I work for an electrical utility in SE Asia. We have experienced failure of 20 Vacuum interrupters used in our electrical circuit breakers; the installed base of interrupters is approximately 3000. The mode of failure is apparently due to corrosion of the stainless steel bellows on the interrupter, which is effectively a moving seal between the vacuum chamber and the external contact drive. The steel is grade 316L and the bellows are 0.13mm thick. An investigation by the OEM concluded that the failures can be attributed to small holes in the bellows caused by chloride corrosion – this is backed up by SEM analysis which found chlorine around the holes; they postulate that the prevailing high temperatures and humidity combined with close proximity to the sea results in condensation occurring on the bellows, which dissolves chlorides thereby liberating elemental chlorine, which sets up corrosive pitting of the bellows; subsequent evaporation leaves a higher concentration of chlorine thereby accelerating the process. The area in which the bellows are contained has a plastic %91end plate’ which restricts the airflow through a clearance of 0.15 – 0.2 mm therefore the area is not well ventilated. During the investigation we installed temperature and humidity loggers within the circuit breaker enclosures and considering the results obtained it is doubtful as to whether condensation is taking place – max RH approximately 75% and approximately 2-3 degrees difference in temp throughout 24-hr period (this is due to internal heaters installed within the enclosure, which are designed to prevent condensation). The conditions are pretty constant throughout the year. There is no heavy industry to speak of therefore the environmental issues can be attributed to the effects of the sea only. Since I have no knowledge of this subject, I would appreciate any comments as to whether the above explanation is feasible and if so should we not have experienced a more widespread failure rate if this is due to uniform environmental conditions. Thanks