Problem with Negative Terminal Corrosion

[DefinitelyMe]

Greetings! I have a problem with corrosion on and around the negative battery terminals only. Furthermore, this corrosion only appears to affect certain metals such as the bolt and wing-nut that secure the wires to the terminals. If i can figure out how to attach a picture i will.......

Has anyone come across this problem before?

Bit of background:

It's been ongoing for about a year and a half now but nothing has changed in the battery system for about 4 years. Both batteries are showing this problem. Initially the boat was out of the water and batteries were charging via shore power. I figured it was a grounding issue but it didn't go away when it was re-launched. Now charging via solar. Batteries are 2 x lifeline 8Ds (AGMs). Battery system is simple. Batteries in parallel, grounded through engine (no corrosion at the engine terminal and the zinc on the end of the prop shaft is fine. The one in the heat exchanger always disappears pretty quickly so i replace that about twice a year and have been doing so since before this problem occurred) and connected to panel via a negative collection plate. On the positive side it goes from the batteries to a switch (which is always left on) and then to the engine and panel distribution. Any ideas at all would be much appreciated! Battery performance is still excellent, but something bad must be happening surely? My chief concern is that more reactive metals in the engine are corroding like this but i just can't see it.