Oxidation of Aluminum

Oxidation of Aluminum

?Aluminum oxidizes very readily but this is advantageous as the aluminum oxide is tougher than the aluminum metal as so the oxide protects the metal. The

aluminum oxide is also less dense than the metal and so takes up more space than the metal and so as the metal is oxidized it puts the aluminum oxide in compression and this helps provide the surface with a good barrier layer that reduces the rate of any further oxidation.

?Thus, the first monolayer of oxidation is virtually instantaneous dependent only on the arrival rate of oxygen. Beyond this the oxidation rate is dependent upon both the oxygen arrival rate and the rate of diffusion through the existing oxide layer.

Typically, the oxide could be expected to be 1.5nm – 2nm almost immediately and 2nm – 4.5nm in anywhere from 1 month to 1 year dependent upon conditions.

In most metallizers the residual gas within the vacuum system, even where the system can reach a good base pressure, means that the rate of arrival of oxygen to any surface is faster than 1 monolayer per second. Hence there is always plenty of oxygen available to oxidize the aluminum surface even in a vacuum system. It is also worth bearing in mind that even at the high aluminum deposition rates available in modern aluminum metallizers it is still common for there to be 1% - 2% oxygen contained within the coatings.

?Where oxygen is not available as a gas the aluminum is happy to strip the oxygen out of water molecules and as water is the dominant background gas of all

vacuum systems there is no shortage of oxygen for oxidation to occur.