By David Chalk, PhD
Photo credit: Keystone Koating, LLC
A number of strategies for cleaning and pretreatment of various metals in preparation for painting are available to powder coaters. In addition to the legacy phosphate and chromate conversion coatings, more contemporary zirconium and silane approaches are viable options.
With white metals such as aluminum, zinc, titanium, and magnesium becoming increasingly prevalent in job shops and manufacturing plants, strategies and considerations for pretreating this wide array of metals will be discussed, with special emphasis on how to handle the more popular combinations. Three specific scenarios will be presented and addressed as well as hints and tips useful for job shops and custom coaters.
Three Typical Mixed-Metal Scenarios
Scenario 1 - A powder coating custom job shop: “We have a real situation here. Our biggest customer sent us these fabricated parts for powder coating. They’re steel and aluminum. Some pieces are fastened by rivets and some look like they’re lock seamed together. No matter what we do, we can’t get the powder to stick where these two metals are joined together.”
Scenario 2 - A manufacturer of commercial toolboxes and metal enclosures: “Our buyer just got a great deal on some stuff called galvanneal. We bought a whole load of it from a warehouse for less than what we pay for our regular class one cold rolled steel. When I run it through our washer (set up for cold rolled steel only) the surface turns black and the paint won’t stick!”
Scenario 3 – A contract coating shop: “We’ve been coating aluminum castings for years, but now we’re receiving parts made of cast magnesium and titanium, too. The paint’s coming off those in our crosshatch test. What can we do?”
All three scenarios have one characteristic in common: multiple non-ferrous metals that require pretreatment and coating. With the growing popularity of aluminum, zinc, magnesium, titanium, and their alloys now being presented to our coaters, the challenge to effectively clean and pretreat them increases.
In this article the strategies for handling this wide array of metals will be discussed, with special emphasis on how to deal with the more popular combinations. The metals themselves will be categorized according to the best approaches for cleaning and pretreatment, along with strategies for their preparation for coating. Finally, the solutions for the three scenarios presented above will be shared.
Cleaning and Pretreatment
The mechanisms by which paint and powder coatings adhere to metallic substrates are physical and chemical in nature. The physical adhesion of the coating is enhanced by increasing the surface area available on the metallic substrate for bonding. Optimally, the coating will also chemically bond to the metal. While there are various pretreatments available to the modern custom coating shop, from legacy phosphates to modern silane polymers and zirconium nanocoatings, the importance of thorough cleaning is common across all of them.
The Importance of Cleaning
Surface-modifying chemical reactions and subsequent adhesion of organic coatings require an uncontaminated, clean as possible surface for best results. Nothing will happen if the work is not clean. As we progress from legacy phosphate conversion coatings to the more contemporary silane polymers and zirconium pretreatments and sealers, the surface cleanliness requirements are even greater. These last are forming polymer networks or nanoassemblies at the work surface. Unremoved soils will inhibit or prevent these chemical reactions before they can start.
Hints and Tips for the Modern Coater
The selection of an appropriate cleaning and pretreatment process is governed not only by the metal(s) being cleaned, but also by the soils, corrosion resistance requirements, coating performance requirements, equipment available, and environmental/disposal considerations. Undesirable reactions by the cleaner on the metal, such as hydrolysis (reactions with metal that increase the caustic at the surface) and smutting (formation of a carbide salt or increase of grain carbon at the surface) must be avoided. Both hinder pretreatment and coating adhesion.
When offered an opportunity to bid on a given job, here are some questions to ask the prospect that will be very helpful when consulting with your pretreatment supplier. Gathering this information first will make your bidding process more efficient.
What metals comprise the parts? Steel only? Aluminum only? Mixes of metals?
Remember that there may be inserts, fittings, and components of the parts that are not made of the same metal as the rest of the part. A thorough understanding of the exact mix of metals is required for both cleaner and pretreatment selection.
In general, the more metals that are included in the mix, the greater the challenge for effective cleaning and pretreatment. While the challenge is certainly not insurmountable, greater flexibility with more washer stages and/or availability of on-site waste treatment, for example, can facilitate the selection and implementation of the best technologies.
How are the parts made? Detail all the physical processes the parts undergo before they are presented for coating.
Are parts stamped, drawn, bent, punched, roll-formed, cut, laser cut, heat treated, or annealed? Lock seamed, welded, tapped, threaded, or machined? Are there gaskets or inserts that we need to know about? Is there weld burn, oxidation, or scale? Laser scale? Burnishing or roll marks? How the parts are fabricated and assembled gives direct insight as to the soils that may be on the parts.
Work that has cut-edge corrosion, also known as laser scale, will require either physical abrasion of the surface or application of a strong acid descaler to address the oxidized surfaces. Work with heavy amounts of forming lubricants, rust preventatives or machining lube residues will need both temperature and alkalinity (caustic, if possible) for effective cleaning.
What other soils can typically be expected on the parts?
Will there be metal fines or high levels of carbon on the surface of the work? Is the metal known to be a high carbon grade? If it’s steel, what is the grade and the alloy composition? A copy of the metallurgical report, preferably from the steel mill, would be very helpful.
In general, high carbon grades should be cleaned at near neutral pH, if possible, and pretreated with one of the modern technologies, preferably silane polymers or zirconium. Higher caustic or acidity can concentrate the carbon at the surface and interfere with coating adhesion. Blasting this type of work can be helpful; the metallurgical grain carbon is greater near the surface of the work.
