A new alternative to hexavalent chromium conversion coatings

Dr. Neil Canter, Contributing Editor | TLT Tech Beat October 2009

Researchers discover an environmentally friendly alloy with lower wear and superior corrosion protection. 

KEY CONCEPTS
• Hexavalent chrominum has been very effective as a coating to protect metal alloys for over 60 years, but health and safety concerns have led the metal-finishing industry to look for an alternative.
• A nickel-tungsten alloy has been developed that exhibits lower wear and superior corrosion protection. This alloy is also environmentally friendly.
• The properties of the nickel-tungsten alloy can be tailored for specific applications.

Hexavalent chromium has been a mainstay in protecting metal alloys for more than 60 years. But the metal-finishing industry is looking for alternatives because of health and safety concerns with hexavalent chromium that has been found to be a carcinogen.

The European Union RoHS (Restriction of Hazardous Substances) directive allows for a maximum concentration of hexavalent chromium of 0.1% in conversion coatings. Development of alternatives has been difficult because hexavalent chromium displays a combination of unique characteristics. Christopher Schuh, associate professor of materials science and engineering at MIT, says, “Hexavalent chromium exhibits a unique slate of properties that include a high degree of hardness, excellent corrosion and wear resistance and a shiny visual appearance.” 

The most difficult property to match is hardness, according to Schuh. He adds, “Hardness is a show stopper for most alternatives, though it is not as important a property as wear resistance.” 

Schuh indicates that chromium displays excellent hardness because this metal inherently develops a nanoscale structure when it forms a coating. He adds, “Very few other metals form such a structure naturally during deposition, though nanocrystalline forms of other metals can be prepared, which leads to a boost in hardness.” 

Besides a nanoscale structure, Schuh believes two other steps are required to prepare an alternative with comparable properties to hexavalent chromium. The second step is to make and keep the nano-grain size small. Finally, a process needs to be developed to control the size of the nanocrystals used in preparing the coating.

A suitable alternative that can match the properties of hexavalent chromium without the health and safety problems has not been developed until now.

NICKLE-TUNGSTEN ALLOY
When processed into nanocrystalline forms, nickel coatings display comparable hardness to hexavalent chromium in the short term. But the nickel crystals gradually lose their high hardness characteristics because they expand in size to become microscale grains.

The researchers found that combining nickel with tungsten produces a coating that displays comparable and in some cases superior performance to hexavalent chromium. The coating fulfills the three steps that Schuh indicates are necessary. He says, “Tungsten is mainly used for thermodynamic reasons to enhance processing and to assist with stability of the nanostructure.” 

The reason that a superior performing coating can be developed is due to a unique manufacturing procedure. In a conventional electroplating process used to prepare conventional hexavalent chromium, DC current flows from the anode to the cathode. Metallic ions in the solution are attracted to and plate out on the material that constitutes the cathode.

Preparation of the nickel-tungsten alloy coating is conducted by a pulse-reverse waveform technique. Schuh explains, “We flip the direction of the current which enables us to put down metal and then strip some of the metal off during the plating process. This enables us to precisely manipulate the structure of the nanocrystalline coating.” 

Evaluation of the nickel-tungsten alloy was conducted vs. a hexavalent chromium coating and an electroless nickel coating. A ball-on-disk/ pin-on-disk tribometer was used to evaluate the sliding wear resistance of the coatings. The counterbody involved in the testing was a six-millimeter tungsten carbide sphere.

A wear load of three newtons was employed for the nickel-tungsten alloy and the hexavalent chromium while the electroless nickel could only handle a one-newton load. The latter did not have the characteristics to tolerate the higher load. Schuh says, “Profilometer evaluation of the wear tracks for each coating showed that the nickel-tungsten alloy exhibits a lower wear rate than hexavalent chromium by an order of magnitude and a greater-than-two orders of magnitude lower wear rate as compared to electroless nickel run at the lower load.” 

Corrosion inhibition was assessed through the use of the salt spray test conducted by the ASTM B117 method. As shown in Figure 2, a 12-micron coating of the nickel-tungsten alloy on steel displayed no rust after 1,100 hours of salt spraying. In contrast, the same thickness of hexavalent chromium coating on the same substrate rusted after only 48 hours.


Figure 2. After 1,100 hours of salt spray testing, a steel specimen coated with the nickel-tungsten alloy displays no rust in the top figure. In contrast, the same steel specimen coated with hexavalent chromium shows extensive rust in the bottom figure after only 48 hours. (Courtesy of the Massachusetts Institute of Technology)

Schuh pointed out that performance tradeoffs are seen with coatings such as electroless nickel and hexavalent chromium. He says, “Electroless nickel is typically baked to improve its hardness. But this process causes this coating to crack, which leads to a sacrifice in corrosion protection. Hexavalent chromium loses hardness when heated at elevated temperature. In contrast, the nickel-tungsten alloy hardens substantially after baking without sacrificing other performance characteristics.”

In fact, Schuh indicates that the properties of the nickel-tungsten alloy can be tailored during processing to optimize specific properties. He says, “We have figured out a way to control the structure of the nickel-tungsten alloy coating. This allows us to tailor the properties of the coating to meet specific hardness, wear resistance and corrosion protection requirements for a particular application.” 

Properties are optimized by mixing and matching the size of nanocrystals. Grain sizes between 2 and 200 nanometers can be prepared, according to Schuh.

The performance of the nickel-tungsten alloy coating was evaluated in an ink transfer application. A rotating gravure ink wheel transfers ink to a wire. Mechanical contact wear is generated as embossed letters on the wheel are exposed to an ink and pressed against a substrate. A 12-micron coating of the nickel-tungsten alloy on a wheel facilitated the printing of 135 kilometers of printed wire, prior to the process leading to unacceptable printed letter quality. A hexavalent chromium coating was only able to satisfactorily print 13.5 kilometers of wire.

The nickel-tungsten alloy technology has been licensed to Xtalic Corp., Marlsborough, Mass., which is commercializing the plating process. Further information can be obtained by contacting John Kinne at jkinne@xtalic.com. 

Schuh is working with other metals in an effort to develop high-strength coatings for lightweight nonferrous alloys. Further information on the nickel-tungsten alloy can be found in a recent presentation made at the 2009 Sur/Fin Conference (1).

REFERENCE
1. Jones, A., Hamann, J., Lund, A. and Schuh, C. (2009), “Nanocrystalline Ni-W Alloy Coating for Engineering Applications,” Proceedings of the 2009 SUR/FIN Conference, Louisville, Ky.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.