The only thing I can tell you in regards to the pipe is that you don't want to weld it as it will give off a poisonous gas. Why not use cedar or yellow pine which is naturally rot resistant. It would last a long time. It is treated with CCA which is chromium, copper, aresenic. It will leach into the soil for years and your grapes will take it up.
Galvanized pipe is either iron or steel that is coated with zinc.
The only thing I can tell you in regards to the pipe is that you don't want to weld it as it will give off a poisonous gas. It would last a long time.