What's Cooking In Chemical Pretreatment?

On the television show “Chopped,” accomplished chefs compete by preparing meals using a mystery basket of unorthodox and difficult-to-work-with ingredients. The chefs’ experience and ingenuity is needed to whip up recipes when they don’t have access to a full pantry of ordinary items. Today’s pretreatment chemists face the same kind of challenge. How do they develop effective pretreatment using only a basket of strange new ingredients? Like chefs, these chemists’ years of experience and ingenuity allow them to create the perfect recipe. So, pull up a chair and learn about three specific avenues of research in the chemical pretreatment kitchen.

David Chalk, a principal research scientist at Dubois Chemical, describes the situation as a “shrinking periodic table” that limits the chemical building blocks his team has to work with. Environmental regulations have restricted many elements that have worked well in the past, such as cobalt, nickel, copper, zinc, cadmium, mercury, lead, arsenic, antimony, chlorine, chromium, molybdenum, boron, strontium, barium, nitrogen and phosphorus, and manganese.

Proper pretreatment is an important step, (some think it is the most important step), to obtaining good coating performance. The pretreatment process typically requires cleaning the metal surface to remove contaminants (e.g., rust, smut, carbonization, and oils) and then modifying, or converting, the metal surface chemistry to improve adhesion of the powder coating.

This image shows a scanning electron micrograph (SEM) of a zinc phosphate conversion coating after five minutes of immersion.

Traditionally, conversion coatings have been used to obtain the high performance standards needed for more demanding powder coating applications. These pretreatments chemically convert the metal surface into an inorganic coating whose crystalline structure provides a surface full of nooks and crannies. The increased surface area facilitates adhesion of the powder coating by providing more sites for the powder coating to form mechanical bonds. The result is both improved corrosion protection and better coating adhesion.

For decades, zinc and iron phosphates have been the most popular conversion coatings for powder coaters. Iron phosphates have been used for applications that require tough, durable coatings but when corrosion protection requirements are not as demanding. These iron phosphate systems can have five- or six-stage processes, or a simple two-stage system if the performance requirements are more forgiving. By contrast, zinc phosphates are used when corrosion protection standards are more stringent. “There are still a lot of automotive body lines out there that use zinc,” says Gary Nelson, surface treatment product manager at Chemetall. “The zinc phosphate process differs from an iron phosphate in that additional metal ions are incorporated into the conversion coating along with the metal ions from the substrate itself. Iron phosphate does not provide the same level of corrosion resistance offered by zinc phosphates, but iron phosphate processes are easier to operate and maintain than zinc phosphate systems,” Nelson says.

But the problem is that phosphates cause eutrophication, or hypertrophication—an ecological reaction to the abundance of phosphates, which are nutrients to plant life. Phosphorous causes a rapid and uncontrolled growth of plant life that deprives the water of the oxygen needed for fish and other marine animal life. Excess phosphorus can cause a body of water to be completely overtaken by plant life, leaving marine animal life to die of hypoxia. Eutrophication has prompted regulations on the discharge of phosphates.

Aside from restrictions on phosphorous, zinc phosphates also contain zinc and nickel, two regulated heavy metals considered hazards in the workplace. Some forms of nickel have been classified as carcinogens. So, environmental, health and safety concerns have triggered a movement to replace phosphate conversion coatings with safer, greener, phosphate- free pretreatments.

“This trend toward safer materials began some time ago,” says Nelson. “When I started in the industry, in the mid-1980s most of the industry was using chrome final seals. The most common lab request was to replace chrome final seals. That movement completely changed the market.

Now, in the last 10 to 12 years, there has been a similar push to replace phosphates—not just because of the environmental and safety concerns, but for lots of other good reasons.”

Adding a Pinch of This or That to Zirconium

New phosphate-free conversion coating technologies that have significant benefits over iron and zinc phosphates are now well established. These coatings use simpler and shorter processes that operate at substantially lower temperatures. They perform well on a range of substrates, including aluminum, steel and zinc, and can be formulated to provide corrosion protection equal to—or better than—phosphates but without the dangers and adverse environmental impact. These phosphate replacements also offer powder coaters some attractive capital and operating cost saving opportunities.

