This type of corrosion is caused by dissimilar metals within close proximity to each other in a wet fire sprinkler system. The water chemistry can affect this type of corrosion and should be considered in the design phase of any fire sprinkler system. The galvanic series chart shows the corrosive nature of metals. Those metals at the top of the chart will corrode before those at the bottom. Therefore iron pipe will corrode before brass or cooper. The dissolved iron will react with minerals in the water and precipitate as deposits on piping, values, etc. These deposits can now cause under-deposit corrosion.
This form of corrosion occurs when the metals in brass (copper and zinc) get separated from each other. The zinc will get extracted from the brass due to the water chemistry attacking the zinc. This will leave a white type of deposit on the metal surface. This is commonly known as plug type dezincification. Weeping occurs within the system because of the loss of metal. Pipe failure can occur with this type of corrosion.
The metals commonly attacked by graphitic corrosion are gray cast iron and nodular cast iron. The affected metal is converted to a gray or sometimes black soft material. Once the deposits are removed from the corroded metal, they will have an oily or slippery feeling. This is the same reaction found in a dry cell battery (Electrolysis). This attack is commonly located in cast iron pump impellers, castings and cast iron piping systems.