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Old 25-08-2018, 18:37   #46
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Chainplate how bad ?

I used to have them painted with Sherwin Williams Jet-GLO after finding out that Airforce 1, the Thunderbirds, Blue Angels and Gulfstream use it.
It was changed to save money though.

Sealant was just usually B2, not sure of brand name though, we used B1/2 and A2 and A 1/2 a little too.
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Old 31-08-2018, 07:18   #47
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Re: Chainplate how bad ?

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Originally Posted by NevisDog View Post
Some interesting insight into that brown staining beneath the fibreglass - a good warning to all of us.

Why then do you think bedding in flexible sealer will not harm the stainless steel by oxygen starvation at deck level, while glassing in is considered harmful? What makes the difference?
At the risk of topic creep, I find this question extremely relevant. After reading volumes about bedding, sealing hardware screws, etc. all out of SS, how does the oxygen starvation scenario come into play? It seems we're told to 'seal it good' to prevent water intrusion - but aren't we also sealing out oxygen?
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Old 31-08-2018, 07:31   #48
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Re: Chainplate how bad ?

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From my posted images on the forum. Zoom in on the diagonal line. This is the crack revealed by polishing

That is a line of pits, not a crack. There could be a crack at the base, and it might be a place for a crack to start, but the terminology is wrong. For it to crack there is would also need to be in a high-load and flexing location.



Eddy current and ultrasound are the workhorses. I don't use dye often anymore.


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Old 31-08-2018, 07:31   #49
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Chainplate how bad ?

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Originally Posted by Nightcrawler View Post
At the risk of topic creep, I find this question extremely relevant. After reading volumes about bedding, sealing hardware screws, etc. all out of SS, how does the oxygen starvation scenario come into play? It seems we're told to 'seal it good' to prevent water intrusion - but aren't we also sealing out oxygen?

Well if you donít seal out the oxygen you donít seal out the water either lol. Crevice corrosion occurs in the presence of salt water and the absence of oxygen. Either keep both out or let both in.

If you donít mind rain or salt water pouring into your boat whenever you take water on deck or bury the rail, by all means leave them unsealed.
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Old 31-08-2018, 12:15   #50
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Re: Chainplate how bad ?

As a (retired) aircraft engineer my philosophy was ďwhen in doubt, sort it outĒ. If your washing up sink leaks a bit, or your bunk creaks when you roll over, then not much can happen if they fail. What is the worst that can happen if your chain plate fails? Well I could not hazard a guess but nothing nice I am sure. These forums (fora?) have lots of good and sage advice. But at the end of the day you are in charge and lives, including your own, are at risk with such serious issues as you describe. Take it apart and make a judgement, if it is beyond your skill set then pay someone or ask someone that you trust implicitly.
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Old 31-08-2018, 17:54   #51
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Re: Chainplate how bad ?

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Originally Posted by thinwater View Post
That is a line of pits, not a crack. There could be a crack at the base, and it might be a place for a crack to start, but the terminology is wrong. For it to crack there is would also need to be in a high-load and flexing location.



Eddy current and ultrasound are the workhorses. I don't use dye often anymore.


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Licensed PE, 43 years machine builder, specialist in corrosion and metal failures.

You can’t see the crack in the poor resolution image. I noted you would need magnification to see it. Yes, there are also visible pits. That piece is now used by the marina riggers as a classroom example of the classic failure.

Oxygen starved crevice corrosion happens where sea water evaporates in a small crevice leaving a higher concentration of salt. This initiates a local galvanic cell. The deeper and tighter the crevice, the greater the galvanic attack. Within the crack, some molecules, crystals are less noble than the parent metal and sacrifice themselves much as zinc protects iron. The crack eventually penetrates deep enough that the remaining metal yields. Tension in the parts contributes as well to the available energy and hastens the attack. The small electrical local cell potential can actually be measured with very precise equipment.


Understanding this, it is critical to keep the water, especially salt water, away from the parts. Bedding is necessary to keep the deck from leaking into the cabin and to prevent the crevice from forming in the first place. One reason these failures appear in older boats is that we seldom disassemble, clean and rebed this joint. Flexure of the hull works the chain plates and eventually compromises the seal. Once water is held in between the chain plate and the escutcheon plate the salinity in the joint increases. This is why the crack is almost always exactly in a straight line at the base of that joint.
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Old 31-08-2018, 19:02   #52
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Re: Chainplate how bad ?

While on the topic of crevice corrosion (CC) in SS, can anyone answer the following?

Is there any NDT that can be carried out on say a pit in SS to determine if there is underlying CC? Ultrasound or X-ray or ....?

Can CC occur when SS is kept dry (like really dry and no salt crystals present)?

How much better is 2205 than 316 WRT CC?
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Old 31-08-2018, 20:03   #53
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Chainplate how bad ?

If you have the plate out, X-ray can certainly find a crack, for probably not much more cost than a new chainplate.
I donít think you can eddy current all SS and itís not guaranteed. Pretty sure you cannot eddy current 316 and may can 304?
Ultrasound may require the crack to go through the surface, and no all do?
Never seen Ultrasound used on aircraft for steel, composites, yes of course, but steel?

If you go through all the trouble to remove it, why not replace it?
I do not think you can do a through inspection if itís installed, a lot seem to break just under the surface of the deck, right were you canít inspect without removal.
Of course ones glassed in or just covered by woven roven like an IPís have to be removed at great expense, be foolish to put old ones back.
Replace them, if your taking out 30 yr old ones and replace with new ones the same, logically they ought to last 30 yrs?
That way you start the clock over, a failure mechanism is high cycle fatigue, particularly for a boat sailed hard, to detect s crack before failure requires short inspection intervals, how short? No idea, but not putting in new ones and not worrying about it for decades doesnít make sense to me.

Once salt ever gets to the area, even if you never let any water ever get in again, the salt will absorb enough moisture from the atmosphere to form that Cell. If you think not a bit of salt has ever gotten in, well you have been extraordinarily lucky.
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Old 31-08-2018, 20:19   #54
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Re: Chainplate how bad ?

^^ all good points however I was asking in general about SS, not just chain plates, e.g. a high cost item say a prop shaft or similar?
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