The coatings are based on zirconium and do not contain manganese, molybdenum, nickel, zinc, or phosphates. “People have researched all kinds of metals. A few of them work, but most of them don’t work,” says Nelson. “Some of them are just not available or they’re expensive. Some of them are toxic or regulated, so they are ruled out. Soon, you are left with a small world. So you try to find one or two like zirconium that will work.” Zirconium is not considered a hazardous material in North America.

When applied to the metal substrate, these conversion coatings react to form a thin zirconium oxide film that is only 20-80 nm thick. That is a much thinner coating than iron phosphates, which are typically 250 nm thick, or zinc phosphate coatings, which can be as thick as 1,000 nm. “These systems are environmentally friendly because they have no zinc and either no phosphorous or very perforlow phosphorous. They offer energy savings because they operate at room temperature rather than higher temperatures. They require fewer process stages, so they cost less to build and take up less floor space,” says Nelson. “It costs less to charge the bath because a smaller tank can be used, and the system is easier to maintain because it has a wider operating window than zinc phosphate systems. These zirconium systems produce almost no sludge or scale and have simple waste treatment. They are very flexible processes because they work with most substrates and powder chemistries and can provide equal or better performance than conventional phosphates.”

“With all the advantages these new coatings offer, you’re not likely to find any new customers who want to put in a zinc phosphate line, says Chris Berger, vice president and director of product development for Calvary Industries, “So we’ve put our research dollars into zirconium and that family of products.”

Thus, a good deal of the action in pretreatment research and development has been the hubbub of activity around the development of proprietary chemical additives that improve zirconium’s performance and overcoming some of the inherent problems, such as flash rusting of parts. “Zirconium is probably the one material that ties everyone together— and from there the differences form,” says Nelson.

“Iron phosphates have been around for a hundred years, so the coatings are all fairly similar,” says Berger. “But with zirconium, the technology has only been around since the late 1990s and early 2000s so everyone has gone in a different direction. You can put a zirconium conversion coating on a piece of metal just by washing it with zirconic acid. It’s what you do differently (after) that to make it more user friendly, more robust, to prevent flash rust or get better performance that sets products apart. We’re on our fifth generation of products, and we’ve learned how they operate and what customers want so we can refine them.”

To improve the basic chemistry of zirconium conversion coatings, pretreatment suppliers have developed a number of exotic recipes that combine zirconium with packages containing other metals, e.g., titanium, aluminum, titanium and vanadium, and other sophisticated organic polymers.

A metal part travels through a power washer in a five-stage pretreatment process.

“This work has resulted in performance of these materials approaching the zinc phosphate type performance levels. So we are seeing a lot more action from people currently using zinc phosphate who want to investigate alternatives. Many of them are switching over to the zirconium pretreatments for the energy, maintenance and waste treatment savings,” says Sergio Mancini, sales director for Bulk Chemicals Inc. “It’s not only that our zirconium systems are not hazardous and environmentally friendly that it’s less expensive to operate, and the performance is much, much, better than iron phosphate. There is no sludge produced, it offers faster process time, low water and energy usage and it’s safe for all metals,” says Mancini.

Another area of recent development has been in developing systems that require fewer process stages. Mancini so points out that reducing the number of pretreatment stages can result in a 10 percent to 30 percent reduction in the overall plant footprint when converting from zinc phosphate, and that a savings in water usage also can be realized because of the reduced number of process steps. Most of the conventional zirconium systems eliminate the seal found in phosphate systems, but routinely include two rinse stages prior to the zirconium stage. “Now we’ve developed the cleaner-coater zirconium product so you don’t need a 5-stage system. Coaters can use two-stage and threestage systems and can be used with hand-wand systems,” says Mancini.

But much of the recent work in zirconium coatings has focused on achieving the corrosion specifications once only possible with zinc phosphate systems that were complex and difficult to maintain. Steady progress appears to have paid off. “A couple of years ago, some of our competitors were saying that their zirconium pretreatment was equal to zinc phosphate because it was passing the same specification. For example, many of our customers would say that if you can get a rating of 6 or higher on an ASTM D1654 test that would be considered passing. But just because you can pass the test doesn’t mean the performance is equal. Zinc phosphates were passing with a score of 8. So even though both passed, there’s a significant difference between a 6 or an 8,” says Berger. “But that was a couple of years ago and now we do get equal performance in real terms. We are on our fifth generation of zirconium products and they keep getting better. They will continue to get better as we discover new additives to make these products better. And everyone has learned a lot about how to optimize the product and get the most out of it.”

