- 08 December 2005 -
Nanoceramic-based Conversion Coating
By Patrick Droniou, Manager, Henkel KGaA, Duesseldorf, Germany, and William E. Fristad, Technical Director, and Jeng-Li Liang, Research Scientist, Henkel Corp., Madison Heights, Mich.

In industries such as office furniture, home appliance, automobile, or electrical components construction, metal pretreatment is currently used before paint application¹. Its purposes are the adhesion of the future paint coat and anticorrosion properties. The state of the art consists of applying phosphating salts over the surface during a complete process.

A new generation of conversion has been recently discovered, to replace the phosphating processes, with dramatic improvement in the ecological field as well as in economics. These processes are free of heavy metals such as nickel, manganese, and zinc, subject to restriction in the field of workers’ safety or waste disposal. They are furthermore free of phosphate ions, which are responsible for the eutrophication of surface waters, and thereby lead to a decrease of animal life.

This new conversion allows the production of thin nanometer range coatings, while phosphate layers are within the micron range. They are based on the combination of a nano-structured ceramic-type metallic oxide, with metals like titanium and zirconium.

Table I: Application Process for Analytic Evaluation of the New Conversion (Click to Enlarge)

The Deposition Mechanism of the Nanostructured Conversion
The nanoceramic conversion is deposited over the metal surface in a process, which has been investigated over time with different analytical methods:

• XRF spectroscopy is used to measure titanium and zirconium on the surface.
• Weight loss after stripping the coating with hot potassium hydroxide solution is used for metallic oxide on steel.
• Microbalance operating with a 10-MHz crystal is used for the assessment of total coating weigh.
• Atomic torce microscopy (AFM) micrographs were made in scanning and phase mode.

All samples used for analysis were prepared according to the spray process shown in Table I.

Figure 1: Coating weights of metal oxide nano-particle and titanium or zirconium metal coating on cold-rolled steel as a function of spray time (Click to Enlarge).

The nanoceramic coating is produced in a short time. Figure 1 shows that the deposit of the metal oxide nano-particles reaches a plateau within the first 20 to 30 seconds. After that time, the metal oxide particles remain relatively constant and seem to follow a self-limiting deposition mechanism; the titanium and zirconium components continues to increase linearly². As time goes on during application, it is anticipated that Zr or Ti is deposited through the first metal oxide matrix, sealing the void places between particles.

The overall coating weight was also measured by using a quartz crystal microbalance (QCM), with an iron sputtered crystal³. As clearly seen in Figure 2, not only the total coating weight can be thus measured, but we get a very good idea of the kinetics of the deposition process.

A 35-nanometer iron layer was sputtered onto a gold wafer. This steel-simulating surface was then exposed to the conversion solution, and a film began to form. Under these experimental conditions, 70% of the total coating mass was deposited within one minute. The coating was 87% complete after three minutes, which corresponds roughly to the deposit of one monolayer of nano-oxide particles. It is expected that any further mass increase corresponds to multiple layers of particles.

Figure 2: Mass change (ng) and thickness (nm) as a function of time for a Fe film exposed to a conversion coating solution containing nanoceramic particles (Click to Enlarge).

A DI water rinse was applied after the conversion, which resulted in no loss of the coating, as also visible in the figure. This latter experiment shows the self-limitation of the coating and its tight bonding to the metallic surface.

Atomic force microscopy (AFM) allows the visualization of the coating on cleaned cold-rolled steel. The total picture represents one square micrometer. Before the conversion is applied, the bare steel surface is relatively smooth. After coating with the conversion, a dense packed layer with metal oxide particles can be seen. Individual agglomerates measuring 20 to 30 nanometers in diameter can be distinguished through either the scanning or phase mode of the AFM.

The transformation of this relatively smooth steel surface into a rough one, thanks to the conversion increases of the apparent substrate surface, also contributes to the adhesion of the subsequent paint coating.

The Application Process
The nanoceramic conversion is industrially applied in a multi-stage process. A typical process includes an alkaline cleaning step, rinses, the acid conversion in itself, and a last deionized water rinse. A unique feature is the application at the ambient temperature of this new conversion.

