Just Woody,
To tell if galvanized steel is good for smoking is like trying to decide which came first, the chicken or the egg. It is made of metal sheets that are dipped into a hot bath of melted zinc and some other chemicals. Zinc melts at 787° F (420°C), and boils at 1,665° F (907° C).
Although zinc presents no danger, a metallic element such as cadmium is bad for our health. Determining the exact composition of a particular pipe is difficult. This is why the Food Safety and Inspection Service of the United Stated Department of Agriculture says ‘Donâ€[emoji]8482[/emoji]t smoke foods in makeshift containers such as galvanized steel cans or other materials not intended for smokingâ€[emoji]8482[/emoji].
Before smoking for the first time, any section of pipe that makes direct contact with burning wood in the fire pit should be burned with hot fire. This takes care of initially burning away any deposits. The rest of the pipe is exposed to such low temperatures (traditional smoking: cold smoking below 80° F, hot smoking at less than 140° F, cooking in a smoker below 190° F) that it can not possibly react with any chemicals that were deposited on its surface during the manufacturing process.
Better still, break your pipe into separate sections. The section that makes direct contact with a fire pit can be made of steel, tile pipe, or reinforced concrete, and the rest can be made of galvanized duct.