Nitrogen plasma source ion implantation for corrosion protection of aluminum 6061-T4
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Nitrogen plasma source ion implantation for corrosion protection of aluminum 6061-T4 J. H. Booske, L. Zhang, W. Wang, K. Mente, N. Zjaba, C. Baum, and J. L. Shohet Engineering Research Center for Plasma-Aided Manufacturing, University of Wisconsin– Madison, Madison, Wisconsin 53706-1608 (Received 19 January 1996; accepted 12 December 1996)
It is established that nitridation of aluminum (Al) 6061-T4 by plasma source ion implantation (PSII) can dramatically enhance the pitting corrosion resistance of this alloy in marine environments (i.e., chlorine-ion-bearing aqueous solutions or humid atmospheres). Corrosion tests and microstructure analyses establish that the mechanism for successful passivation against chloride-induced pitting corrosion involves the formation of a multilayered microstructure, including the presence of a continuous layer of aluminum-nitride (AlN). Important process variables are the implantation voltage and the nitrogen dose (or total implantation time), as these two variables establish the implanted nitrogen concentration. Too high or too low an implanted nitrogen concentration will not yield the continuous AlN layer required for good corrosion resistance. PSII is attractive for this application as it provides for uniform, conformal implantation of irregularly shaped objects without masking, beam rastering, or object rotation.
I. INTRODUCTION
Effects of nitrogen ion implantation on the corrosion resistance of aluminum (Al) and Al alloys have been previously investigated using beam-line implantation facilities.1–3 The observed effects include more noble corrosion and pitting potentials. These results suggest that nitrogen ion implantation may be an effective method for modifying the surface properties of Al to inhibit corrosion. In particular, implantation may be preferable for materials (e.g., solution-andprecipitate-hardened Al alloys) for which additional high temperature surface treatment, such as thermal nitriding, is undesirable. However, conventional, line-of-sight, beamline implantation is limited by low throughput, the necessity for rastering when treating a large area, the necessity and inefficiency of when treating nonplanar targets (to avoid excessive sputtering by obliquely incident ions), and the relatively high capital and ownership costs of a high-voltage beamline implanter facility. Plasma source ion implantation (PSII) may provide equivalent material modification capabilities without the above-mentioned constraints of beam-line implanters. Potential advantages of PSII include higher throughput, ability to simultaneously treat large areas (without beam rastering), ability to conformally implant nonplanar targets without masking or rotating the target, and a relatively compact configuration that is compatible with batch or web processing. The principles of PSII operation have been described elsewhere.4 Briefly, PSII involves the repetitive application of a high negative voltage pulse to a target object that is immersed in a plasma, r
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