“Our highest performance products can match zinc phosphate with the right paint system or an e-coat system,” says Nelson. “We have successfully replaced zinc phosphate on major powder production lines for outdoor products, such as fencing and agricultural products. I would say the performance is equal in those cases,” says Berger.

Would You Care for Some DIP?

Nelson points to the important distinction between conversion vs. deposition coatings. Traditional phosphates like zinc phosphate react with the metal surface. Iron is acid pickled off the surface, reacts with zinc phosphate, and is redeposited on the surface. So when the coating is rinsed in the final stages, metals are present in the effluent.

“We have products that can be used in a traditional rinse system, or as a dry in place (DIP) coating that does not get rinsed,” says Berger. “If you are powder coating you can just dry it in place. There’s no rinse effluent, and the product doesn’t dissolve any of the surface metal. The organic components we add to our product make it almost like washing the part with a zirconium-containing surfactant. When you apply the paint, you get a chemical paint-to-metal bond. This bond goes beyond the simple kind of mechanical adhesion you get with iron phosphate,” says Berger. “When we began looking at how to replace phosphates, we looked at a number of additives like the silanes and surface passivators that paint formulators incorporated into coatings and asked how we can incorporate these into our pretreatment so we can get the paint to stick better.

“We have replaced iron phosphate systems with dry in place technology and it works very well,” says Mancini. “Some companies that have taken out their phosphate use a process where they use an alkaline cleaner, followed by a couple of rinse stages and then apply the dry in place pretreatment. They can then declassify themselves as a metal finisher since there is no reaction with the metal. They can say, ‘I am not conversion coating, I just clean metal,’ and in places with a lot of regulatory supervision (such as) California, declassifying themselves as metal finishers is huge because they no longer have to file reports, and they safely put these chemicals down the drain after they are pH-adjusted .”

“There are some definite benefits to dry in place,” says Nelson, “but you have to keep the chemicals very clean and free of contamination. Otherwise, it might work really well for the first couple of days, but start to degrade after a couple of weeks if things get dragged into the coating or hard-water salts begin to form. That’s a risk with dry in place that needs to be addressed with good design and housekeeping.”

Berger also notes that zirconium treatments are not as tolerant to alkaline drag-in from the prior cleaning stages. “So you want to be careful about the pH. A little too much alkalinity will throw off the chemistry more than a traditional phosphate system. With higher performance specifications, the cleaner you keep the zirconium, the better it will perform. We want the system to run as well on Day 300 as it did on Day 5, so we recommend putting in a second rinse.”

“Dry in place can be fine for some applications,” says Nelson, “but there’s also an advantage to have a final rinse if your parts are prone to puddling,” he cautions. “For example, on a large sheet of metal you can have a drip edge where a high concentration of a dry in place chemical can build up and become a potential source of failure.”

Organic Acid Approach

Performance of conversion coatings continues to improve with each new generation. Here, corrosion from neutral salt spray improves dramatically.

David Chalk and his colleagues at Dubois Chemical are investigating a completely different approach to replacing iron and zinc phosphates using strong organic acids. One reason is that Chalk observes that some regulators are starting to take a harder look at another wastewater product—fluorides. “There are really three emerging camps when it comes to pretreatment,” observes Chalk. “One group can still use traditional phosphate coatings. If you can do that, sure, go ahead and use a phosphate coating and be satisfied. Then there’s another group that has a phosphate restriction but can use zirconium. There are lots of ways to use zirconium, and we certainly have a lot of great products in that area. But there’s a third group that is under a restriction both for phosphorous and for fluorides and they cannot use the zirconium approach.” This is because some of the chemicals commonly used in zirconium conversion coatings have fluorides.

