As the demand for non-chromium protection of zinc and zinc alloy plating has increased, trivalent chromium passivation systems have appeared. Early on, an interesting phenomenon was observed: Hexavalent chrome was found on coatings that were not supposed to contain hexavalent chrome. It was thought that the processor must be cheating and using hexavalent chrome in the working solution in order to achieve increased corrosion resistance.
While all of the questions have not been answered, there are some conclusions. Before explaining the various parts of this mystery, it’s important to understand the differences between hexavalent and trivalent coatings.
Hexavalent and Trivalent Conversion Coatings
Hexavalent chromium based conversion coatings have been in use for a very long time. The corrosion preventative properties of hexavalent chromium based conversion coatings cannot be overstated and have been well documented. Hexavalent chrome processes are inexpensive and relatively simple to apply. These systems produce colors that range from clear to yellow to olive drab to black. As a general rule of thumb, the darker the hexavalent chrome finish, the better the corrosion resistance produced by the finish. It is not unusual for a hexavalent chrome conversion coating to provide nearly 150 hours to white corrosion in a neutral salt spray chamber.
But for all of hexavalent chromium’s advantages there are some problems with this coating. Hexavalent chromium is a known human carcinogen and a toxic material. Hexavalent chormium exposure in humans may lead to significant health damage. Hexavalent chrome can be an insidious material that leaches into groundwater systems. And despite its obvious dangers, hexavalent chrome conversion coatings are still applied in many plating shops because of its corrosion preventative properties.
Trivalent chromium passivation systems are relatively new to the surface finishing industry. Their use has been encouraged by increasing regulatory pressures to reduce exposure to hexavalent chromium in both the workplace and the consumer market. This drive has been championed throughout the United States and European Community. While restrictions on worker exposure to chrome has been the tool of choice employed by United States regulatory authorities; outright material bans on hexavalent chromium containing articles have been mandated by the European Union. The effect of these regulatory initiatives has been to restrict the use hexavalent chrome and encourage the use of trivalent chrome and other substances to prevent corrosion of zinc plated substrates.
So what are some of the differences between the finishes? Hexavalent chromium based conversion coatings are the result of the contact with the zinc surface by a hexavalent Chromium based conversion coating solution. The contact actually converts the zinc surface to a gelatinous film consisting of zinc chromate, trivalent chromium compounds and water.
Trivalent chromium passivates work differently. In the trivalent chromium process, the zinc surface is not converted to a gelatinous film. Rather, the surface is passivated by the exposure to the trivalent solution. A gelatinous film is formed, but the water content and morphology are quite different. Because of the passivation mechanism, the trivalent chromium based coating does not produce similar corrosion resistance. A dark trivalent coating does not necessarily equate to a heavier or more corrosion resistant finish. In virtually all cases, a top coat is required to provide the required corrosion resistance.
Understanding the Mystery
In early formulations, some trivalent chromium solutions began as either chromium nitrate, or hexavalent chrome which was reacted with oxidizing agents at controlled pH ranges to convert the hexavalent chromium to a trivalent state. After processing, not only was hexavalent chrome found on the surface of these coated parts, it was possible to find evidence of hexavalent chrome in the working trivalent bath. Unfortunately, what was thought to be a compliant trivalent chromium process produced non compliant results. Hexavalent chromium was commonly found in trivalent chromium operating baths. Needless to say this puzzled more than a few line operators.
How does trivalent chrome convert to hexavalent chrome? Nobody is entirely certain. However, there are a few theories. The first theory holds that because trivalent chrome is not completely stable it is possible that in the presence of oxidizing agents, at certain ranges of acidity, trivalent chrome simply converts to a hexavalent state. Some early trivalent chromium applications included the use of peroxide compounds in a step subsequent to the application of the trivalent chromate. This post treatment step was used to make the trivalent passivation film more corrosion resistant. After exposure to these aggressive oxidizing agents it was found that trivalent chromium compounds had actually converted to hexavalent chromium compounds. Unfortunately while the resultant coating is more corrosion resistant, it is not free of hexavalent chrome and did not meet the standards which were specified.
An alternative theory is postulated that when a trivalent coating is placed in an accelerated corrosion testing chamber, such as a salt spray chamber, the trivalent film actually converts to a hexavalent chromium film. This postulate is based upon a finding of hexavalent chromium present on a part which showed no presence of hexavalent chrome prior to placement in the salt spray chamber. The mechanism of this reaction is unknown.
In some cases, parts which had been exposed to environmental conditions in their expected use, such as brake parts removed from a car, showed evidence of hexavalent chrome where none was expected.
Needless to say, the presence of latent hexavalent chrome on a trivalent chrome finished part has caused some serious explaining!
In order to eliminate hexavalent chromium both in the bath and on the finished parts, suppliers have developed newer and more robust technologies to produce trivalent chromium finishes. These processes do not rely on any oxidizers, and use other ROHS compliant chemicals in the composition of the trivalent chromate solution. These multi-part baths and finishing processes have gone a long way to eliminating the presence of hexavalent chrome on parts finished in a trivalent passivation process.