Have the parts, or the metal(s) that comprise them, been subjected to a chromate passivation process?
If so, then organic coatings will not adhere. If the parts are made of zinc or galvanized steel, and they are presented dry (i.e., not oiled) and not white rusted, there is a good chance they have been chromate passivated. A quick test to check for chromate passivation is to expose the work in question to a condensing humidity environment. A test panel covering a beaker of freshly boiled water for about a half hour, then allowed to dry in air, should white rust if it is not chromate passivated.
What are the corrosion resistance requirements of the finished work?
Is there a salt spray or cyclic corrosion test specification? Some other industry standard specification, like automotive, heavy equipment, architectural? Is there expectation of superior corrosion performance, for example, as required in an outdoor marine environment?
There has been much energy expended in increasing accelerated corrosion performance, and that’s a good subject for investigation in another venue. The knee-jerk response of zinc phosphate and chromate seal as a pretreatment regimen is available to very few coaters, and likely not those in the readership of this article. The environmental and regulatory burdens are daunting. Nevertheless, a contemporary approach to pretreatment, featuring a zirconium and/or silane approach, perhaps as a pretreatment and sealer, or as a dry-in-place, can give excellent corrosion performance. Carefully matching the silane to the expected paint/coating can yield impressive results.
Are there coating appearance and durability expectations?
There may be a formability specification, especially for post-formed coated parts. Impact resistance, gloss, and other coating appearance factors are influenced by pretreatment selection. Zirconate coatings do not perform as well as iron phosphate or silanes regarding coating flexibility, particularly considering impact resistance. However, the nanotechnologies and silanes do particularly well for applications with special distinctness of image or reflectance requirements.
Pretreatment Strategies for Different Metals
When determining an appropriate pretreatment regimen for a mixture of metals, it is important to remember these axioms:
Axiom 1: Any metal that is insoluble in dilute phosphoric acid cannot be phosphate conversion coated.
Axiom 2: Non-ferrous metals (i.e., those that do not contain iron) cannot be iron phosphated even if they are soluble in dilute phosphoric acid.
Axiom 3: Any metal that can be acid etched can likely also take a zirconium coating.
Axiom 4: Any metal that can be deoxidized can likely also accept a silane type conversion coating.
How To Use The Tables
Identify the metal(s) of interest in Table 1 and determine common applicable pretreatments. For example, if steel, zinc die cast and aluminum extrusions must all be pretreated in the same system, then potential choices are 1) zinc phosphatizing, 2) etching, 3) silane-treating, 4) zirconium-treating, and 5) chromating.
It is interesting to note that some of the more commonly seen combinations of metals, like steel and aluminum, or steel and zinc, can be prepared with two very different surface modification reactions simultaneously. It is possible to deposit an iron phosphate conversion coating on steel, for example, while etching aluminum and/or zinc in the same chemical process. As a practical matter, this etching/iron phosphatizing choice is almost exclusively used when pretreating mixes of steel and aluminum or steel and zinc (galvanized).
What if the metals are among some of the more exotic ones found in Table 2? While these metals are resistant to chemical pretreatment, those that are not passivated can be prepared to hold a coat of paint just fine. However, if it is necessary to enhance adhesion, then physical, rather than chemical means must be used. Such steps as blasting with grit, shot, or beads, or otherwise physically abrading the surface to increase surface area are usually required to ensure good paint bonding to these metals.
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Here are the solutions for the problems presented in the three scenarios that opened this article:
1) Dissimilar metals in direct physical contact. When dissimilar metals are in direct electrical contact, and conditions for electrons to flow are appropriate, one of the metals will inevitably oxidize, or corrode, sacrificially to the other. In this case, the steel in direct contact with the aluminum is protected by the sacrificial oxidation of the aluminum. Since that iron is needed for iron phosphate, no iron phosphate coating can form in the area close to the aluminum. Hence, there is poor coating adhesion in this area.
The solution to this problem is to coat the metals before assembly, so that the phosphatizing of the steel is uninhibited.
2) Galvannealed steel in a steel-only washer. Here is a clear-cut mismatch between chemical processes and substrate. The strong caustic cleaner designed for a steel-only application was attacking the galvannealed surface, turning it black. Moreover, the iron phosphatizing solution contained no fluoride to inhibit further attack on the substrate.
The solution to this problem involves use of a silicate-bearing caustic cleaner and a fluoride-bearing iron phosphatizing solution to protect the white metal zinc from undesirable reactions. This scenario further illustrates the importance of not changing substrates in your washer without first consulting with your pretreatment chemical professional.
3) Magnesium and titanium in the aluminum shop. These two metals are starting to show up in certain applications where increased physical strength and lighter weight are important. Examples would be laptop housings and aerospace applications. While magnesium can be etched with a sufficiently strong acid, titanium is fairly inert to any chemical approach.
The ultimate solution to this problem involves a modification of the chemical process (more acidity and use of a fluoride additive) to handle the magnesium parts, and a light bead blasting to “open the grain” of the titanium parts so the paint will have greater surface area with which to adhere.
As always, when challenged with a situation in which a multiple metal problem might be implicated, it’s best to consult with your chemical pretreatment professional. With the correct application of chemical and physical processes, and with the flexibility inherent to the contemporary silane and zirconium pretreatments, almost any combination of metals can be run simultaneously with satisfactory results.
David B. Chalk, Ph.D. (retired) was principal research scientist with DuBois Chemicals.