In terms of process length, or complexity, iron phosphate can include a further passivation step when an extended performance level under paint is needed. The new nanoceramic conversion process is, therefore, shorter than the iron phosphate one.

Zinc phosphate is an even longer process, as it also includes an activation step for the nucleation of the zinc phosphate crystals over the metallic substrate⁴.

Nanoceramic Coating Performance
Panels and parts were treated with different industrial spray application processes in the U.S. and Europe, compared with standard spray iron and zinc phosphatation systems, and evaluated for performance. These latter comparison systems were applied in a passivation-free sequence, as it is rather common in appliance industries. The different panels and parts were then coated with various paints.

Table II: Anticorrosion Performance of Conversions Under High Solids Paint (Values are total scribe creep, In mm.) (Click to Enlarge)

High-solids acrylic paint produced in the U.S., and polyester powder paint without TGIC crosslinking, produced in Europe, were used. Cold-rolled steel (CRS), electrogalvanized steel (EG), and aluminium (AL) of standard quality, produced in the U.S. and Germany, were used as panels.

Neutral salt spray (NSS) according to ASTM B-117 or ISO 9227, and cyclic corrosion testing according to the GM 9540 P standard were used for the assessment of corrosion resistance. Cross hatch according to ISO 2409 was used to measure the paint adhesion.

The nanoceramic conversion was used under the name of Bonderite NT-1. Bonderite 1080 HMF and 1090 were used as iron phosphating products, and Bonderite 958 was the zinc phosphating product.

The performance of the same nanoceramic conversion applied over galvanized or aluminum substrates was also quite acceptable, especially when comparing the more relevant cyclic corrosion test results.

A comparison was also made with polyester TGIC-free powder paint, which is currently used in general industrial businesses in Europe. All three CRS, EG, and aluminum substrates were converted with either nanoceramic conversion or iron phosphate. The comparison was made on different customer lines, and paints came from different suppliers.

Table II: Anticorrosion and Adhesion Performance Under TGIC-Free Powder Paint (Values are half-scribe creep, in mm.) (Click to Enlarge)

The reported corrosion resistance and paint adhesion show both minimum and maximum values, according to the test location and paint. Results are gathered in Table III. Whatever the substrate, the nanoceramic conversion shows very narrow creep from scribe and better performance than the standard phosphate coating, as far as the NSS test is concerned.

On steel substrate, a corrosion propagation of one tenth of the phosphate corrosion level (1.5 mm versus up to 14 mm) was even observed in extreme conditions. Paint adhesion was good in almost all conditions, but, here again, there is a clear advantage in the reliability of the performance for the nanoceramic conversion.

Conclusion
A new nano-structured metal oxide, titanic, and/or zirconic composite coating has now been made available, which can be deposited on metal substrates. This coating and process provide phosphate-free and environmentally friendly pretreatment, which has corrosion protection and paint adhesion properties at least equal to iron phosphate. In certain instances, it is even anticipated that this new conversion would reach the level of a shortened zinc phosphating process.

References

1. Vorbehandlung von AL-Karosserien, Wichelhaus. Euroforum; March 2002.
2. Mit neuen Ideen an die Oberfläche, Droniou, Krömer, Willers, Euroforum; February 2003.
3. Liang, Fristad, Meagher, Bryden, Murphy, Nanostructured Inorganic Conversion Coatings, Am. Chem. Soc.; September 2004.
4. Droniou, Wichelhaus, Quellhorst, Pretreatment and Corrosion Protection, Interfinish; September 2000.

Patrick Droniou, manager for the European Product Development of Metal Treatment Processes, can be contacted at (e-mail) Patrick.Droniou@henkel.com.

William E. Fristad, technical director responsible for all cleaners, pretreatment, and thin organic coating development in North America for automotive and general industrial markets, can be contacted at (e-mail) bill.fristad@henkel.com.

Jeng-Li Liang, research scientist working on the development of new metal pretreatments in the Auto-motive Department of Henkel Corp. in the U.S., can be reached at (e-mail) jeng-li.liang@us.henkel.com.