Calling metallurgy experts! Mode of failure? Galvanic corrosion or something else?

[MagicalArmchair]

The rudder on Mirage (Bavaria 40) got really really stiff and creaky coming back over the estuary on our latest adventure. To cut a long story (very long in fact (Whitlock steering squeak)) short, I had to replace the lower bearings as they had been crushed onto the stock by corrosion of the aluminum housing. The acetal bearings needed to be carefully chiseled out they were so wedged in there. As soon as they were out, with a bit of sanding clouds of white dust, came from the housing, and it looked as below:

Questions:

1. There was copper-based antifouling right up to the edge of the stern tube, some might have got onto the tube itself (it was painted with Trilux (non copper based antifouling)). My conclusion was that this created the above corrosion, so after replacing the bearings I have stripped it right back 10mm clear of the tube. Is that a sound conclusion? Does that look like corrosion from
2. Do I need to bond the stern tube to an anode? By doing this, it will electrically connect the stern tube to the anode and other steel items on the boat.
3. How else can I prevent this corrosion from happening in the future and crushing my new (expensive) bearing? (I will now be dropping the rudder every three years to eyeball the bearings)

 

[Tranona]

Yes, it is the action of the copper with the aluminium, hence the advice not to use it. No an anode will not make any difference as once you remove the copper there is only one metal so no galvanic circuit can form. That is why rudder tubes (I assume that is what you mean by stern tube) tend to be GRP and bearing housings aluminium.

There is no need for any anode on the boat apart from the saildrive housing and maybe the prop if it is a folder/featherer. You will, however find a second anode on later boats that is used as a ground for the 240v system, brought in to comply with the RCD revisions in I think 2011 - but not connected to anything else.

You have done the job properly and unlikely there will be any further corrosion.