Here is my rationale:
When I bought my boat, the thru hulls were bonded by the original owner. He owned the boat nearly 40 years and did everything right. It's become my rule of thumb on this ongoing project to just copy what the old guy did and I'll be fine. So, last year, a different guy who brokered the sale to me told me that bonding will not be necessary once we replaced the thru hulls (Groco). I went with that, and now 18 months later, I see that there is some green on a couple of the thru hulls. This tells me there is corrosion, so I bonded them yesterday. I will watch closely to see if anything changes. The question is, if they are correctly protected, then how long would it be before I begin to see the signs of corrosion? Also, since I am basically getting the boat ready for the next owner, I have to think in terms of what the surveyor will report. Maybe I should run this by a local who knows (Los Angeles).
Green is not necessarily corrosion at all. It is "verdigris" the green patina all bronze, copper and brasses can undergo when exposed to wet salty environments. The statue of liberty is a prime example of verdegris...
If your seacocks start to turn "pink" they you need to start worrying.. If you mixed yellow brass or any dissimilar metals in your seacock system then you can begin to worry. The most often "mixed" metal in a seacok system is when people try to cut corners on the male adapters and use Home Depot quality 60/40 yellow brass.
The thru-hull, seacock or ball valve body AND the male adapter or ANY nipples or elbows should all be of 85-5-5-5 bronze.
The green, in and of itself, is not a concern and is normal. It is what may be under the green that would be cause for concern. Green is ok but green can hide real corrosion so green can be bad if the seacocks are not cleaned and examined every now and then. Heck look at the bronze portlights of any older Cape Dory they are green with verdigris but are not submerged in an electrolyte.... If you want to minimize verdigris it slather a clean seacock with Lanocoat...
This is a good example of why articles like the WM Advisor/Honey article are so dangerous. We don't even understand the difference between a natural patina of bronze in a marine environment and what real corrosion looks like.
Below is a picture of dealloying or dezincification of brass. This is real corrosion and quite dangerous. Note the pink color to what used to be a goldish yellow piece of brass. This is why brass should not be used below waterline for seacocks, bonded or not. The "green" on this valve had nothing to do with this corrosion. The "zinc" content in the brass, upwards of 40%, clearly became the sacrificial metal to the 85-5-5-5 bronze thru-hull fitting or other more noble metals.
These were not bonded and if they were they would have likely survived better, in this particular application. Again, no one size fits all... When the wrench was put to this valve it literally crumbled because there was no zinc left in it. Proper 85-5-5-5 bronze seacocks are VERY, VERY, VERY resistant to dezincification. I have many customers with 40+ year old tapered cone 85-5-5-5 bronze seacocks in as good a condition, corrosion wise, as when they left the factory