What is the black dust that forms on antennas?

If your antenna has been on the roof for a while you might notice that it’s covered with a fine black dust. This may worry you, it may even keep you up at night. But rest assured, it’s not a terrible thing and believe it or not it’s better to have that layer of dust than not have it.

The black dust on your antenna is aluminum oxide. All metals have a tendency to combine with oxygen to form another substance. In the case of iron, iron oxide is not as strong as regular iron, and you’ve seen it yourself — the common name for iron oxide is rust and you’ve seen how a rusty piece of iron can fall apart easily. However, not all oxides are the same. Silver oxide (otherwise known as tarnish) is ugly but harmless, and aluminum oxide is fairly similar.

The black aluminum oxide coating on an antenna doesn’t affect the antenna’s ability to pick up signal, because aluminum oxide is roughly as conductive as regular aluminum. Unlike rust, once an oxide layer forms on an aluminum surface, the oxide itself acts as a barrier to stop more oxides from forming. So it’s actually protecting the antenna from becoming corroded. Common silver polish will take off the coating, but repeatedly removing it will eventually weaken the aluminum.

Given that aluminum oxide is harmless and actually prevents further corrosion, you might ask why all antennas aren’t shipped “pre-oxidized” with the fine black coating already in place. Blame that, I suppose, on Americans’ predilection for shiny things; in the chrome-plated world of 1940s and 1950s USA, it simply wouldn’t do to send you something brand new that was already dull grey and black No, a shiny new antenna is what you wanted and that’s what you got. Not to mention, pre-oxidized antennas would cost more to make because you would have to actually do the oxidizing.

Bottom line though… don’t worry about that fine black dust. Believe it or not, it’s your friend.