The corrosion of stainless steel 1.4301 and 1.4401 in low chloride drinking water is
unexpected as from a chemical point of view the water quality would normally not cause
corrosion. However, natural waters give rise to ennoblement due to biological activity and
this in combination with a metallurgical/geometrically sensitive area increase the risk of
corrosion. And finally under specific process parameters (standstill, low exchange rate,
contaminations e.g. due to repair work) the critical conditions can occur that leads to crevice
corrosion in press fittings or pitting in the vicinity of welds. So which of these parameters –
biological activity, material sensitivity, and process conditions - can be controlled or
predicted?
It is important that water as clean as possible is used with low amounts of solids, chlorides,
manganese, iron and organic substances. Ennoblement will occur in time in all water types,
except if water treatment like chlorination is used, which is rarely done in Denmark. Presence
of a specific organism has been suspected for causing failure, but as long as this has not been
identified, the only indication is that often more cases appear in the same town or area during
a specific time frame thereby linking the failures to the specific feature of the water quality at
that time. It seems clear that strong microbial activity is a risk, but whether in future this may
lead to a prediction based on a water sample is so far not likely unless major mechanistic
breakthroughs take place. Prevention of contamination of the systems with microorganism
can be obtained by e.g. using ultrafiltration of pressure testing water and chlorination at
commissioning of installations. Correct design and service may limit biofilm formation and
growth of microorganisms (including pathogen bacteria like Legionella). In short this means
avoiding dead ends and keeping hot and cold water separated as well as sufficiently hot and
cold. Furthermore, organic compounds that can be utilized as nutrients must be limited and
focus must be on ensuring that O-rings are inert and are not leaking possible nutrients.