“The problem with fluoride is that people in health and safety departments may see fluoride on a safety data sheet (SDS) and think that it’s hydrofluoric (HF) acid, which is nasty stuff,” says Nelson. “But the truth is that there are very low levels of HF. They are more of a reaction byproduct from mixing the zirconium material with water, and the concentration is very low. It’s actually kind of ironic that there is concern over fluorides when you consider that many cities put fluoride into their drinking water because they think there are health benefits.”

But aside from the potential outlawing of fluorides, Chalk sees other compelling reasons to develop the organic acid approach, including spiraling costs and the increased use of variable quality imported steel, which can exhibit inconsistent surface condition. “The oils used on steel shipped from overseas are not always what we expect, and the conventional use of alkaline cleaners is not as effective as acid cleaning on these unusual soils,” says Chalk.

Berger agrees that steel quality can be a problem. “We see a lot of buyers who buy the cheapest hot rolled steel they can get that meets the spec. It is pickled and oiled but you don’t know what kind of oil you’re going to get. It could come from China, or it could come from Chicago—you just don’t know. So you need a robust process.”

“A primary task in the modern specialty chemical supplier’s laboratory is to creatively develop and deploy technology to overcome regulatory hurdles, high process costs and increasingly poor substrate quality,” notes Chalk.

Dubois’ process, dubbed acid driven organic pretreatment—or AOP— uses a combination of an organic film former and passivator along with an acid package containing mineral and organic acids and was patented in late 2014. According to Chalk, the purely organic AOP system outperforms zirconium, zirconium-silane systems, and competitive dry in place pretreatments by 20 percent in salt spray testing, exceeds all iron phosphate systems by a large margin, and comes close to the performance of zinc phosphate systems. Dubois has targeted applications for heavy steels like pickled and oiled or blasted hot rolled steel because they are particularly good candidates for the AOP process. An important key to the process is the unique passivator that controls the tendency to rust in process, making strong acid cleaning much more user- friendly.

The process is operating commercially in a number of plants, says Chalk, including a large Midwest agricultural equipment manufacturer who adopted the process because they use laser cutting on a large portion of their material. “AOP is an ideal solution for parts like square tubing made of hot rolled steel, which has smut, carbon rich or carbide materials and where a good strong acid cleaner is effective at removing these soils,” says Chalk.

Chalk says, “If tighter regulation comes to pass, you can put in a waste water treatment system to handle your own waste and be compliant. But if you can’t do that, you could find yourself being unable to use phosphorous and now you can’t use fluoride.”

“Fluoro acids are particularly nasty—especially hydrofluoric acid, for example—but many of the products that are used in these processes like fluorozirconic acids have really minimal free fluorides,” says Mancini. But we don’t want fluorides in the process, so we minimize free fluorides anyway.”

“Organic acids can be effective, but you need to be cautious. They can decrease the robustness of the process since you need to be careful about flash rust in the rinse afterwards, and dragging acids down the line. So it makes the process a little more difficult to control,” says Nelson.

Thorough rinsing of these extrusions is critical for maintaining the integrity of the conversion coating in the next stage.

“Right now organic acids are also somewhat more expensive than zirconium, so performance will have to take off to the point where it outperforms zirconium, or zirconium will have to be regulated, or the cost of the organic acids will need to come down,” says Berger. “Besides, I don’t see zirconium becoming a regulatory issue. Because even if they say you can’t put zirconium down the drain anymore, it’s super easy to treat it since it precipitates out very easily.”

Taken together, three avenues of research, 1) novel additives that enhance the performance of zirconium pretreatments, 2) the advent of dry in place coatings that enable powder coaters to declassify themselves and avoid costly wastewater treatment and red tape, and 3) organic acids that provide a metal-free alternative, provide powder coaters with a number of new pretreatment alternatives. Which one is best? That’s a bit like asking the waiter which meal to order. The answer depends on composition and quality of your substrate, what soils are present, the chemistry of the powder coating, and your performance requirements. Fortunately, there are plenty of good specialty chemical suppliers willing serve a winning recipe.

Paul Mills is a marketing and business development consultant to industry chemistry and equipment suppliers. He has been a writer for the powder coating industry since 1994. Paul can be reached at 440-570-5228 or via email at pmillsoh@aol.com.