stainless steel keel bolts

There've been a few posts on keel bolts - notably Lydia's on dropping and refurbing SS34 Defiance's keel www.seabreeze.com.au/forums/Sailing/General/Defiance-story-continues-?page=2

and also www.seabreeze.com.au/forums/Sailing/General/Is-your-keel-really-about-to-fall-off--Featuring-Marine-Surveyor-Ben-Sutcliffe-Davis?page=1#19

As the owner of a boat with stainless steel keel bolts, I've been looking into this a bit.

We tend to think of stainless steel on boats as 316. But we also know that even 316 stainless rusts ("tea stains" on pulpits and so on), and in particular that it can suffer crevice corrosion, which is corrosion of stainless that occurs in the absence of oxygen. 316 is not intended to be constantly submerged in saltwater.

For those with stainless keel bolts, crevice corrosion isn't an issue if the bolts are, and always have been, kept dry. But of course while the keel may be well sealed on the outside, bolts are often exposed in bilges, and so there is a possibility in older boats that seawater has worked its way down the bolt shaft over a decade or three.

In looking into this I've become aware that there's a lot more to stainless steel than 304 and 316. And in particular there are stainless steels that have been specifically developed to survive constant submersion in saltwater - for example in oil and gas rigs and desalination plants - and even more corrosive environments (eg some kinds of chemical plants).

The measure or resistance to corrosion in stainless steel is the pitting resistance equivalent number (PREN). Pitting resistance is also relevant to resistance to stress and crevice corrosion. It is generally agreed that alloys with a PREN of 40 or more are able to resist pitting and crevice corrosion long term (for many years).

PREN for 304 is generally 17-21, which is why it isn't much use in marine environments.
For 316 PREN is generally about 23 to 29. This is good enough for exposure to sea water, but it will still rust a bit, and it is not intended to be constantly submerged.

Both 304 and 316 are austenitic steels. This relates to the nature of their crystalline structure.

Another type of stainless is known as Duplex. The name comes because it has a hybrid crystalline structure - both ferritic and austenitic.
Duplex 2205 generally has a PREN of 31 to 38, and there is a subset of Duplex known as Super Duplex 2507 which has a PREN generally between 38 to 45. There is a trade marked Super Duplex alloy - Zeron 100 (UNS S32760) - which includes chromium, nickel, molybdenum, nitrogen, copper and tungsten and has a PREN greater than 40.

And there is also a "super austenitic" stainless - AL-6XN (UNS N08367) - which has a much higher nickel content (24%), while still having 20 -22% chromium, 6% molybdenum, plus traces of Nitrogen (0.18-0.25%). AL-6XN has a PREN of 43 to 46.

It is possible to source stainless steel bolts made of Duplex 2205, Super Duplex 2507, Zeron 100 and AL-6XN. These are much superior to 316 in terms of corrosion resistance, including resistance to crevice corrosion. And as luck would have it, they are also significantly stronger than 316 - some are twice as strong.

Another consideration is that duplex stainless steels are magnetic. They are not as strongly magnetic as iron or ordinary steel, but they are not as non-magnetic as 316. If this is an issue, then consider AL-6XN (UNS N08367) which as a type of austenitic stainless, has the same non-magnetic properties as 316.

Of course these varieties of stainless steel are not generally available in your local shop, and they are more expensive than 316.

However, if you are nagged by doubts about your stainless steel keel bolts, such that you plan to replace them, then you may want to consider getting replacements made of one of these stronger and much more corrosion resistant alloys.

If you stick with 316, another consideration is passivation. This is treatment - often with acid (usually nitric, but can be citric), but can also be done in other ways - which removes surface iron and causes formation of a thicker chromium oxide layer that gives stainless its resistance to corrosion. It is used where 316 stainless (and other stainless steels) will be exposed to a more corrosive environment. Done properly, it increases resistance to corrosion including to pitting and crevice corrosion. It doesn't completely prevent corrosion.

PS I should note there are other stainless steel alloys than those mentioned that also have higher corrosion resistance than 316. One is "hyper austenitic" UNS 32707 which has a PREN of 44 to 53. Other examples include the various "Incoloy" alloys produced by the Special Metals Corporation in the US (www.specialmetals.com/documents/aqueous-corrosion-handbook.pdf).

Obtaining bolts in many of these specialist alloys would likely require finding an overseas supplier willing to fill a small order, which may prove a challenge.

Cammd did say both bolts and frame in keel on Wapiti were 2205. 2205 is a duplex stainless which is more corrosion resistant and stronger than 316. Not magnetically neutral however.

Duplex steel has been found to be more corrosion resistant in seawater than naval bronze (NES747) which is in turn more corrosion resistant than 316.

The 2024 article (Lyons et al) you referenced in lydia's keel thread suggested bolts should be tightened to apply pre stress force FPS: FPS=0.7 sigma-y Sn where sigma-y is the yield stress of the bolts, and Sn is the cross-sectional area of the bolt's neck.

Select to expand quote

r13 said..
IIRC Cammd's keel and presumably keel bolts are SAF 2205 - uts 800MPa.

Typically A4-80 and 2205 bolts have such markings on the head.

No markings on my keels bolts but they look they have been custom made.

The nuts in the bilge have 316 stamped on them. I had chat with Graham Radford about the wisdom of dropping the keel to do an inspection on the bolts. He didnt think it would be necessary for a long long time unless there are signs of a problem. The bolts are basically welded to the frame.

His personal yacht has a similar arrangement and he didnt expect to ever require dropping the keel whilst he owned and sailed it so unless I see signs of something going on I don't plan to drop it anytime soon either.

Mechanical engineering perspective (I do not own a yacht, just have a keen interest in sailing when i can)

If given the choice on a new yacht i would be going 2507 for anything critical that is difficult to inspect or replace.

Prop shaft - somewhat serviceable consider 2205 of ss316 and inspect regularly if cost of 2507 os prohibitive
Rudder stock - as above
Keel bolts - 2507 if inspection and replacement regime cannot be established or if any doubts as to integrity of keel to hull seal.

The limitations of ss316 can be overcome with correct design such as keeping it dry or making it a serviceable component. In this way we balance cost and servicability.

Ss316 is not really adequate for chloride levels found in seawater for long term serviceability. If you can access it, clean it regularly with fresh water and it is exposed to oxygen, 316 can maintain its passive chromium layer for a certain time and be ok.
Stagnant and low oxygen wet environments kill ss316.

If you have ss316 bolts priority would be maintaining the integrity of the seal between the keel and hul and keeping bilges dry

We do not use ss316 in desalination plants because it is no good for chloride levels above about 700 at marginally elevated temps. There is a graph on this you can google.

I have seen ss316 pipes fail with 500 to 800ppm chloride due to water sitting stagnant in them and or being heated by the sun.

With 2507 you must make sure that you do not contaminate it with ferrous or lower grade ss, overheat it or use incorrect shielding gas.

As with anything the material limitations need to be understood by the designer. Consult with a naval architect and metallurgist for specific application advice.


Cheers

Edit, I just double checked and it is 2507 super duplex we use on desalination. That is what the materials guys specify on seawater side piping.

But of course, if money's no object, why not build the entire hull out of duplex stainless!? au.boats.com/sailing-boats/2024-hoek-78-7339052/ tanielle.com.au/