Corroded Leaky Stainless Steel Pipes

Steel pipes, used for years in a food factory soft water preheater, were found to leak as a result of corrosion. The pipes, made of 18/8 steel, were immersed in steam maintained at 0.5 atm and 150 deg C. They carried desalinated process water, heating it to approximately 100 deg C. Inspection revealed a reddish-brown coating on the outside of the pipe with a few flat pitting holes and incipient cracks. Corrosion was also observed on the inner walls of the pipe, consisting of rust patches with pitting scars, branching out to predominantly transcrystalline cracks. In this case, leaking appears to be the result of a combination of pitting and stress corrosion, most likely due to chlorides. The factory was recommended to use molybdenum-alloyed steels (type 18/10 or 18/12) which are more resistant to local disruption of passivating films and pitting than molybdenum-free types such as 18/8.