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Hybrid Sol-gel corrosion inhibitors a novel approach to corrosion inhibitors for coatings.

Corrosion inhibitors provide an indispensable function to protective coatings. Heavy metal-based corrosion inhibitors were widely acclaimed as the best materials to provide corrosion protection to coatings. The performance of a coating under corrosive conditions requires that corrosion inhibitors provide reliable and sustainable protection during the coatings lifetime. Today, the industry is challenged to be more conscious of the impact that toxic metals have on human health and on other ecosystems. Sol-gel technology offers an eco-friendly alternative to the use of toxic metals (Cr VI, Cd, Pb, Cu, Ni, and Zn). This article highlights research in new sol-gel based corrosion inhibitors for use in paint as well as for use in the pretreatment of bare metals (e.g., steel, aluminum, silver). Sol-gel technology particularly affords superior corrosion protection in thin film coatings (< 25 [micro]m dry), an area that up to now has been dominated by Cr-based anticorrosives. This technology is applicable to most waterborne and solventborne finishes, high gloss direct-to-metal coatings, conventional primers, acrylic clearcoats, fast drying lacquers, wash primers, anti-fingerprint, and conversion coatings.

INTRODUCTION

The economic cost of corrosion throughout the world is often estimated on the basis of maintenance and safety. The costs of corrosion come in the form of either premature deterioration or failure resulting in the need for maintenance, repair, and replacement of damaged equipment. (1) According to a study recently conducted by CC Technologies for the Federal Highway Administration (FHWA), these costs can be divided into direct costs (i.e., those associated with design, manufacturing, and construction) and indirect costs (those associated with management of corrosion). The costs of designing, manufacturing, and construction include material selection, such as stainless steel to replace carbon steel; additional material, such as increased wall thickness for corrosion allowance; material used to mitigate or prevent corrosion, such as coatings, sealants, corrosion inhibitors, and cathodic protection; and application, including the cost of labor and equipment. The cost of management includes corrosion-related inspection, corrosion-related maintenance, repairs due to corrosion, replacement of corroded parts, and the loss of productive time. (2) The authors of that study concluded that more industries ought to take a hard look at corrosion from a preventative standpoint as opposed to a simply maintenance and repair viewpoint. Several corrosion control methods come into play. Among them are organic and metallic protective coatings, corrosion resistant metals and alloys, polymers, anodic or cathodic protection, and the use of corrosion inhibitor additives.

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The use of corrosion inhibitor additives in coatings has long been established as an effective preventative option to combat corrosion. The use of coatings containing either chromates (Cr) or lead (Pb) based anti-corrosives continues to come under heavy scrutiny because of the harmful effects of these products to humans and the environment. Heavy metal containing products continue to be phased out as the green initiative takes root. As a result of directives such as RoHS and WEEE, the arsenal of dangerous substances available as corrosion inhibitors continues to shrink. Paint formulators are discouraged from using products based on heavy metals which can be bioaccumulative, hazardous, or toxic to humans and the environment.

Hybrid sol-gel technologies offer an environmentally friendly alternative to using heavy metal-based corrosion inhibitors. Sol-gels offer an opportunity to replace Cr-based pretreatments for metal. Sol-gel is a process of preparing novel metal oxide glasses or ceramics by hydrolyzing a chemical precursor or mixture of chemical precursors that pass sequentially through a solution state and a gel state before dehydrating to a glass or ceramic. (3) The chemical nature of silanes allows them to combine with organic and inorganic materials. This article highlights the use of a new sol-gel technology which can be used as both a corrosion inhibitor additive for paint as well as a corrosion-inhibiting pretreatment for bare metal. There are innumerable ways in which sol-gels can be designed or tailored to provide the corrosion protection similar to toxic substances like chromate-based compounds.

