Corrosion Protection Of Aluminum By Silane Based Surface Treatment

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Download or read book Corrosion Protection of Metals by Silane Surface Treatment written by and published by . This book was released on 2005 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The need for toxic chromate replacements in metal-finishing industries has prompted an intensive search for replacement technologies in recent years. Among the replacements that have been proposed, those that are based upon the use of organofunctional silanes rank very high in terms of performance, broad applicability as well as ease of application. This dissertation presents a four-part work: (1) structural characterization of silane films on metals, (2) mechanism studies of silane-treated metal systems, (3) development of water-based silane systems, and (4) measurements of other properties of silane films. In part 1, silane films, i.e., bis-[triethoxysilylpropyl]tetrasulfide (bis-sulfur silane) and bis-[trimethoxysilylpropyl]amine (bis-amino silane) were deposited on AA 2024-T3 and were characterized mainly using reflection-absorption Fourier-transform infrared spectroscopy (FTIR-RA) and electrochemical impedance spectroscopy (EIS) techniques. The results showed that further hydrolysis and crosslinking occurs in the applied bis-sulfur silane films in the presence of water and moisture. This is because the bis-sulfur silane is difficult to hydrolyze completely in its water/ethanol solution. The remaining ester groups would hydrolyze to silanols when water or moisture presents (e.g., in the processes of curing in the atmosphere and immersion in an aqueous solution). The as-formed silanols would further condense either with themselves or with aluminum hydroxyl groups at the alloy surface, forming siloxanes and aluminum-siloxanes. Three different regions were clearly detected by EIS in the bis-sulfur silane-treated AA 2024-T3 system, and were further observed in the SEM/EDX studies. According to these studies, the three regions are assigned to, from outside to inside, outermost silane film dominated with siloxanes (SiOSi), interfacial layer with both siloxanes and aluminum-siloxanes (AlOSi), and innermost aluminum oxide. In part 2, the mechanistic study of corrosion protection of AA 2024-T3 by bis-sulfur silane film was carried out. In summation, the following two factors play critical roles in the corrosion protection of AA 2024-T3: (1) the formation of a highly crosslinked interfacial layer, and (2) high water resistance of silane films. The former inhibits corrosion in the following two ways: (1) blocking favorable sites for water adsorption by the formation of AlOSi bonds at the interface which effectively reduces the tendency of aqueous corrosion; and (2) bonding tightly to the metal and thus restricting transportation of the existing corrosion products away from their original sites which hinders pit growth. It should be noted that a high density of AlOSi bonds can be obtained employing bis-silanes rather than mono-silanes. A high water resistance makes water penetration difficult in silane films. This is essential for preventing AlOSi bonds from hydrolysis. The results also indicated that hydrophilic bis-amino silane can not provide a good corrosion protection for metals. This is because the bis-amino silane film tends to be positively charged due to the protonation of hydrophilic amino groups. As a result of electrostatic attraction, the transportation of detrimental anions like chloride (Cl- ) ions as well as water is promoted in the film. Without sufficient water resistance of the film, AlOSi bonds at the interface are readily hydrolyzed. Consequently, the interfacial adhesion is lost. Corrosion thus readily proceeds at the alloy surface. Although bis-sulfur silane performs well on Al alloys, it fails on HDG. This is caused by poor film coverage on HDG due to the insufficient wetting of Zn oxide by bis-sulfur silane solution. A mixture at the bis-sulfur/bis-amino ratio of 3/1 enhances the corrosion resistance of both AA 2024-T3 and HDG. A small portion of bis-amino silane makes the mixture solution hydrophilic enough to wet Zn oxide on HDG, which facilitates the formation of a homogenous film on HDG. On the other hand, a large portion of bis-sulfur silane enhances the hydrophobicity of the mixture film, which is the basis for good protective performance of the mixture. In part 3, test results for newly-developed water-based silane systems were reported. The major advantage of these silane systems is that they are highly miscible with water, which makes them more industrially acceptable than alcohol-based silanes. Test results demonstrated that these silanes provide excellent corrosion protection as well as paint adhesion on a variety metals including, Al alloys, Zn-coated steels, carbon steels, and stainless steels. Part 4 reported several other properties of silane films, such as resistivity/conductivity, mechanical properties, and thermal stabilities of silane films.