What causes transom bolts to corrode, and not the Anodes?

[dorelse]

So...the 18 appears to be staying with me...going to keep progressing.  (Honestly, for $350, I thought it'd find a good home before winter.  Thinking I scared everyone off with the 'whole package' price...anyway...back to the project.)

The 2 bottom transom bolts have anodes on the ends.  They look pristine, no corrosion at all.  Both bolts however have quite a bit of corrosion on them...and its not rust, the metal has been eaten away.  Enough that they've been weakened quite a bit, and are unusable.

I thought the whole point was that the anodes would protect the bolts?

[Rich_V174SS]

Those two bottom bolts always corrode because of water that gets trapped in between from the inside of the boat. I think they're aluminum and sometimes they just break when you try to remove them pulling the transom assembly apart. It's no big deal to get new ones and I don't think they're expensive.

[dorelse]

Thanks Rich...there was a leak there, so that makes perfect sense...water getting trapped and allowing the bolts to corrode.

[Rich_V174SS]

There's also supposed to be a rubber "gasket" washer between the square end of the bolt and the transom shield to keep water out, make sure you get those as well. I know Mercury sells an installation kit which contains all the bolts, washers, and nuts for re-attaching the transom assembly, I don't know the price though.