Chicago, IL.