Corrosion Prevention in Metals Using Layered Semi-conductor/Insulator Structures Forming an Interfacial Electronic Barrier

Abstract

A new approach to corrosion prevention involving the use of layered semiconductor/insulator films on metal surfaces is described. It is shown that the improved corrosion protection is due to the existence of a built-in electronic barrier at the metal-semiconductor (MS) or metal(thin) insulator-semiconductor (MIS) interfaces. This is in contrast to the conventional techniques which rely on physical barriers (e.g., paints) or high resistivity oxide/nitride films at the exposed metal surfaces. The electronic barrier, which arises due to charge redistribution at the MS or MIS interface, serves to impede the transfer of electrons from the metal surface to foreign oxidizing species, thereby preventing oxidation. Specific structures fabricated and tested include: Al-Indium Tin Oxide (ITO) for MS and Al-Si02-ITO for MIS configurations. A comparison with Al-S13N4 (passive barrier) is also made. High purity (single and polycrystalline) and commercial purity aluminum and aluminum alloy (7075-T6) samples were used in this study. Cathodic and anodic polarization data, weight-loss measurements, and the results of physical, optical and electronic characterizations are presented for numerous aluminum samples. It is shown that the magnitude of the electronic barrier height, and the resultant corrosion protection, is enhanced by: (1) the presence of a thin (20-100A) Si02 (insulator) layer, and (2) an increased indium/tin ratio in ITO films, which results in a larger energy gap. The application of the active electronic barrier concept to semiconducting polymers such as doped polyacetylene, phthalocyanine and chlorophyll is also discussed.