How to evaluate dezincification of bronze fittings?

[Sonrisa]

Hi All,

Our boat has a bowsprit with a bronze bobstay and bobstay attachment fitting (the attachment point at the water line).

We hauled out yesterday, and upon inspection, the bronze clevis pin holding it all together had deteriorated badly. When I tried to remove the stainless cotter pin, it just ripped through the end of clevis pin, as shown in the attached pictures.

I'm wondering how to evaluate the integrity of the other fittings. They're all covered in old green and blue bottom paint in the photos so it's hard to evaluate the surface.

When bronze loses integrity, would it be visible in the surface layers? Any other tips for things to look for, or tests I can do?

I'm going to have some professionals in the yard take a look next week, but always appreciate more opinions.

Thanks in advance for any advice.

[Manateeman]

Hi. There are a lot of bronze alloys. Some better than others. First thing I see is a stainless split pin. Mixed metals underwater...somebody has got to loose.
I built a 32’ cutter with a bobstay connected to the hull with two 3/8 inch thick plates. 1/2” button head bolts to the hull and a 1/2” through bolt between the plates. Merriman toggles to the bobstay. They’ve been immersed in the sea non stop for 20 years. Green, but no indication they have lost any significant amount of metal. All the Apollo seacocks work. The boat has been at anchor for years.
Her exterior chainplates are silicon bronze. Nice patina. That’s it.
I’m sure someone here will post a complete list of all the bronze formulations. I just ask for silicon or navy bronze and while I can see a slight difference in color, I’ve never had an issue.
Mark

[tkeithlu]

First and foremost, get down to the metal. If there is a problem, it will be visible post cleaning. My bet is that you'll find that the damage is limited to the pin, and that Manatee Man is right on switching to a bronze cotter key.

[s/v Jedi]

Like said above, remove some paint and corrosion to inspect the color of the bronze as well as surface texture.

There are many better choices than stainless steel in contact with bronze. While bronze itself is the logically best choice, Monel (cupro-nickel) would probably have been fine as well.

[BillKny]

Just looking at the photos of the pin, tells the story. That dull, copper color with a grainy appearance is all you need to see to understand that whatever alloy that "bronze" pin was made of was totally unsuited to the environment it was in.

The other parts look good, but a bit of polish will tell you. Any copper color that you find under the green surface is bad.

I will diverge from the other posters and say it is very unlikely that the stainless cotter pin could be the cause of the issue here. The problem was poor material in the pin. Proper bronze alloy and stainless are put together in lots of places on a boat without issues. Think bronze props on stainless shafts, among many, many other places. What might be missing here is good anode protection. Where is the nearest zinc? And is it bonded to these parts?