Stainless steel screws under epoxy !?

**** I better pull my prop shaft out lest it get scratched. Gimme me a break... This applies to fastenings in certain situations was all I was saying, you seem to want to miss read that. Keel bolts are another widely used example of stainless in critical below water line application.

Stainless can be anti fouled and is anti fouled regularly, you just gotta use the right product and have anodes in place if dissimilar metals are present.

Man if you have stagnant water in and epoxy strip plank hull then you have bigger issues than a little bit of corrosion in what will be redundant fastenings by then... after all the damn thing is supposed to be held together by the epoxy.

Dogmatic little... aren't we.

 

Need I say, that if I had loads of cash, I'd have everything metal done in titanium? It's beatiful, and it's inert - even in saltwater. Did I say it was beautiful?

 

Man your rude, work on your comprehension where did I say anything denying crevice corrosion?

 

Where the hell did I say that?

 

Excuse me, but since we're talking about screws and fittings below the waterline, I'm hardly the one misinterpreting anything. On the other hand, you don't seem to understand that a fast moving shaft will hardly suffer from crevice corrossion, and as such, your motoring example is less than valid.

Yes, and Lloyds even demand them, if I recall correctly. But try doing a google search on it. Or, hell, merely a search on this site about SS keel-bolts. They're a problem.

Someone has STILL not read, nor understood crevice corrossion. I think it's about time you did some actual research. People also install nylon through-hulls, that doesn't mean it's great. You're using the exact same argument now: That because people antifoul their stainless, there's no such thing as crevice corrossion. Yet, you refuse to do even some basic research.

Yes, but a scratch will do it. Please do read some of my links, and otherwise se google.

Not really. I'm not the one making unfounded claims about crevice corrossion only being a problem in old-school water-ridden wood boats. You are.

 

Haha, I'm rude!? Seriously, you're claiming that it's not a problem having 316 stainless below the waterline. You refuse that water can get behind a fitting, and state that it's only a problem in a water-sogged wooden hull.

Speaking of having to work on ones "comprehension". I'm not the one refusing to read up on the topic, refusing to understand it and refuse to accept that stainless isn't a wonder material. The smaller the crevise, the bigger chance of having stagnant sea water.

When the topic is crevise corrosion, I think this constitutes an answer:

Btw, galvanic corrosion is a whole 'nother matter, just to make sure, we're on the level on that point.

 

You made this blanket statement with no reference to fastenings, that is what I responded to.

Man your rabid... all I did was point out that SS is widely used below the water with good service life and success. I conceded that fastenings in certain circumstances are prone to corrosion. In your circumstance it sounds like you have pulled your knickers over your head.

On the 304 SS thing, it was always drummed into us never to sell it for below waterline use. That I have never done so yeah that might be acceptable, I don't know that for sure but we did as our suppliers instructed and I have always taken that as correct. You live you learn....

I'm not refusing anything.... your just a dogmatic fecker to have a conversation with. I think its a cultural thing... CYA

 

And you're the one trying to ride the high horse about comprehension? How about a little context? You're in a thread that's dealing with crevice corrosion - something that corrodes SS in stagnant seawater, and when a crevice is there. "Stagnant" of course doesn't mean water around a fast turning prop-shaft. Sheesh! About the fittings, I'm talking about a small crevice on the rear, for instance. You're in essense making a strawman argument now.

Yes, you used examples like your prop-shaft, denied that there could be a problem with stainless through-hulls by way of linking to one, completely ignoring that a through-hull has to sit in something, opening up the potential for crevise corrosion, and so forth. You even went as far as making claims that one shouldn't worry about crevice corrosion in an epoxy-hull, as you would have bigger problems by then (insinuating that by then, the hull must have come apart), completely ignoring that a scratch is all it takes for crevice corrossion to take place.
Further, you confuse crevise corrossion with galvanic corrossion and make statements that it's perfectly fine painting stainless because people do it.

Ah, yes, what a concession. You made a blanket statement that only saltwater sogged wooden hulls would need to worry about crevice corrosion.

Well, it's better than putting my head up my rear.

No, it's sort of correct. The 304 will corrode (as in corrode where you can see it) faster than 316. However, 316 corrodes even faster than 304 if oxygen starved (and that happens where you cannot see: In the oxygen starved places).
[edit: 316 is better at resisting galvanic corrosion, but that's an entirely different matter]

Oh, haha. That's bloody excellent, calling me a dogmatic ****** after saying and reacting like you have done in this thread - especially since you have refused to read the links where crevice corrosion is explained. Making one nonsense-statement after another. You may not calling that "refusing", but I do - otherwise you would have read them.

