Leaks usually occur where air or water is trapped in the system.
In wet pipe systems, corrosion occurs most frequently where the air is trapped in the system piping. The oxygen in the air dissolves in the water and immediately reacts with the first iron that it contacts. This generally occurs at the air/water interface in high branch lines.
In dry pipe systems, corrosion always occurs under pools of trapped water, primarily at low point mains. Water is introduced to the dry pipe system in one of three ways: during hydraulic testing of the piping, as condensate from the air compressor, and during periodic testing of the fire sprinkler system to satisfy code requirements.
In order for oxygen to cause corrosion in fire sprinkler piping, the gas must first dissolve into liquid water. Then the oxygen accelerates the cathodic half of the corrosion reaction that causes the metal (in this case, iron or zinc) to give up its electrons in the anodic half of the reaction. This forms a pit in the pipe wall where the metal has been converted into a positively charged ion in the water. The iron ion reacts with oxygen in the water to form iron oxide (rust).