By Cynthia A. Gosselin, The ChemQuest Group

Journalist Colin Mason once wrote, “A pristine superyacht in the marina, with shining paint work, sparkling stainless steel elements, gleaming brightwork and flawless teak decks is not the result of luck. Paintwork in particular involves a complex process that is tough to get right and requires a perfectly well-ordered and clean environment.”1

His discussion centered upon keeping the yacht workplace consistently tidy, clean, and free of dust, dirt, and residue to eliminate expensive repainting and other corrections during the building of these expensive boats.

There are two primary “pillars” of the surface preparation process; adhering to these procedures will help ensure the best result in the painting and coating process.

The First Pillar: Cleanliness

The first essential element for the application of a defect-free durable coating is that of a clean substrate. A large percentage of in-service coating failures are the result of the inadequate removal of surface contaminants such as mill oils, greases, fingerprints, oxides, mill scale, metallic fines, weldment residue, molding release compounds, and other soils. These “dirty” surfaces inhibit adequate pretreatment and ultimately lead to ugly paint delamination.

Even do-it-yourself (DIY) painters are cautioned about the need for a clean painting surface, be it a wall, floor, or metal yard furniture. Instructions on every paint can stress that before painting, the surface must be free of dirt, oil, mold, mildew, and any other pertinent contaminant that has the propensity to lead to a poor aesthetic, or even worse, paint voids, or delamination.

In the industrial world, the ideal cleaning agent is one that removes contaminants from the metal surface. The process must be robust enough to inhibit any detrimental reaction products from forming and at the same time prevent redeposition of soils onto the surface.

Typical chemical cleaning methods range from alkaline cleaning, solvent cleaning, vapor degreasing, ultrasonic cleaning, and pickling. Mechanical cleaning can involve brushing, grit blasting, wire brushing, chipping, and sanding. In addition to removing soils, cleaners can be tailored to activate the substrate surface to uniformly accept either a pretreatment, adhesive, or paint film.

It is important to pair the proper cleaner with the proper type of contaminant. Generally, soils are either organic or inorganic. Mill oils, rolling solutions, lubricants, and rust-preventative oils are typical examples of organic soils. Alkaline cleaner chemistries are most effective in removing organic soils. These cleaners can lift soils off the substrate surface and disperse them into the bath. The soils are then removed by overflowing the bath, mechanically skimming residue from the bath surface or by filtration.

Conversely, mill scale, heavy oxides, and metallic fines are considered inorganic soils. Acid cleaners are used to remove them. Pickling is a typical acid cleaning process.

Most cleaning sections are followed by water rinses to remove any loosened residue or remaining cleaning solution. In the laboratory, panels are said to be “water-break free” when the surface is adequately cleaned. This is a quick visual approach to monitoring cleanliness of the surface. More sophisticated approaches involve measuring the contract angle and even stating the optimum measurement on a technical data sheet.2

A contact angle is a quantitative way to measure the degree of wetting of a solid by a liquid. Surfaces can be identified as perfectly wetting with a strong solid-liquid interaction strength (θ = 0°) to a completely non-wetting interaction (θ = 180°). Typically, a contact angle between 0° and 90° will exhibit good wettability (Figure 1).3 The better the wetting, the more uniform the pretreatment, adhesive, or paint deposition onto a surface.

The Second Pillar: Pretreatment

Once the metal surface is adequately cleaned, the second essential element governing durability and corrosion resistance of a painted substrate is the deposition of a pretreatment onto the clean, activated surface. This technology has evolved significantly since the first phosphate bath was commercialized in 1906. In fact, today, a microcrystalline phosphate pretreatment can even serve as a functional interim coating in conjunction with mill oil to facilitate formability. This is a common approach for forming automotive panels.

In other cases, pretreatment can be a permanent thin film organic that imparts fingerprint resistance or a temporary coating that enhances uniform formability while at the same time keeping the substrate surface pristine as the film “comes off in the (alkaline) wash,” exposing a perfectly active, undamaged surface ready for pretreating. There are even some thin film organic coatings that act as pretreatment/primers that are viable adhesion promoters for topcoats as long as two years after application.

However, the traditional construct of a pretreatment is to convert substrate surface oxide molecules in such a way as to promote adhesion, enhance long-term durability, and increase corrosion resistance of a painted or adhesive-bonded product.

