The zinc is leached out of the copper. It is the remaining copper that gives the pink colour, not the other way around. I'm also going to guess that your bronze is less bronze than brass like. Good bronzes have a very small % of zinc, maybe 5%, to none. Good bronzes replace the zinc with tin and other metal which are much closer on the galvanic scale to copper.
Manganese 'bronze' is about 20-40% zinc, is really a brass, and it is likely what you have for the strut. Some brasses can be good if used properly and protected. Witness many manganese bronze propellors which last for years IF they are protected.
Nonetheless the metals should be zinced. There is lots of metal in the strut so if it looks like mostly surface then you have years to go, zinced.
I see you have installed a prop shaft zinc - good. You might even go fo a second one. Just keep it at least two to three shaft diameters from the cutlass bearing.
I'll wager that you lost electrical continuity between the various parts. Even if the bolts were tight eventually seawater can creep into the joints and create enough oxidation to cut the continuity through the joint. The joints should be disassembled, cleaned and redone. I may be off base here but I think you could use a sealer or caulk to keep out the water for a long time. The bolting pressure will squeeze the bulk out leaving a metal to metal contact but close off the remaining gaps, and they willl be there, to water entry. Confirm it with an ohmmeter.
All the connections internal should be cleaned and checked. If any are the least bit dirty or oxidized they may lose continuity and thus protection
The voltages here are quite low so it doesn't take much resistance to upset things. An ohmmeter, at any connection, should show less than 1 ohm.
You might also do some quick checks to ensure you don't have some DC electrical leakage from something like a faulty bilge pump or switch. They can leak into standing bilge water if the insulation or seals are compromised in any way. From your description this may not be the case but be sure.
If any bolts from the forward parts of skeg and strut come through the hull and you have access to them you may be able to check the zinc protection levels over the year. Silver-silver chloride half cells are available over the net and with a dmm can be used to monitor. I think I posted a link which included how to make one by going to a jewellers store for a few bucks.
This subject can be huge and the variables widely variable
so if you have any further questions and it appears that you still have a problem get someone to give you a hand and repost.
Your transom zinc should be checked also for CLEAN mounting and continuity to the internal bonding. I had a fellow do mine for a couple haulouts and one of the zincs didn't waste away. Next H.O. ohmed it - isolated - so I pulled it and cleaned the mount with the grinder and a disc. I've also found that sometime I have to grind the zincs, the contact area, as any zinc oxide will interfere with a good connection.
Hopefully a full cleanup of all bolted connections and wire connections will take care of it. But you can do a lot to check in the meantime, before next H.O., to see if you have the problem under control.
Good luck any good boating.
Clark
EDIT:
*** There are microohmeters but the last time I checked they were pricey enough that even though I drooled , I decided I would have to do without.* Too close to retirement even though I could have used it a bit at work, I couldn't justify it.** Take a look if any are interested as a few years have now passed and some amazing tools and meters are 'affordable' compared to just a few years ago.
-- Edited by C lectric on Friday 25th of February 2011 11:30:16 PM