This article introduces readers to Zirconization, a process that utilizes transition metal pretreatment, low-temperature cleaning technology, and rinse water quality maximization techniques. Among other benefits, Zirconization enables finishers to minimize the need for process heat; minimizes labor-associated phosphate sludge; eliminates phosphates; and maintains or improves quality. More importantly, this article—which represents the culmination of laboratory work, field testing, and collaborations with end users—will demonstrate how the process can save finishers a significant amount of time and money, while removing chemicals from the waste stream and reducing rinse water consumption.

With water—as well as energy—seen as precious resources, being green is increasingly becoming a requirement to operate competitively, profitably, and legally. A senior manufacturing manager recently stated, “There is no doubt that waste treatment standards are changing, and industries in some communities are facing increases in sewer rates for industrial discharges, which is an additional consideration for reduction or elimination of the waste stream, if possible.”

Due to these increasing environmental restrictions and global competition, manufacturing facilities have been forced to consider alternatives to conventional processes. A good example is the European Union. With a population twice as dense as the United States and with fewer freshwater resources, it has, by necessity, invoked stringent water regulations. In early 2007, the European Parliament approved legislation that revised regulations on phosphates with a goal of completely phasing them out within three years. This forced a major shift in the coating pretreatment industry, where best-practice chemistry has traditionally consisted of iron and zinc phosphate treatments.

Consequently, work began on developing non-phosphate-based chemistry—specifically in the very challenging area of metal preparation for coating. These efforts have been relatively successful, and the base active raw materials of these chemistries are elements from the transition metal group that include zirconium, titanium, and vanadium (Fig. 6). These elements exhibit chemical properties similar to chromium, but they do not have toxilogical concerns. As with any new process, there were drawbacks. Most significantly, the requirement for higher water quality, new precision cleaning, and issues associated with flash rusting.

However, recent technological advances have resulted in a new chemistry that addresses these drawbacks, while providing an effective replacement for phosphate chemistries in many pretreatment operations. This chemical process, known as Zirconization, not only matches or exceeds the ease of use, paint adhesion, and corrosion resistance provided by typical phosphate-based chemistries, but it also provides additional benefits such as less chemical usage and lower operating temperatures.

History of Phosphate Pretreatment Chemistries

Historians refer to the period from 1865–1900 as the Second Industrial Revolution. During this period, advances in the chemical, petroleum, electrical, and steel industries resulted in a new phenomena: the mass production of consumer goods. A need arose to protect these new steel goods from rusting.

British inventor William Alexander Ross provided some of the earliest work on phosphate coatings, filing patents in 1869. In 1906, Thomas Watts Coslett, also from Great Britain, pioneered iron phosphate coatings when he dissolved iron chips into a phosphoric acid bath. Coslett subsequently patented his process in both Great Britain and the United States.

By 1912, inventor Frank Rupert Granville Richards improved upon the Coslett iron phosphate process with the development of a manganese phosphate—a process patented in 1913. Several improvements were made to the manganese phosphate process over the following 20 years. Although this process provided adequate corrosion resistance and paint adhesion capabilities, it required the use of manganese, which was fairly expensive and—with the onset of World War II—difficult to obtain in the United States.

In the 1930s, employees of the American Chemical Paint Company developed the zinc phosphate process. This process was more energy efficient than manganese, and it became the primary process used on American war materials during World War II. In the latter half of the 20th century, improvements were made to iron and zinc phosphate chemistries primarily through the addition of various coating accelerators (organic and inorganic) as well as other additives such as cleaning packages (surfactants and solvents), but the base process and chemistry remained the same.

Iron vs. Zinc Phosphate

Although manganese phosphate is still used in limited cases, paint pretreatment chemistry today primarily consists of either iron or zinc phosphate. Both have the same goal: to produce a coating on the part that improves paint adhesion and corrosion resistance.

While the goal is the same for each of these chemistries, they produce very different coatings with various strengths and weaknesses. Understanding these relative strengths and weaknesses will allow manufacturers to determine which process is best for their operation. General knowledge will also provide a foundation to better understand how the Zirconization process can be used to replace either iron or zinc.

Iron phosphate produces a non-crystalline, amorphous coating on steel substrates. The color of the coating can range anywhere from gray, gold, blue, or purple (Fig. 1). It generally produces lower coating weights than zinc and is easy to control, flexible, robust, and economical.

Iron phosphate processes can be either single- or multiple-stage processes, with cleaning and phosphating occurring in the same stage. Application can be accomplished through spray, soak, wipe, steam, or wand processes. In general, iron phosphate coatings provide excellent physical adhesion and good corrosion resistance (Fig. 2).

