Innovative, Practical Corrosion-Resistant Coatings

The valve industry, however, has focused on the development of coating applications over the last decade, and today’s coatings offer much-improved protection. For example, a common pretreatment that has been used to improve corrosion properties of aluminum alloy is the application of chromate conversion coatings (CCC). These coatings provide good corrosion resistance as well as proper foundations for various primers and paints. Also, hexavalent chromium salt is often used to form an oxide layer on metal as well as to leave a film of chromium (III) oxide.

Among the chrome-free conversion coatings that have been developed for aluminum alloys, organic-inorganic coatings derived by the sol–gel process are promising. These systems have emerged as an efficient, environmentally friendly, and sustainable alternative to toxic heavy metal-based systems.

SOL-GEL COATINGS

Sol-gel coatings are produced by hydrolyzing inorganic alkoxide particles within water and alcohol ‘sol’, allowing the sol to condense, and producing a ‘gel.’ Upon application to a surface and curing, the gel produces a hard and dense film with coating thickness typically less than 10 microns. (Figure 2)

The chemistry of silanes and the interaction of these molecules with a metallic substrate and organic coating show that silanes will not only confirm adhesion between metal substrates and organic coatings but also provide a thin barrier film. The film guards against diffusion of corrosive elements to the metal interface.

A variety of approaches are used to enhance performance of such OIH systems so they match or surpass the performance of chromate-based systems. The use of colloidal nano-particles for improved corrosion resistance of OIH films is promising. Creating a dense OIH network using nano-particles provides films that are resistant to the diffusion of electrolytes. This is because of high crosslinking density. Also, improved adhesion to metal surfaces that results from the coupling effect between silane and the surface metal-hydroxyls creates resistance to water entrance along the interface by reducing microporosity. The theoretical concept of the condensation reaction of alkoxy silanol with colloidal silica is shown in Figure 3. The dense network can present fewer defects, such as micro cracks, pinholes, microspores or areas of low cross-link density, all of which provide the initial path for the electrolyte uptake into the system (which starts corrosion).

IMPROVING PROTECTION

Compared to traditional chromate conversion treatments, the problem with silanes is they do not actively protect the metallic substrate. In fact, when water and aggressive ions reach the surface of the metal, silane layers cannot ensure an inhibition of the corrosion process as active as chromate compounds can be.

To improve the protection properties, silane layers have been made by adding organic or inorganic inhibitors to the silane films. Adding corrosion inhibitors to sol–gel coatings, for example, can enhance the interface stability by delaying corrosion-induced delamination at the damage site. Many kinds of inhibitors have been used to increase the corrosion-protective properties of sol–gel coatings. One promising development is adding inhibitors directly to the inside of the sol–gel coatings. Inorganic inhibitors, such as chromates, have been shown to have a positive influence on the corrosion protection of aluminum alloys. However, these inhibitors negatively influence the stability time of the sol–gel network. (Figure 4)

CONCLUSION

An overview of the scientific literature shows hybrid materials have a range of different properties that allow them to be used in many different fields. The corrosion protection performance of environmentally friendly OIH coating has been evaluated by electrochemical analysis methods and conventional corrosion tests, such as salt spray. The results of the electrochemical analysis highlight the good barrier properties of the innovative silane film in comparison to conventional CCC.

Additionally, the high cross-linking density and mechanical properties for this type of coating confirm plausible hardness and strength, which are the two most important qualities in valve application. Different types of curing methods, excellent adhesion and flexibility of coating during metal shaping are other impressive properties for the valve industry. In addition, high thermal stability of OIH coatings may have potential application in protective coatings at elevated temperatures, which can be important in the valve industry.

Sol-gel based silane systems have been designed for most metal surfaces. Preferred substrates include aluminum, steel, zinc, phosphatized and other treated metal surfaces for a multitude of applications and uses. The corrosion protection properties are equal to or better than traditional pre-treatment systems.

Although OIH coatings have some limitations compared to chromate, the outlook for chromate replacement is promising. Also, some of the limitations can be overcome by optimizing the treatment process and conducting further research and development to find the most cost-effective solutions.

Mahshid Niknahad is a Ph.D. candidate at Eastern Michigan University with a concentration on polymers and coatings. She works on sol-gel derived organic-inorganic metal pretreatments and their corrosion resistance. Mahshid also has eight years of research and development experience in coating for the automotive industry. Reach her at mniknaha@emich.edu.