Chromate conversion coatings have been widely used as a corrosion protection for metal surfaces due to the strong oxidation properties of hexavalent chromium (Cr⁶⁺). However, Cr⁶⁺ presents health and ecological concerns and is subjected to strict environmental regulations. Sol-gel coatings without chromium have emerged as a possible alternative for corrosion protection, but these do not electrochemically protect the metal. Rather, they provide only a dense barrier against electrolyte diffusion to the metal surface. Other advantages of sol-gel coatings include their simple processing conditions, strong adhesion with metal surfaces, and ability to covalently bond with most organic “top-coats,” if appropriately functionalized. Unfortunately, at this point the corrosion protection provided by sol-gel systems is much inferior to the chromium-based coatings. The failure may be traced to the existence of micro-pores, formation of cracks, and uneven distribution of cross-link density in the coating. It has been shown that improvements to sol-gel coatings may be made by incorporation of inorganic (metal oxide) nano-particles [1]. For example, various oxide particles can increase the moisture resistance of the coatings as they are known to be effective water scavengers [2]. They also have been shown to lower the porosity of the coating and toughen the film such that production of a thicker barrier layer is possible. Furthermore, they have been used as a carrier for corrosion inhibitor compounds (e.g. cerium salts) in sol-gel coatings to enhance long-term corrosion protection. However, aggregation of these particles presents a critical problem because for all of the properties mentioned, the optimum improvement is obtained only when the particles are uniformly distributed throughout the volume of the film.

The objective of this investigation is to understand and control the state of dispersion of a variety of oxide nano-particles in a SiO₂-ZrO₂-based sol-gel coating during curing or gelation. The state of aggregation of unmodified oxide particles during cure is tracked by dynamic and static light scattering. It is then correlated with the particle's surface electronic properties and the state of the gel structure formation, as determined by rheological measurements, in order to identify critical conditions for aggregation. Here we show that stabilization of these particles with appropriate block copolymer adsorption or by modifications in processing conditions might produce uniform spatial distribution of the particles. The resulting improvement in sol-gel coating properties is discussed.

Reference:

1. Zheludkevich M.L. et al. “Corrosion Protective Properties of Nanostructured Sol-Gel Hybrid Coatings to AA2024-T3.” Surface and Coatings Technology. 200(2006) 3084-3094

2. Vogt B.G. et al. “Control of Moisture at Buried Polymer/Alumina Interfaces through Substrate Surface Modification” Langmuir. 21(2005) 2460-2464