Zinc phosphate, on the other hand, forms a crystalline coating that is usually gray in appearance (Fig. 3). The formation and size of these crystals can be manipulated based on various additives and operating parameters. When compared to iron, zinc-phosphate baths produce higher coating weights, are more expensive to run, and are harder to maintain.

Zinc-phosphate processes must involve multiple stages and require cleaning and conditioning steps prior to the zinc phosphate stage. Zinc phosphate is applied either by spray or soak. Zinc phosphate provides good physical adhesion and excellent corrosion resistance.

Phosphate Alternatives

Despite their track record of success over the last century, phosphates have come under increasing scrutiny due to their environmental impact on water sources. Simply stated, phosphates cause eutrophication, the increase of chemical nutrients in water that cause excessive plant growth. This plant growth and ultimate decay will lower the oxygen content of water and result in depleted fish populations due to lack of oxygen (Fig. 4).
Both local sewer districts (MSDs) and the Environmental Protection Agency (EPA) have increasingly forced industries to severely limit—or, in some cases, completely eliminate—phosphate discharges. These restrictions are only expected to further increase in the future. If you are lucky enough to not currently face phosphate discharge restrictions, chances are likely that you will be forced to deal with them within the next decade.

In response to the growing concerns over phosphates, chemical suppliers began introducing phosphate-free pretreatment processes to the marketplace. These chemistries included polymeric technology, alternative acids, solvents, primers, and metallic-based coatings. The most popular of these phosphate alternatives has been silane technology.

Silanes are molecules that have multi-functionality in that one end of the silane will have an affinity for paint, while the other end of the molecule will react with a metal surface. They work for paint pretreatment by “linking” the paint to the metal surface. Although successful in some specific cases, silanes have several disadvantages when compared to phosphates. First, the silane’s reaction with the paint occurs with only certain specific resins. That means manufacturers who use several different paint resins in the same paint line typically have had limited success with silane technology.

Silanes are also more difficult to control and monitor in tank solutions than iron phosphates. Additionally, silanes are often clear on the part, as opposed to the color produced by phosphate coatings, and it is nearly impossible to look at a part and know if the silane coating was properly applied. Finally, silane-treated parts are more susceptible to flash rusting than phosphated parts, and a silane bath has a significantly shorter bath life than phosphates.

Due to these limitations, silanes cannot be effectively used to completely replace iron or zinc phosphate in most pretreatment systems. As a result, chemical companies continued to investigate other possible phosphate replacements. The latest development, seen throughout the pretreatment industry, is the use of fluorozirconium-based chemistry to create a thin coating on parts.

The Path to Zirconization

The use of fluoro-based acids (such as fluorozirconic or fluorotitanic acid) have been used for metal treatment for decades. The most common use for fluoro-acids has been in aluminum treatment. However, making the jump from treating just aluminum to prepping parts usually coated with phosphates (which include cold- or hot-rolled steel, aluminum, galvanized and glavaneal steel, and other soft metals) was not easily done. Initial attempts at using fluoro-acids to replace iron phosphate resulted in unstable baths, rusty parts, and corrosion resistance less than that of iron phosphate.

Improvements continued to be made in fluorozirconic-based formulations, resulting in the introduction of European-developed nanotechnology into the American marketplace. Nanotechnology promised to exceed the corrosion resistance of iron phosphate, and in some cases could meet the corrosion resistance of zinc phosphate. However, initial nanotechnology formulations had limitations. First, the chemicals tended to be corrosive to mild steel washers, forcing users to modify their washers to run the product. Also, parts were very susceptible to flash rusting after treatment.

Work continued toward developing an effective phosphate replacement that would not cause rusting of steel parts. The result can be seen in DuBois Chemicals’ recently launched Zirconization process. Like other chemistries used as phosphate replacements, Zirconization is based on fluorozirconic acid. However, it also includes a unique blend of additives and accelerators that help promote a quicker and tighter coating on parts. This coating helps protect the parts from rusting while minimizing sludge creation.

Setting Up and Running a Zirconization Bath

In order to properly show how Zirconization can be used as a replacement for phosphate pretreatment, it is helpful to examine how a Zirconization bath works. A Zirconization bath actually runs very similar to an iron phosphate bath and can be used in existing washers that use iron phosphate, often with no physical changes to the washer itself. Typical use concentration in Zirconization is 1–5%, which is similar to iron phosphate, and some accounts have had success using concentrations as low as 0.5%.