SOL-GEL CHEMISTRY

The complete hydrolysis of tetralkoxysilanes under highly controlled conditions usually without the presence of fillers is associated with the sol-gel technology. (3)Sol-gel represents a process of fabricating stable oxides (glasses, metal oxide, mixed oxides, and ceramics) by hydrolyzing a chemical precursor or mixture of chemical precursors that pass through a solution state and a gel state before being dehydrated to form a coating as shown in Scheme 1 (courtesy of Evonik). (4)

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The sol-gel process was originally used to obtain ceramic masses or glasses from initially solvent compounds via the intermediate stage of a gel. A particular advantage of this process is the production of far more homogenous products as opposed to the classic ceramic methods. In addition, the production of superior coatings can be realized, wherein an alcoholic solution of hydrolyzable alcoholates is applied with polyvalent metal ions on a surface and a metallic hydroxyl network is formed as the alcoholic solvent evaporates. This coating, which contains numerous MOH groups, is hydrophilic and antistatic. As the temperature rises, the MOH groups react under dehydration to metal oxide groups so that the surfaces become hard and scratch-resistant. Often, such materials find application as binder materials. Production of stable phases (sols) typically involves the catalytic hydrolysis of alkoxy silanes with acids or bases in aqueous solution up to a certain degree, filling with siliceous sol or other particles, and adjustment with alcohols as solvent to a predetermined solids content. (5) This article highlights unique properties of sol-gel chemistry as applied in coatings, especially waterborne and solventborne primers. Several examples of the application of sol-gel technologies will be highlighted. More noteworthy, the authors will demonstrate the ability to replace toxic heavy metal corrosion inhibitors using a room-temperature applied sol-gel process in combination with environmentally friendly corrosion inhibitors.

APPLICATION OF SOL-GEL AS PRETREATMENTS ON METAL

Hydrophobic Film Formation

The sol-gel chemistry described here is modified using a proprietary approach to allow it to be used as a metal pretreatment solution or as a stable liquid corrosion inhibitor in paint (primers). The use of the solgel chemistry described here is an extension of silane surface modification processes. The sol-gel is deposited from an aqueous alcohol-containing solution. It is our understanding that these silanes couple with the hydroxyl-rich surfaces of the metal substrate (Fe, Zn, Ag, Al), allowing it to form a film. Stable oxane bonds are formed by the reaction of the silane with the substrate as shown in Figure 1. Upon further gelation or cross-linking, a three-dimensional film is formed across the metal substrate. This film is capable of improving adhesion of the primer layer because of its organic functionality. The byproducts formed during the sol-gel process are non-corrosive and volatile in nature. The formation of this network imparts improved corrosion resistance to the substrate by impeding the transportation of ions through this dense non-polar network.

[FIGURE 1 OMITTED]

Linear Polarization

Cold-rolled steel substrates were dipcoated with the sol-gel coating made from a 10% dilution of the sol material in a de-ionized water/alcohol mixture. Panels were immersed in this solution for 30-60 sec and then air dried under room temperature conditions for 24 hr. After drying, these panels were exposed to a 5% NaCl solution in a three-electrode cell assembly consisting of a Saturated Calomel (SCE) reference electrode, a graphite counter electrode, and the coated or uncoated steel substrate as the working electrode. This setup was used for all of the polarization evaluations described here. Tafel plots of the bare and sol-gel coated substrates were used to determine the extent of corrosion protection afforded by this technology. From the comparative analysis it was verified that the sol-gel coating caused a decrease in the anodic and cathodic currents compared to the bare substrate. The sol-gel coating acts as a barrier to the ingression of moisture and chloride ions from the electrolyte.

Bonderite[R] 952 is a zinc phosphate conversion coating formulated for a spray application on substrates such as steel, galvanized and electrogalvanized, zincalloy, and aluminum. Bonderite 1000 is an iron phosphate conversion coating. (6)

The analysis of the Tafel plot (Figure 2) shows that the bare steel panel exhibits a non-passivating response with the anodic current increasing linearly with potential. The Tafel scan of the sol-gel coated substrate labeled (C) shows a plateau behavior similar to that of zinc and iron phosphate steel panels, which indicates passivation. When a low voltage potential is applied to the uncoated substrate, the current is highly indicative of uninhibited anodic and cathodic processes. On the contrary, the sol-gel coating shows a decreased current density due to the insulating behavior of the coating itself. Tafel scans can be used to quantitatively exploit the differences between various forms of pretreatments.