 

Got this in my mail, but it's not here …

Isn't this swell? You think it's hard to figure out if someone knows something about a given subject? Go back and read the thread, and it's obvious to most people, how much you understood in the course of this thread, and that you didn't read the links provided when they were provided. One doesn't have to be brilliant to "figure" that. Of course, I am brilliant, but that's not the point …

 

The simple explanation about stainless under water is, oxygen is available in two forms, captured and free. Captured oxygen is when the molecule is attached to another, like in water, where it's attached to a hydrogen molecule. Free oxygen is as the name suggests and isn't attached to anything.

Free oxygen permits stainless steel to form a oxide coating on it's surface which helps prevent rust formation (as has been discussed). With captured oxygen in water, the stainless is prone to rust. In a shaft/strut situations, I suspect there is sufficient turbulence to allow some free oxygen to react with the stainless, which is why they seem to work. If you left the boat berthed without using the boat (assuming it's new and hasn't develop much of a coating), it will rust about as quickly as most hardened steels. Clean it off and expose it to free oxygen by use or haul out and the coating will form, protecting it.

Stainless fasteners in wet wood are a different matter. A tightened down stainless fastener in wet wood, is subject to the corrosive effects of water, again like any hardened steel. This time without any chance of getting turbulence born free oxygen to develop a protective coating.

Buried in epoxy, say in a bonded hole, technically the fastener should be okay. In reality, they do live longer then mild steel, but seem to last about as well as galvanized steel, weeping rust after a few years of emersion.

Stainless fasteners are sized much the same way steel plating is on metal hull shells, which respect paid to the amount of material lose you could expect, over the service life of the fastener. In other words, they're big enough or spec'd sufficiently over size to tolerate the corrosion, while retaining the strength requirements for it's loading.

 

Thank you PAR, hopefully things will settle down till the next time......

 

Thanks, Par.

 

Is it worth the worry

Hello Danish

Speaking with my Chemistry teacher hat on I think you will find that electrons are involved in all corrosion as in all chemical reactions. Cathodic protection is actually a form of preferential electron donation - the metal higher on the activity series reacts with oxygen or another electon acceptor instead of the baser metal. Dezincification is much the same thing - the more active zinc reacts and goes from being a non charged metal solid into a charged metal ion donating 2 electrons. The oxide coating on stainless acts as a barrier to chemical reactions. Think of the epoxy as a really big bubble of oxygen making lots of impermeable oxide.

Burying the whole lot under an insulator and a lump of muck is a great way to stop any chemical reaction.

Anyway I think your problem is not that much of an issue. If the outside laminate gets compromised -cracked or chipped - that is bad. Water could move along the face of the cedar. If any water gets into the end grain of the cedar - that is very bad. These would be higher on the priority list for most builders and repairers, and for me than a worry about the screws. As Meanz says if water is getting into your laminate then screws are the least of your worries.

Phil Thompson

 

Phil, moisture tends to be a problem below the waterline, whether you like it or not, and that doesn't mean that one would have "bigger problems to worry about". Again, I bid you to look up crevice corrosion. It should be pretty easy for you to understand, being a chemistry teacher and all

About the electrons. Yes, I guess one could say that any corrosion is because of that, but the problem is, that most people only mention that when they think of dissimlar metals in need of anodes (bronze and aluminium together, for instance).
You also write this:

Yes, in theory at least. However, stainless need to have access to oxygen in order to protect itself. If not, you will have oxygen starvation (i.e. NO protective layer), and just a little seawater moisture added to that, and it will corrode very fast.
In my mind, it continues to be a ridiculously ignorant "argument" to state that if I worry about crevice corrosion below the waterline, I will have much bigger problems on my hand. It simply shows, that the ones putting forth such pseudoargumentation really don't understand what crevice corrossion is and why it's something to "worry about". In fact, on my own boat, I got around that problem by simply using bronze screws = No crevice corrosion.

 

Don't get it

I am not enjoying this thread as usually contributions in good spirit do not usually elicit overly assertive responses. I only contributed because I thought you wrote your boat had stainless screws and felt that any angst you had would be better spent worrying about the protecting the laminate. I know assume you built your own sistership. I didn't find this clear in the thread.

As a builder of 4 composite boats, a long term cruiser and a teacher I thought you had better things to worry about than stainless screws in your own boat. I could go on about the Chemistry of boatbuilding metals and I could even invite you to do some classes in it. I teach a course in shipwrecks and salvage and well understand the mechanics of corrosion and I come up with a different perspective from you. That does not make me wrong or stupid - just different.

Phil Thompson