Toxic Fumes! Please Read!

[Titus]

Single wall galvanized smokepipe, that's changed color. Nothing was spilled on it or got on it. Was "galvanized" appearing, now splotches on it have turned gray. The top side of one elbow and the top of a slanted run, the hottest areas.

If the zinc didn't burn off, changing the color of the pipe, what happened?

Did someone mention oxidation?

The outer layer of galvanized steel is nearly pure zinc. As you progress through the metal, you find a series of iron-zinc alloys until you finally reach pure steel. Here's a nice picture:

With new galvanized steel, you see the crystalline zinc outer layer.... the spangle. When exposed to the atmosphere, pure zinc reacts with oxygen to form zinc oxide, which further reacts with carbon dioxide to form zinc carbonate, a dull grey, fairly strong material that stops further corrosion in many circumstances, protecting the steel below from the elements.

In other words.... losing the spangle and getting a dull grey is a good thing!