I would second metengr; First figure shows that elbo material is more suseptable to pitting than the pipe materials and external conditions are promoting the corrosion; probably under deposit.
I have pitting corrosion, possibaly initiated by stress corrosion crack under chloride ion effect, on external side of 316 stainless steel pipework. It is also suggested that it could have been started due to deposit on the surface, which i cant confirm. It was used under open environmental...
