Originally Posted by Streetcar
Water injection is below the pic.
Insulation is non metallic.
Surely the ss elbow would have to be in contact with a 'higher up metal' for there to be electrolysis? If so what would that be?
Electrolysis is the term usually reserved for corrosion arising purely from stray currents from an external voltage source, and does not require a 'higher up metal'. In fact, if the voltage is high enough in the correct direction, a 'lower' metal can consume a 'higher' metal.
I'm guessing you're thinking of what's usually termed "galvanic corrosion", where the voltage source is provided by metals with higher and lower galvanic potential.
Carbon, rather surprisingly is (for the purposes of galvanic corrosion) a "higher up metal" than almost anything else, certainly higher than both active and passive conditions of stainless steels. It is often overlooked.
Carbon attaches to the interior
of the elbow at the point where the cooling
water causes the exhaust gases to condense. I have seen instances where this has caused a stainless elbow to pit from the inside outwards at this location.
Not sure if that's what you're seeing, though: it doesn't look like random pitting.
If I had to hazard a guess from the photo
I'd probably try to establish whether a crack (perhaps arising from incorrect weld filler rod, or procedures) has allowed the passage
of moist gases from the inside to the outside, where they have crystallised.