read that one it explains pitting ....

I just hope its not electrolosis which can be cause by lead etc sitting on the ali and the dissimilar metals reacting.

1) The alloy is not good quality marine grade. We have this on one half of a Ramco hull. One side perfect, one side with pitting that we are sorting out

2) There were pencil lines left on the hull. Apparently lead pencil if not cleaned off can cause pitting

3) There has been a sinker or some other foreign metal inside the hull.

Are you looking under the floor, or in the open bilge? I would also send the photos to Marco and get their feedback as well.

So photos would be good mate.

 

I also wonder if sacrificial Zinc Anodes would be sensible on an alloy hull... anyone know?

 

Other theories are an earthing wire off the hull to a ground rod when the boat is parked/stored.

 

If it is chemical corrosion of the ali (as in cases where a non-marine grade plate found it's way into the stack going thorugh the marking machine and ended up getting sent out with the rest of the 5083) then you end up with areas that look like white powdery worms have chewed into the ali.  You grind them out for welding and end up with slots to weld up.  Real ugly...

 

In your case as it appears to be inside the bilge in an area that doesn't get exposed to the full seawater effects of electrical conductivity and electrolysis I would be interested to find out what you use to clean the boat or if there has ever been any unusual cleaning products or chemicals spilt inside the boat.  As the chemical corrosion of ali normally results in white powdery deposits I would put forward the opinion that something has been spilt in the boat that has reacted with the ali as it flowed down into the stern, hence the amount of corrosion increasing towards the arse.

 

Once the white powdery deposits have been cleaned off you end up with pitting.  I would measure the pits, if any are more that 15% of the plate thickness consider puddle weld repairs.  If they are shallow, clean back with a wire buff in a drill and treat with an ali corrosion preventer and monitor them. 

 

 

Alright there ya go!

 

The last couple are of bubbling paint i have on a quite a few patches around the boat. Im not sure if this has anything to do with it or not.. I have a feeling I dont want to know whats actualy going on.......

busted, could you elaborate  a bit on where to get this corosion preventer and what its called ..