For steel products, the most widely used pretreatments are zinc or iron phosphate. Iron phosphate pretreatment systems are alkali metal phosphates. The structure is amorphous and best suited for cold-rolled steel parts that do not require performance in highly corrosive environments. A sealing rinse is generally applied after iron phosphating.

For decades, a chrome rinse was the sealer of choice. However, stricter regulations surrounding hexavalent chrome-containing products have caused many industries to turn to other sealing technologies. The prepainted lighting fixture market is a good example where iron phosphate pretreatments are widely used.

Zinc and zinc alloy coated steels are typically pretreated with zinc phosphate systems. These systems are more complex, involving a conditioning step where the surface is prepared (or seeded) to accept phosphate, regulate coating weight, and determine crystal morphology. During the time that the surface is in contact with the acidic phosphate solution, it dissolves a tiny amount of the metallic coating. At that surface, the acidic zinc phosphate produces a localized increase in pH that causes precipitation and deposition of insoluble zinc phosphate crystals onto the substrate surface. These coatings remain crystalline in nature, varying from large 25µ boulder-like particles with incomplete coverage to tightly packed acicular crystals, 5-10µ in size. Following a hot-water rinse to remove excess zinc, a sealer is applied as the final step in enhancing corrosion protection.

Over the years, various additives have been added to zinc phosphate baths to accommodate new zinc alloy coatings, optimize crystal size and coverage, or make the phosphating process more efficient in mixed metal applications. Adding free fluoride to the solution allowed for better optimization of phosphate coatings on aluminum substrates. Elements such as nickel were removed in order to reduce the amount of metal ions in wastewater and reduce costs.

Sealers have traditionally been hexavalent chromic acid solutions because of their electrochemical advantage over other treatments. With the advent of more stringent effluent guidelines and the move toward eliminating hexavalent chromium wherever possible, most systems have effectively converted to trivalent chrome or non-chrome sealers.

For decades, the complaint against non-chrome sealers was that nothing provided the level of durability and corrosion protection imparted by hexavalent chromium solutions. Aqueous silane solutions, even though excellent for both enhanced adhesion and corrosion inhibition, were inherently unstable solutions in production environments.

Finally, in 1997, an organo-metallic polymer was synthesized that provided a very useful bifunctionality. One end of the polymer chain contained an organic molecule that was well suited to bond with liquid or powder paint resins that contained hydroxyl, carboxyl, or amino functionality. The other end of the chain reacted with metal oxides or pretreatments providing a reactive chemical bridge between the metal and the paint.4 This allowed the heretofore mostly mechanical bond between the high-surface-area pretreatment and paint to become a mechanical-chemical bond with enhanced physical properties.

Today, because of significant advances in this base sol-gel technology, several pretreatment manufacturers have polymeric-
based dry-in-place sealer options that appear to perform as well as those containing hexavalent chromium.

In the “old days,” complex chrome oxide conversion coatings (CrO4) often laced with cobalt or other trace elements followed by hexavalent chrome rinses were used to pretreat aluminum substrates or aluminized steel. These coatings provided excellent active corrosion protection, self-healing properties and promoted good adhesion with many topcoats. The toxicity of hexavalent chromium along with the high cost of wastewater treatment was a strong impetus for examining other technologies.

The push to exchange steel for aluminum for autobody panels (as seen, for example, with the flagship Ford F-150 pickup truck) accelerated pretreatment developments that would be compatible with aluminum alloys. The trick was to provide the level of durability and corrosion protection required by the cost-conscious automotive industry.

As a result, there has been a flurry of activity to examine new technologies that will reduce or eliminate the use of zinc and nickel and at the same time, make pretreatment processes shorter (fewer stages) and more cost effective without compromising durability and corrosion resistance.

One option that was examined was a zirconium/titanium pretreatment conversion coating.5 The bath solution consisted of a mixture of fluorozirconic and fluorotitanic acids. The pretreatment coating was two layers—a 30µ thick hydrated ZrO2/TiO2 outer layer and a 60-90 nm interfacial layer. In this case, the titanium levels were twice the zirconium levels in the coating. Unfortunately, electrochemical examination of these coatings indicated that there was limited corrosion protection, probably due to the thin and porous structure of the two layers.