Also similar to iron phosphate, the pH of a Zirconization bath is maintained between 4.0 and 5.0 (Fig. 7). Adjustments can be made from any common alkalinity source such as potassium, sodium, or ammonium hydroxide. If the pH is run too low, a coating will not accumulate on the part and rust will develop. If the pH is run too high, it can cause poor coating formation and result in bath instability (Fig. 8). A Zirconization bath will form a coating from any temperature from ambient to 115 ºF, and most users will run the system at ambient temperature. However, running at a slightly higher temperature will result in higher coating weights.

Zirconization can be used to replace a four-stage or more phosphate system and can be used in either spray or soak systems. The parts will need a cleaning stage prior to Zirconization, as a clean part is essential for the process to work. Also, the parts will need to be rinsed following Zirconization. Therefore, the typical washer requirement would be a four-stage system (clean, rinse, Zirconize, and rinse). In some instances, a low temperature surfactant cleaning additive can be used to allow the user to clean and coat in one step, thus processes with fewer than four stages are possible. A typical five-stage system would be: clean, rinse, Zirconize, rinse, and final rinse or seal.

One negative of Zirconization when compared to iron phosphate is that a Zirconization bath is more susceptible to contamination than a typical iron phosphate bath. To prevent contamination from the cleaning stage, a possible five-stage setup would be: clean, rinse, rinse, Zirconize, and rinse. It is possible to run the Zirconization bath in the fourth stage, which is typically used for rinsing in an iron phosphate system and, therefore, is not equipped with a burner—since Zirconizing is effective when run at ambient temperatures. Since Zirconization typically needs less contact time than iron phosphate, it can be run successfully in the rinse stages typical of most designs currently found in the marketplace. In cases where the contact time is severely reduced (less than 20 seconds) chemical concentration can be increased to compensate.

Zirconization vs. Iron Phosphate

As previously discussed, the recommended use parameters for Zirconization are very similar to iron phosphate. In most circumstances, Zirconization can be used as a drop-in replacement for iron phosphate. That begs the question: Even though they may be operated in a similar manner, how will the other properties of Zirconization compare to iron phosphate?

One difference between iron phosphate and Zirconization is the resulting color of the parts after treatment (Figs. 9 and 10). Iron phosphate coatings will, for the most part, be gold, blue, or purple. Zirconized parts will be gray with a light coating, or a light blue or gold with a heavier coating. In most cases, this color will not be a factor. However, it is possible that, when using a light, thin paint, that the color of the part may show through differently than before.

Although the Zirconization process is specially formulated to minimize flash rust, Zirconized parts are slightly more susceptible to flash rusting than iron phosphate. Due to the higher presence of fluorides (a strong acid), a high iron content in an old bath, and a thinner coating, flash rust can be caused by frequent line stops and slow line speeds. Despite this, it is possible to run a Zirconization process with no rusting at all if the bath is operated properly. Running a bath at the appropriate concentrations and pH, maximizing post-cleaner rinse water quality through simple counterblow and freshwater make-up techniques, running at low temperature in both the Zirconization and the cleaning stages, and minimizing line stoppages will all help to eliminate rust.

The most obvious strength of Zirconization over iron phosphate is that the Zirconization process is phosphate-free, eliminating environmental issues associated with phosphates and producing a “green” product. As environmental issues such as global warming gain more and more prevalence in television and print news, the demand for green products from consumers will likewise grow. In addition to being phosphate free, Zirconization is also an environmentally friendly process due to its lower operating temperatures, helping to reduce the energy consumption and the carbon footprint of the plant where it is used. Zirconization can further be classified as being green due to the fact that the chemical consumption will be less than with iron phosphate.

Zirconization will also greatly reduce the sludge generated in the bath. Sludge is a common problem in phosphate baths (Fig. 11) and results in additional maintenance costs to keep the bath running by unplugging clogged nozzles and cleaning tanks filled with sludge once a bath is dumped (Fig. 12). Sludge can also reduce part quality, as parts will have a powdery coating that will reduce paint adhesion and corrosion resistance.

The results of a scientific sludge study have proven how much less sludge is generated with a Zirconization process than with an iron phosphate process. This study was conducted by first preparing a 3% Zirconization bath, a 3% iron phosphate bath, and a 3% nanotechnology transition metal competitive product for comparison. All solutions were adjusted to the midpoint in their recommended pH range. The test was run at ambient temperature. Every day for thirty days, a 4" × 6" cold-rolled steel panel was placed into each solution. After 24 hours, the panel was removed and a new one placed into each solution. After 30 days, the test was concluded (Figs. 13, 14, and 15 show the resultant solutions).

Whereas nanotechnology certainly produces drastically less sludge than even the lowest sludging iron phosphate chemistry, it produces significantly more sludge than the improved chemistry now commercially available—DuBois Dura-TEC 100. The minimal sludge produced as a by-product of the Zirconization process will result in lower maintenance costs, less downtime, longer bath life, and more consistent quality.

