#20
Author: Heiner Grünwald,
Email:
Subject: Vacuum deposited Aluminium does not necessarily tarnish under office conditions.
Due to my experience, vacuum deposited Aluminium does not necessarily tarnish under office conditions. I kept a piece of aluminium coated PC without any corrosion protection under these conditions over years without tarnishing in my office as well. The corrosion protection may be due to the native oxide on top of the aluminium that may have a thickness of 3 to nm only. Also, your aluminium may be protected with a thin (20 nm or so) plasma polymer coating that can be deposited in most modern Al evaporation or sputtering systems. It is unlikely that you will be able to identify either of these layers since their thickness is well below the information depth or sensitivity of SEM/EDX and FT-IR. If you put your part into NaOH (1%) solution, you will observe formation of little bubbles within a couple of seconds if the Al is protected by the native oxide only whereas in case of a plasma polymer top coat no bubbles should occur within 3 min if the coating is done properly. If the information is important for you, an ESCA (or XPS) analysis that is sensitive to a layer thickness of 2 to 10 nm should help.
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