Since then, there have been more advancements that use zirconium or titanium as base elements with a number of clever organometallic and other additions that have become commercial successes.6,7 Unique aqueous formulations are replacing very old and traditional chemistries such as iron and zinc phosphates.

One formulation yields homogenous inorganic coatings that form a thin nano-metallic matrix layer on metal surfaces. These films are extremely uniform in composition and significantly thinner than iron and zinc phosphates. The thickness of this organometallic coating is only 40-80 nm, while iron and zinc phosphates are 200-300 nm and 3,000-4,000 nm thick respectively.

Another advantage of these organometallic pretreatments is that the baths operate in the alkaline range (pH 7-11), unlike the very acidic baths typical of phosphate solutions (pH 2-3). This not only improves the efficacy of the silane deposition around adhesion, but the problem of flash rusting or blush is significantly reduced. An added bonus is that no additional sealer is required.

Aluminum oxides provide a challenge for paint adhesion. A new sol-gel chemistry produces a very thin 50-500 nm film. The coating composition is an aqueous sol of cerium oxide and/or silica particles and a ureido silane compound. Rather than removing the protective oxide layer naturally present on aluminum, the molecules are designed to penetrate deeply into the nano-pores within the oxide layer and self-assemble up to 100 layers of crosslinked polymer.

Bath concentrations are used to control coating thickness. Organofunctional molecules on the non-oxide side of the polymer chain facilitate bonding with the organic paint film.8 Corrosion testing indicated that these new technologies exhibited excellent corrosion resistance, especially as seen in creepback from scribe testing after exposure to corrosive environments.

These no-rinse pretreatments comprised of reactive liquids leave reaction products on the surface of the substrate. These conversion coatings are extremely uniform, thin and generally absent of phosphates, hexavalent chromium compounds, molybdates, tungstate or vanadates. This allows for shorter application cycles (usually with less steps than traditional pretreatments), improved effluent cleanliness and adhesion and corrosion resistance characteristics as good as traditional pretreatments.9

About the author: Cynthia A. Gosselin, Ph.D., is director at The ChemQuest Group/ChemQuest Technology Institute/ChemQuest Powder Coating Research; cgosselin@chemquest.com; www.chemquest.com.

 

References

  1. Mason, Colin. Cleanliness is next to Godliness. Coating Consultants for Superyachts. May 24, 2019. https://ccsyacht.com/ccs-news/cleanliness-is-next-to-godliness/ (accessed Feb 28, 2022).
  2. Epstein, Marce. Henkel and BTG Labs Partner to Establish Surface Measurement Standards. June 2021. https://www.brighton-science.com/blog/henkel-and-btg-
    labs-partner-to-establish-surface-measurement-standards     (accessed Feb 28, 2022).
  3. Gatenby, Art. Initiation to Contact Angle. CSC Scientific Blog. Aug 11, 2016.
    https://www.cscscientific.com/csc-scientific-blog/initiation-to-contact-angle (accessed Jan 25, 2022).
  4. Kuhnen, T., et al. Using Hydrosilylation to Assemble Organometallic Polymers Containing Combinations of Silicon-based Functional Groups. Organometallics.
    Nov 11, 1997, 16(23) 5042-5047.
  5. Li, Liangliang, Whitman, Brandon W., and Swain, Greg M. Characterization and Performance of a Zr/Ti Pretreatment Conversion Coating on AA2024-T3. Journal of the Electrochemical Society. 2015, 162 (6) C279-C284.
  6. Schlosser, Ted M., and Rivera, Jose B. Process and Seal Coat for Improving Paint Adhesion. Patent No. US 9,073,083 B2. July 7, 2015
  7. Astra website. Zirca brochure. https://www.astrachemicals.com/zirca_brochure.pdf (accessed Jan 15, 2022).
  8. General Electric Company. No-Rinse Pretreatment Methods and Compositions for Metal Surfaces. International Patent Publication Number WO 2006/110328 A1. PCT/US2006/011528.
  9. Shimizu, Akio, et al. Agent for Producing a Primer on Metallic Surfaces and Method for Treatment. Henkel AG and CoKGaA. AU20222361029A1.