Pitting Corrosion Restraint Mechanism of Aluminum with the Silane Coupling Agent

Conventionally, surface treatment based on the use of chromate conversion layer is efficient for preventing corrosion of metals, but it releases highly toxic Cr(VI) into environment. For this reason, a new surface treatment method using the Cr(III) was developed. This replacement process is based on a corrosion control mechanism by formation of reaction products and oxide that are formed by a base metal dissolving.When this surface treatment method is applied to a thin metal film, there is concern that a base metal film may disappear during this treatment. Thus, this treatment cannot be applied to a thin metal film. Recently, surface treatment using a silane coupling agent such as bis(3-triethoxy Cyril) ethane (C₂H₅O)₃Si(CH₂)₂Si(OC₂H₅)₃ (BTSE) has evolved as a promising alternative to replace the toxic chromate-based treatments. There are many reports about the improvement in general corrosion resistance of the Al by the silane coupling treatment^1)-4). However, the improvement in pitting corrosion resistance by the silane coupling treatment is not necessarily obvious. To improve the pitting-corrosion resistance of thin Al film, this work reports the optimum of the silane coupling treatment.

An Al film was immersed in ethanol solution with several silane coupling agents in 0.5-96h. Polarization curves of the Al film treated by silane coupling agents were measured in 3.5% NaCl solutions. Pitting and general corrosion were evaluated using pitting potential and corrosion current density, respectively. Ten kinds of silane coupling agents that can be divided two major categories, i.e. mono-silyfunctional silanes (‘mono silanes’) and bis-silylfunctional silaned (‘bis silanes’), were used as treatment solutions. Surface observation and surface analysis of Al surface treated silane coupling agents were performed by using AFM, SEM and XPS, respectively.

The difference in corrosion resistance behavior between use of mono silanes and bis silanes is not confirmed. The treatment of Al using a silane agent including the N element does not inhibit general corrosion or pitting corrosion. On the other hand, the treatment of Al by using a silane agent including the O or S element inhibits both general corrosion and pitting corrosion. In this paper, the relationship structure of the silane coupling layer and corrosion resistance is discussed.

Reference

1)Danqing Zhu, Wim J. van Ooji, CORROSON SCIAENCE 45 (2003)2177-2197

2)M. Teo, J. Kim, P.C. Wong, K.A.R. Mitchell, Applied Surface Science, 252, (2005) 1293-1304

3)DANQING ZHU and WIM J. VAN OOJI, J. Adhesion Sci. Technol., Vol. 16, No.9 (2002) 1235-1260

4) A. Frignani, F. Zucchi, G. Trabaneli, V. Grassi, CORROSION SCIENCE, 48 (2006) 2258-2273