The trend toward lightweight materials in transportation means more aluminum, magnesium, and titanium are being used. With more aluminum involved, there’s more coupling of aluminum and steel parts, so eliminating galvanic corrosion is becoming a large part of design, Patrick Scalera, Henkel’s technology manager for general industrial surface treatments, told us.
For example, there’s his company’s Alodine EC2, a new base coating for protecting parts made of aluminum, titanium, and related alloys in severe environments. The coating process is electrolytic, producing a tough, flexible titanium oxide coating with high resistance to abrasion, corrosion, extreme temperatures, and chemicals, Scalera said. Its primary applications are in the marine and industrial sectors. The coating is also used in automotive applications, such as piston and intake manifold coatings.
Polysiloxanes combine the environmental toughness of silicone polymers with the functional attributes of different organic compounds, providing solventless high-performance protective coatings.
Shawn Dolan, Henkel’s senior research scientist, told us that, in addition to protecting inboard and outboard marine engines, the coating can be used for cast aluminum lights on large bridge structures spanning salt water. “The corrosion on these bridges is very bad. In that environment, we’ve actually tested our coating for as long as 8,700 hours, which is a full year.”
Automotive internal combustion engines are shifting to more turbocharged types, such as those that use exhaust gas recycling, Dolan said. “That means a lot of hot, corrosive gases recirculating -- a very aggressive environment. Temperatures on the crown of the piston can reach over 537C. Our coating protects the pistons and the whole engine at those temperatures.” The EC2 coating is also used in military engines, and NASA is testing it for rocket engines.
The coating itself is stable to 900C, but the metal it coats melts at 600C to 660C, depending on the alloy.
Matt Hagemeyer, application engineering and technical service specialist for industrial assembly and maintenance at Dow Corning, said one trend it sees is demand for anti-friction coatings that last longer under various conditions.
As automotive manufacturers try to achieve federal mileage goals, there will be a sizable increase in the use of anti-friction coatings, such as Dow Corning’s Molykote. These coatings are lubricating paints, also called dry-film bonded lubricants, which may be used to coat pistons and other rotating parts of an engine. They are replacing grease and other wet lubricants, because they are easier to handle and less messy, don’t attract dust, and look better.
“Sometimes they replace coatings designed to resist corrosion,” Hagemeyer said. “Dry-film anti-friction coatings can be used everywhere in assembly, from fasteners to clips and springs to threaded connections.”
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?
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, 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.
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.
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?
Thanks for the feedback, guys. Chuck, I specifically looked at coatings that are targeted for harsh environments. And I didn't look for raw number volumes, just specific apps where these are used. Within those parameters, I got the impression that anti-corrosion is a big deal, maybe the biggest deal, for various reasons.
Your impression is VERY accurate. Stainless steel ISN'T. It's corrosion-resistant. Coatings can do better, but when put to use they must be monitored for wear. As soon as a penetration occurs, be it through wear or external damage, the part must be replaced or recoated.
Ann, will you be covering FDA approval of coatings in any of your reports?
Thanks, TJ. You're right, stainless is well-known for being corrosion resistant. One thing I would have liked to explore was exactly what you point out--the fact that penetration and wear do occur. This was addressed to some extent by my sources, but monitoring processes and techniques are really a separate subject.
TJ, I don't have any present plans to cover EPA approval or regulations of coatings in a major feature, but might report on this in a blog or news story if something newsworthy happens. What do you have in mind?
TJ, the coatings I reported on are designed for use in harsh environments, such as aerospace and transportation, as well as bridges and other large structures exposed to weather, and some industrial environments. To my knowledge, none of these objects or their coatings are involved in, or designed for, direct food contact, and so AFAIK are not regulated by the FDA. However, I did not ask that question since it was not the focus of the article. Did you have something specific in mind?
I have nothing specific in mind, Ann. I've dealt with food processing environments quite a bit, and am amazed at what the food alone can do, let alone the caustics used to clean machinery.
Thanks for the reply, TJ. I don't tend to think of foods as a harsh substance, more of a messy one, but you've certainly got a point. At least I can easily think of ketchup, mustard, and pickles, for example, which are all highly acidic.
Ann, don't forget about all the concentrates from juices to sauces to soda that might be fine when you consume them but are in a totally different state in the plant. There are also the unpronouncable ingredients that are in a small measure in the final product but great quantities there. Also, the main ingredient in just about everything seems to be salt. If it can take out an unprotected car over a winter, just imagine what it can do in quantities inside a moist food plant.
Thanks, Jack. I'm not familiar with food processing--and each time I learn something about it, I must say the "Ecchh" factor tends to apply. In fact, I basically don't eat processed foods because of all the additives and excessive amounts of salt. Anyway, your description certainly fills out the picture of how harsh an environment processed food ingredients create in a plant.
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.
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.
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.
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