[FIGURE 2 OMITTED]

APPLICATION OF THE SOL-GEL TO PAINT SYSTEMS

The sol-gel chemistry was applied to several coating systems as a corrosion inhibiting additive. Table 1 briefly describes the coatings evaluated for anticorrosion performance. The authors discovered the unique synergistic effects of the sol-gel coating when combined with several commercial non-toxic anticorrosive pigments. The typical modified zinc phosphate inhibitive pigments are effective as anodic and cathodic passivators in coatings. These pigments rely on partial solubility to release their corrosion ions (typically phosphate, zinc, strontium, or calcium ions) in order to impart corrosion resistance to coatings.
Table 1--Sol-Gel Incorporated in Coating Systems

 Type Sol-Gel Outcome
 Incorporated

Waterborne Water reducible Added in grind Boosted salt spray
 alkyd direct-to- Boosted blister
 metal (DTM) enamel resistance

Waterborne Acrylic lacquer Added in base Reduced black
 sealant corrosion

Solventborne Vinyl butyral Added in grind Replacement for
 wash primer Cr-based corrosion
 inhibitor

Solventborne Medium oil alkyd Added in grind Increased adhesion on
 primer galvanized steel


Figure 3 shows a water reducible alkyd enamel in which the sol-gel technology was evaluated as a stand-alone corrosion inhibitor additive. The inhibitor was post added at a dosage of 2% by total weight of paint. The panels were room-temperature cured for one week, scribed, and placed in a 5% NaCl salt fog chamber for two weeks. The sol-gel additive provided several advantages to this direct-to-metal (DTM) enamel coating, among which were gloss retention, improved adhesion, and blister resistance to the unpolished cold-rolled steel test panels. The coatings were applied at 2.5 mils (~60 [micro]m) dry film thickness.

[FIGURE 3 OMITTED]

The next example in Figure 4 depicts the evaluation of this technology in a waterborne clear lacquer applied to Galvalume * substrates. The sol-gel technology was tested alone and in synergy with a corrosion-inhibitive pigment. As shown in the figure, the corrosion resistance was remarkably improved when both inhibitors were used in synergy in the lacquer formulation. Galvalume is a carbon steel substrate with 55% Al and 45% Zn alloy. A small addition of silicon is included in the metal alloy, not to enhance the corrosion performance, but to provide good coating adhesion to the steel substrate when the product is roll-formed, drawn, or bent during fabrication. (7) This substrate is particularly prone to zinc galvanic corrosion manifested in the form of dark pits across the face of the panel.

[FIGURE 4 OMITTED]

The following example is based on the evaluation of this technology in a solventborne vinyl wash primer. Wash primers, also referred to as etch primers, have been used since the 1940s and represent one of the most aggressive application conditions for chromate pigments. Chromate pigment powders dispersed in an alcohol/resin mixture are blended with an aqueous phosphoric acid catalyst. The acid roughens the metal surface and initiates crosslinking of the resin to form a pigment-filled polymeric film. The chromate pigment may also be dispersed in other carriers that are not as harsh as the wash primer. However, if a corrosion-inhibiting pigment can survive the harsh conditions of acid diluent, then it can usually be successfully incorporated within other paint, polymeric, or barrier film systems for corrosion inhibition. Wash primers are applied to metal surfaces under acidic conditions where the primer is cured as a corrosion-inhibiting film. The authors discovered that the Cr-based corrosion-inhibiting pigment can be replaced using the synergy of the sol-gel product and non-toxic corrosion-inhibiting pigments. (8)

As seen in Figure 5, the sol-gel technology is effective as a synergist for non-toxic corrosion-inhibiting pigments at low dosages. Not only does it provide adhesion improvement to the coating, it helps to reduce the electrolyte (NaCl) attack of the substrate in the scribed areas.

