Coatings in harsh environments already have a tough job protecting metal and plastic components in cars, ships, planes, and industrial machinery. But a combination of globalized markets, more extreme operating requirements, and the desire for a cleaner environment are demanding more from the coatings.
Some manufacturers are combining the product’s basic function, whether it’s resistance to salt water, chemicals, or extreme temperatures, with additional features such as increased durability, reduced friction, lower cost, or less wasteful application. Some classes of coatings are replacing a competing class in certain applications. In other application areas, new coating types are gaining ground.
Field-applied coatings are used as topcoats on metal structures with tanks or piping exposed to outdoor conditions, such as bridges, oil tankers, or chemical plants. Traditionally, a zinc-rich primer coat was used to protect metal from corrosion, Jerry Witucki, senior coatings specialist for Dow Corning’s coatings and composites group, told us. The zinc powder protects the steel via sacrificial oxidation. An epoxy coating is applied to hold down the zinc and provide corrosion protection, followed by a polyurethane topcoat.
Henkel’s electrodeposited Alodine EC2 titanium oxide coating can be used on a variety of aluminum castings, extrusions, and wrought alloy parts for superior corrosion resistance in harsh environments.
“It’s possible to cut down the number of coatings and get the same level of protection for less labor cost and downtime,” Witucki said. “Siloxane-based coatings have superior weather and chemical resistance. The same zinc-rich primer is covered by a silicone acrylic or silicone epoxy topcoat. These are being used on offshore oil platforms, wind farms, radio towers, transmission lines, and anywhere steel is outside.”
Because of industry consolidation, field-applied coating systems have become commoditized, he said. Now coatings suppliers are differentiating their products by incorporating a new benefit or performance enhancement, such as durability, easy cleaning, or chemical resistance. “Historically, the silicones were relegated to niche applications due to cost, but as the price of organic resins increases with the price of oil, siloxanes have become a more attractive option.”
One of the fastest-growing coating technologies in industrial maintenance is the silicone hybrid coating, which combines the best of organics and silicones. In particular, polysiloxane hybrids combine silicone with epoxy, acrylic, or urethane. “Polysiloxanes have inherently low viscosity, so there’s low solvent requirements. They have good thermal and UV stability, excellent chemical resistance, and excellent compatibility with organics.” Their share of the coatings market is still small, but engineers increasingly prefer hybrids, so rapid growth in polysiloxane use is expected.
This is a great roundup, Ann. As from the considerable utility of the stuff you're writing about, your coverage is making this stuff interesting besides! Quite an achivement.
Good job detailing all the reasons for use of coatings, Ann. In the coatings market, is there one application for coatings that stands out above the others, in terms of raw numbers? Would it be anti-corrosion?
Dave, thanks for weighing in on coatings, especially the health issues in chromate conversion coatings. You've got valuable information since it comes from an independent testing lab instead of from the suppliers. Although the suppliers of course want to tout their own products, I was pleased to find that most of them could be objective about many technical and industry issues.
Ann, thanks for a whirlwind tour through the world of coatings. You covered a tremendous amount of ground in this article.
We have been doing a lot of work on finding suitable alternatives to chromate conversion coatings for marine applications. Chromate conversion coatings do a great job of inhibiting corrosion on aluminum. Rather than sealing the aluminum off from the environment, the presence of the chromate ion actually alters the chemistry of the corrosion reaction, slowing it down significantly. Chromate conversion coatings are self-healing, and provide a great base for paint. Unfortunately, hexavalent chromium is also a potent carcinogen. Because of this, there has been an industry-wide effort to phase it out.
We have had some good success using the Alodine EC² coating described in the article. This coating has many excellent properties. However, there are also a number of challenges which come from the fact that it is an electrodeposited coating. Among other things, this makes it difficult to coat the inside of tight passages. So, while EC² has many advantages, it is not necessarily a "silver bullet" for every application.
We are continuing to look at some of Henkel's other non-chromate coatings. Among these is Alodine 5200. Like EC², it is a titanium oxide coating, but unlike EC², it is not electrodeposited. Unfortunately, our initial findings were that the Alodine 5200 did not have as much corrosion resistance as a chromate coating.
However, this testing was done using an epoxy primer. Since doing this testing, I've seen some data which suggests that Alodine 5200 may perform better than chromate when an acetoacetate primer is used, rather than an epoxy. This points to the fact that it's important to look at the entire coating system (substrate, coating, primer, and paint), rather than just one component in isolation.
Of course, besides the many chrome-free products which Henkel makes, there are also a number of chrome-free products on the market from other manufacturers, including Metalast, Macdermid, and many others.
Coatings and corrosion are complex topics, and I continue to try to educate myself about them. Just about anyone who is actually an expert on coatings works for a coating supplier, so it can be hard to find an unbiased opinion. It's important to build up enough general knowledge to be able to evaluate the suppliers' claims about their products. And, of course, there's no substitute for doing your own testing.
Al, I think you bring up an important point about the multiple disciplines an engineer must master, especially relating to materials: coatings and paints, fasteners vs or plus adhesives, metal and/or plastic/composite materials for the box or body of the object and if mixing those body materials, what should go where? On a larger scale, much of this complexity and these interacting decisions reminds me of PCB design issues.
Beth, for many (if not most) of these harsh environment apps, coatings are not an afterthought or seen as an addition, but are considered an integral part of the product. As such, the time and cost involved of particular coatings are weighed just like the time and cost of other aspects of manufacturing the product. The question isn't usually to coat nor not to coat, but which type of coating to use and how many of them.
Ann, Exellent article. One of the interesting things with advancing technology in almost every discipline is making the connection between innovations and applications. It just makes sense that multifunction coatings will increase performance and reduce costs, as long as engineers are educated on what's possible. That's a significant problem for personnel asked to wear many hats. Thanks.
As you very adeptly lay out in this piece, Ann, coatings are playing a much bigger role in product designs of all sorts for a variety of reasons--for durability, for environmental reasons, for performance. That said, I imagine they add significant cost to a design, not to mention, require additional time on the part of the engineer to properly research and figure out what type of coating best addresses their particular design challenge. Any thoughts on how engineers can mitigate these additional costs with better design choices or processes?
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