Understanding Galvanic Corrosion - help

[Tranona]

I was wondering about protecting bronze rudders by coating in epoxy and anti fouling over the top. But I was told this would concentrate galvanic action on the tiny gap on the shaft where the rudder enters the hull, as this must remain uncoated if the rudder is still to move easily. Is this true?

If coating works to help prevent galvanic action, should the covered parts still be connected to a separate anode in the normal way - just to be on the safe side?

It is not possible to answer your question without knowing the actual alloy used in your rudders. The brass types such as manganese bronze (which is commonly used for P brackets) have high zinc contents and will dezincify. However, isolating from seawater as described in this thread can reduce or eliminate dezincification. Other alloys do not and will not suffer from galvanic corrosion on their own, but may do if mixed with other metals. So, I think the answer is that there is no clear answer.

 

[MYStargazer]

It is not possible to answer your question without knowing the actual alloy used in your rudders. The brass types such as manganese bronze (which is commonly used for P brackets) have high zinc contents and will dezincify. However, isolating from seawater as described in this thread can reduce or eliminate dezincification. Other alloys do not and will not suffer from galvanic corrosion on their own, but may do if mixed with other metals. So, I think the answer is that there is no clear answer.

Thanks for that. There's no way to find out which type of bronze was used, but it's been an education to learn that ordinary brass will self-destruct in seawater by itself (which explains why brass plumbing fittings are such a no-no).

Assuming that those areas which has been epoxied will remain sealed, the only concern is a concentration of galvanic action around the unepoxied shaft as it enters the hull. I doubt there's been any research on this, but there must be a reason why bronze rudders (and bronze props and stainless shafts for that matter) are left unepoxied...?

I have mild steel p-brackets, which I plan to have shot-blasted, galvanised, epoxy primed, epoxy top coated, then copper coated.

 

[vyv_cox]

Here is the simple example of galvanic effects, used on our training courses. If you put an aluminium rivet into a steel sheet, immersed in seawater, the rivet will corrode quickly whilst protecting the steel. If you put a steel rivet into a sheet of aluminium and immerse that, the steel will again be protected by the aluminium. Corrosion of the aluminium will be very slow thanks to its much greater size.

Illustration of Tranona's post.

 

[viago]

Almost exactly what I do. Bonding paint to manganese bronze seems to be quite difficult and my first two attempts were unsuccessful. Then i found that Hammerite special metals primer sticks to it very well, with prop antifoul over that.

so, can i assume that using cheap brass skin fittings could well be ok if painted prior to fitting with hammerite smp and antifouled after fitting?