[FIGURE 5 OMITTED]

In the last example, the authors investigated the effectiveness of sol-gel technology in a solventborne medium oil alkyd primer applied over hot dip galvanized substrates. Painting galvanized metal usually requires making sure the surface of the metal is completely free of any alkaline buildup that may have settled onto the zinc coating. Because of the nature of the coating, oil-or alkyd-based paints are generally not recommended for use with galvanized steel. When alkyd paints are applied directly to a galvanized surface, they tend to lose adhesion because of the saponification of the resin with the alkaline surface. Saponification is a chemical degradation of the alkyd resin in which the oils react with the zinc used in the galvanizing process, causing the alkyd to peel away or lose adhesion to the substrate. (9) An example of this loss of adhesion is shown in Figure 6.

[FIGURE 6 OMITTED]

SOL-GEL: SYNERGY WITH ANTICORROSIVE PIGMENTS

The sol-gel technology described thus far has been shown to complement other non-toxic inhibitive pigments. This unique synergy offers the flexibility of combining multiple mechanisms of anodic or cathodic passivation within the sol-gel network. It is our belief that the corrosion inhibitors are trapped close to the substrate surface, improving delivery of anticorrosive ions to the metal. The sol-gel may in fact be acting as a controlled release film as illustrated by Scheme 2. (10)

[ILLUSTRATION OMITTED]

CONCLUSION

Hybrid sol-gel based corrosion inhibitors are an environmentally friendly alternative to the use of heavy metals or Cr-based corrosion inhibitors. The sol-gel chemistry described here can be used in two ways: as a pretreatment (i.e., a stand-alone coating for corrodible metal) or as a corrosion-inhibiting additive for paint and coatings. The improved corrosion resistance was demonstrated when the sol-gel was combined with non-toxic anticorrosive pigments implying that the corrosion inhibitors appear to be entrapped within the gel network enhancing their leaching capabilities. The use of sol-gels and corrosion inhibitors in coatings allows one to effectively replace toxic Crbased corrosion inhibitors.

References

(1) http://www.corrosionsource.com/cost/CorrosionCostUS.htm.

(2) http://www.corrosioncost.com/corrosioncosts.htm.

(3) Arkles, B., "Silicon Esters," Reprinted from Kirk-Othmer Encyclopedia of Chemical Technology, 4th Ed., Vol. 22, p 69-81 1997.

(4) http://www.dynasylan.com.

(5) Sepeur, S., et al., "Solvent-poor sol-gel systems," US Patent 7,247, 350.

(6) http://www.henkelna.com/cps/ (Product Finder).

(7) http://www.ussteel.com/corp/sheet/coated/coatvalu.htm.

(8) Phelps, A.W., el al, "Non-toxic corrosion-protection pigments based on rare earth elements," U.S. Patent 7291217.

(9) Cusumano, B., "Salt of the Sky," Painting & Wallcovering Contractor, Nov./Dec. 2004.

(10) Blohowiak, K.Y., et al., "Hybrid Nanostructured Sol-Gel Coating Systems," Proc. Smart Coatings Symposium, Orlando, FL, 2007.

by Tony Gichuhi, Andrew Balgeman, Amanda Adams, and Shonda Prince HALOX *

Presented at FutureCoat! 2008, sponsored by FSCT, October 15-16, 2008, in Chicago, IL. * 1340 Summer St., Hammond, IN 46320.
COPYRIGHT 2009 Federation of Societies for Coatings Technology
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Author:Gichuhi, Tony; Balgeman, Andrew; Adams, Amanda; Prince, Shonda
Publication:JCT CoatingsTech
Date:Apr 1, 2009
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