If galvanized iron fittings are connected to copper pipe in moist ground, what will happen first?

When galvanized iron fittings are connected to copper pipes in moist ground, the first event that occurs is the corrosion of the galvanized iron. This is due to a galvanic reaction that takes place when two dissimilar metals are placed in an electrolyte, such as moist soil.

In this scenario, copper, being a more noble metal, will not corrode easily when in contact with iron. Instead, the more reactive metal, which is galvanized iron (iron coated with zinc), will corrode preferentially. This happens because the zinc offers some level of protection to the iron underneath, but over time the zinc will sacrifice itself to protect the iron. Eventually, if there is enough moisture present, the galvanized coating may degrade and expose the iron to corrosion.

As the reaction continues, the zinc on the galvanized fittings will wear away, leading to the eventual corrosion of the iron itself if the exposure is sustained. The presence of moisture increases the conductivity of the soil, accelerating the corrosion process.

This phenomenon is a common concern in plumbing and water distribution systems where different metals are used in conjunction, particularly in buried applications.