Perhaps the most important reason to consider Zirconization is that, at a minimum, it will meet the same quality standards as iron phosphate. In many cases, Zirconization will result in an increase in paint adhesion and corrosion resistance. At a major custom coater, a Zirconization bath was maintained at 1–2%, a pH of 4–4.5, and at ambient temperature. The washer process was as follows:

Stage 1. Non-phosphate low-temperature cleaner (DuBois Met-ALL TERJ)
Stage 2. Freshwater rinse
Stage 3. Freshwater rinse
Stage 4. Patent-pending Zirconization process (DuBois DuraTEC 100)
Stage 5. DI water rinse

After running through the washer, the panels had a light blue appearance and were rust-free. Two different powder coatings were used on the Zirconized parts, and the same powder coatings were applied to Bonderite B-1000 (with DI rinse) panels for comparison. The panels were tested in salt spray (ASTM B117) and were considered to have failed when their rating was below 7. The Zirconized panels averaged 876 hours prior to failure (Fig. 16). The B-1000 panels averaged 624 hours.

Prior to using Zirconization, the facility had used a nanotechnology process that averaged 576 hours of salt spray prior to failure. The Zirconized panels were also subjected to cross-hatch adhesion testing and received 5B adhesion ratings.

Zirconization vs. Zinc Phosphate

The benefits of using Zirconization as a replacement to zinc phosphate are even greater than they are with iron phosphate. First, there is the extra green benefit of removing zinc from the waste stream. Second, Zirconization does not require a conditioning step, allowing for further chemical reduction and a decrease in water usage. Because zinc phosphate generally sludges more than iron phosphate, the reduced sludge generated from Zirconization will be even more pronounced.

Zinc phosphate is as susceptible to contamination as a Zirconization solution. So, a washer that runs zinc phosphate should have no problem making the switch to Zirconization. Additionally, with fewer titrations and chemicals required, a Zirconization bath will be easier to run than zinc. The key to deciding whether or not Zirconization can be used in place of zinc phosphate depends on the quality requirements. Zinc phosphate coatings are generally used when high salt spray results are required. In some cases, Zirconization can meet these same results. In other cases, Zirconization may fall short. Testing should be performed prior to switching to Zirconization to determine if your quality standards can be maintained when switching chemistry.

Summary

Zirconization creates a sustainable manufacturing process that is free of phosphates and minimizes water, chemical, and energy usage while maintaining and/or improving quality standards. Certainly, some people will contend there are some advantages to traditional phosphate chemistry. First, it is readily available and is likely what a company is using now, and there is a comfort zone in dealing with a proven process. Second, the number of people available in the marketplace that have worked with traditional phosphate chemistry is large compared to those who have any experience in the newer transitional metal chemistry. Finally, change is difficult because people inherently do not like it and there may be circumstances where it is difficult because of approvals or specifications. As an example, contracts may call for resubmission of parts if a new process is adopted that could affect the quality of the parts. If the change is for something that is too new, the customer may also have misgivings, which makes it complicated for the supplier to make an alteration.

However, the benefits will likely outweigh the costs of change. Hank Jeanneret, group vice president, Commercial Vehicle Products, Leggett & Platt, a pioneer in the use of transition metal–based chemistry, put it this way: “It is a much less expensive approach to the preparation of our parts for finishing at an increased throughput, with benefits in reduced energy and water consumption and the closure of source of liquid discharge and solid waste from the plant, thereby simplifying our environmental reporting and reducing compliance costs.” In one five-month study on a production line operating five and a half days a week at 20 hours a day, one user realized an annualized reduction in total process costs from $55,000 to $21,000 (Fig. 17).

Ted Schreyer, vice president of operations for a custom coater, has experienced the benefits of utilizing the Ziroconization process. “Given the process efficiencies, environmental and cost trade-offs, I cannot foresee any reason to ever switch back to iron phosphate,” he said. To be certain, the Zirconization process will not be appropriate for all manufacturers, but it is a process that you can ill afford to not consider.

About the Authors

Richard Moore, chemical engineer, and Bruce Dunham, director-innovation, are both from DuBois Chemicals, Sharonville, Ohio. They may be reached at (513) 326-8800. For more information, please visit www.duboischemicals.com.

Acknowledgments

Contributors to this article included: Hank Jeanneret, group vice president, Commercial Vehicle Products, Leggett & Platt; Ted Schreyer, vice president, operations, Associated Finishing, Inc.; and Ted Shader, president, Cincinnati Industrial Machinery.