Like anything made of metal, valves are subject to corrosion that compromises performance. Aluminum alloys are widely used for valves because of their low density and positive mechanical properties. Unfortunately, serious corrosion problems can occur when aluminum alloys are exposed to corrosive environments.
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.
Although chromate-based pretreatments provide excellent corrosion inhibition, they are toxic, and Cr (VI) is known to be a carcinogen. Additionally, Occupational Safety and Health Administration (OSHA) studies have found that applying hexavalent chromium presents significant medical risks. The environmental and human health impacts of such treatments have forced manufacturers to search for alternative methods.
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 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)
Among the benefits of inorganic and organic components, organic-inorganic hybrid (OIH) structures provide specific coating properties, such as hardness, corrosion and wear resistance, biological functionality and thermal stability, along with mechanical toughness and flexibility. In recent years, such OIH systems, which are based on silane, zirconium and titanium, have been successfully commercialized for pretreatment of aluminum alloys, galvanized steel, cold-rolled steel and many other metals and alloys. These pretreatment are used for improving adhesion and corrosion resistance performance. Additionally, by properly choosing the organic component of the sol-gel precursor, hydrophobicity can be controlled and different surface functionalities can be provided to create a barrier film on the metal surface.
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).
The presence of the nanoparticles also increases the thickness of the silane layer and contributes to blocking conductive pathways through the film, which hinders the electrolyte uptake. Some studies have proposed that silica particles react with the cathodically generated OH− ions, which could control the cathodic reaction. The silicate ions would react with Al3+ ions, forming a stable passive silicate film at the anode.
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)
On the other hand, most organic inhibitors suppress aluminum corrosion by covering the surface with passive films and by forming complexes with the alloy without any negative effect on the stability time of the sol-gel network. To confer active corrosion protection for pre-treatments, organic inhibitors also can be added to the sol–gel system during a synthesis procedure. Because of their inhibiting action towards the cathodic process, triazole and thiazole derivatives, as well as 8-hydroxyquinoline, have been studied as possible corrosion inhibitors for aluminum alloys.
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.