"Aggressive Pitting in Hull"

Question:

"I would greatly appreciate some input as to why our dredgeboat has suddenly developped several pits in her bilge. These pits are about 2" diameter and 1/4" deep and have an exposed white metal surface. The hull metal is 3/8" ABS grade A-36 steel plate. These pits have appeared suddenly (within the last 2 months) - and one of them has already sprung a leak. The pits are all located in the same area - in the deepest part of her hull - this part of the bilge is always filled with oily water. Question 1: Is it possible for a bilge microbe (eg. sulfate reducing bacteria) to be this aggressive? Do they ever leave pits this large? Does microbe induced corrosion typically leave a near white metal surface? Question 2: If stray current corrosion is the culprit - Would pits on the interior of the hull be consistent with an outside current passing thru the boat's bilge before finding its way to ground. (As opposed to a stray current originating within the boat). The vessel is docked in brackish waters. Interestingly, the metal components in the bulkhead have also undergone rapid corrosion. Several vessels are docked nearby, including one with an aluminum hull protected by an impressed current system (continuously on). There is also a fleet of tugs docked nearby. Any one of these boats could have a continuous or intermittent "stray current leak" - which could find its way to the surrounding waters."

Answer:

Microbiological induced corrosion (MIC) can be aggressive to carbon steel, especially in water containing hydrocarbons, as the hydrocarbons act as nutrients. Yes, they have been known to leave a deposit free surface. You may want to test for MIC activity. I.D. pits would not be consistent with stray current corrosion from an external source, unless the stray current was able to leave the metal at the I.D. Typically, this would not occur as the resistance of the metal is much less than that of the water.

 

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