I had a steel boat I recently purchased hauled out. The bottom paint looked excellent and there were no obvious signs of electrolysis anywhere on the vessel. The vessel had been immaculately maintained.
While going over the boat with the boatyard manager we noticed a small area of flaking bottom paint, which was unusual given the condition of the rest of the bottom paint. Upon inspection it appeared to be a small plate of stainless steel inserted into the hull. The boat yard manager with decades of experience agreed it appeared to be stainless and a couple other very experienced people came by and agreed it looked like a stainless insert. The consensus was the bottom paint and primer coatings had not bonded properly to the stainless, but that left the question of why a stainless insert?
I contacted the owner, curious about why someone would insert a small area of stainless. There are numerous stainless rails, rub rails and other wear surfaces added in stainless on the vessel but usually any small steel repairs on the hull are done in the parent material.
He replied that it was not stainless, but electrolysis, and he suspected a transformer he had installed recently was causing it. The transformer is located directly above the affected area.
Upon ultrasound survey the affected area is just as sound as the rest of the hull. I have never seen anything like this, and I thought I would see if anyone on Gcaptain had come across it before.
Here is the affected area.
It is magnetic, but scraping will not dislodge the plated material at any thickness I wished to go. I theoretically understand how this could happen, similar to electroplating, but the mechanism in this situation escapes me.