electrolysis on mast/boom connection

Danny and Yvonne SIMMS

Hi all,
I have a problem I would like your collective wisdom on.
For a couple of years I have had electrolysis/corrosion occurring on the plates on the mast that receive the boom.
These two horizontal plates have a vertical bolt passing through them and through an alloy block through which a horizontal bolt passes attaching the boom.
The corrosion is exclusively on the plates on the mast.
I started by cleaning off and applying the appropriate aluminum paint systems. (And have done several times)
At the same time I did my best to isolate the stainless bolts from the alloy components with plastic washers. There are fiber inserts through which the bolts pass.
 This had no effect and the corrosion continued. Investigating with a multi meter I found up to 0.7 of a volt between the mast and the vertical bolt and the vertical bolt and the alloy block it passes through, however not the horizontal bolt.
 This voltage is enough to cause the damage.
On the advice of a marine electrician I drilled and tapped the various components (mast bolts, alloy block and boom) and connected them with electric cable which of course eliminated the differential but didn't seem to stop the corrosion. Logic seemed to suggest a "battery" was being created between this bolt and the block perhaps caused by degradation of the fiber inserts and invasion by salt crystals. 

So I had new ones made by a marine engineer using material he believed robust and with good insulating properties. After installing these I removed the connecting wires and joy... no differential. That was a few weeks ago. Now I find a differential of 0.4 of a volt...woe.
There is no differential between the two electric motors and the mast at rest. The corrosion is exclusively occurring on the plates on the mast that secure the boom

So now I have replaced the connecting wires and of course there is no differential. Question. Should I connect the mast to the bonding system that connects to the zinc anodes on the rudder.
A thought, does Amel connect the mast to the bonding system? if so perhaps I have lost a connection.
The corrosion is getting serious so I need to find a solution

Kind Regards
SM 299 Ocean Pearl

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