Titanium with steel bimetallic corrosion in chainplates

[roaringgirl]

Our boat's chainplates are stainless steel. They're long plates that are glassed into the hull and protrude up through the cabin and protrude up through holes in the GRP deck to attach to the rigging. I've re-sikaflexed the holes in the deck a few times over the years and I've observed some markings on the steel at deck level. I think it's probably time to replace the chainplates. I'm certain that the below decks sections are totally fine and also a lot of hassle to replace, so I'm thinking about replacing the through-deck section by grinding off the top few inches and replacing it with short section attached to the old main-body with a yoke and pin.

My questions are:
1) can anyone see anything wrong with this approach?
2) should I make the new short top section out of stainless, or would there be an advantage to using something more corrosion resistant (eg Ti).
3) if the answer to (2) is Ti, would there be any corrosion issue, having it in (dry) contact with stainless?

 

[vyv_cox]

I am not much of an authority on titanium but know it has remarkable corrosion resistance. Assuming the section you want can be purchased at a reasonable price and has sufficient strength I see no problem with it, although not sure of the advantage. My stainless chainplates are almost 40 years old and are not corroded at all.

There are no corrosion issues between any metal pair in dry conditions.

 

[roaringgirl]

Assuming they're original, mine are of a similar vintage, and are half way through their second circumnavigation. I'm concerned with the lines and ridges that are visible when I dig out the old Sikaflex in the deck-seal.

 

[roaringgirl]

Ah, I just realised the top will be in the elements where it sticks through the deck and attaches to the bottlescrew. Does this change your answer?

 

[Spirit (of Glenans)]

Even though it's below decks it's probably a good idea to take the precaution of using Tefgel or similar, when joining the new parts to the old, even if only to prevent the fixings galling.

 

[Tranona]

Assuming they're original, mine are of a similar vintage, and are half way through their second circumnavigation. I'm concerned with the lines and ridges that are visible when I dig out the old Sikaflex in the deck-seal.

That may well be crevice corrosion from water seeping in past the sealant over the years. Quite common in chain plates of that design. Some designs have caps over the plates to reduce the chances of leakage from the sealant breaking down. Not sure about your plan as any serious corrosion will be lower down where the chain plate is actually in the deck.

 

[roaringgirl]

That may well be crevice corrosion from water seeping in past the sealant over the years. Quite common in chain plates of that design. Some designs have caps over the plates to reduce the chances of leakage from the sealant breaking down. Not sure about your plan as any serious corrosion will be lower down where the chain plate is actually in the deck.

My plan is to grind the top few inches off the plate, a few inches below the deck, in the saloon, then attach a new through-deck section (perhaps of Ti) securing it to the old stub using pins.