Case 1. Fire protection system
In a fire protection system in a power plant perforating pits were found in stainless steel (EN 1.4401) pipes ø140 mm of 3 mm wall thickness after two years of service [10, 11]. The system included both older parts of carbon steel and cast iron and newer, replaced parts of stainless steel 1.4401 and 1.4301. The water was tap water (drinking water) with low chloride content of 20-30 mg/l and the system was at ambient temperature. The water was chloramine treated when the system was filled, and water was exchanged typically once a year. The pits were situated in the welds or in the heat affected zones. Pipes of EN 1.4301 were also attacked and leaks detected after two years of service. Typically the pits were very small having large subsurface cavities. In general the pits were situated close to or in the transversal on- site welds. There were clear indications of oxidation of the heat affected zone (heat tints) but no other signs for welding defects were found. Reddish brown deposits were found and the pits were surrounded by dark brown circular deposit rings and the inner surfaces of the pipes were mostly covered with a thin and rather smooth layer of a yellowish brown deposit.