Stainless steel corrosion.

Titanium

vyv_cox said:
Titanium has excellent resistance to corrosion by seawater. Bolts made from it would be ideal for sub-sea use. The only case I know in which the metal corroded, on a ball valve in seawater service, occurred when the electric actuator failed, passing current through the valve.
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Excellent, thanks vyv thats what I was hoping, Ben also sent me a useful link http://www.keytometals.com/Article24.htm that seems to indicate that it would be a good choice especially if not placed next to stainless.

So billet machined plastic trim tabs held on with ti bolts and nuts it is then. We'd better start drawing them up in Solidworks and try to find a spare production slot on a CNC milling machine - Home jobs always take priority over real customer jobs don't you know :-]
 
Scarron said:
Excellent, thanks vyv thats what I was hoping, Ben also sent me a useful link http://www.keytometals.com/Article24.htm that seems to indicate that it would be a good choice especially if not placed next to stainless.

So billet machined plastic trim tabs held on with ti bolts and nuts it is then. We'd better start drawing them up in Solidworks and try to find a spare production slot on a CNC milling machine - Home jobs always take priority over real customer jobs don't you know :-]
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Sounds like a lot of work to no real advantage if weight is not important. Why not use bronze bolts available off the shelf?
You will also find titanium liable to suffer crevice corrosion similar to stainless steel in a situation where oxygen is excluded. Bronze is not liable to crevice corrosion at all as far as I am aware.

What you are not seeming to understand is that it's high corrosion resistance relies very heavily on a titanium oxide film that protects the surface. If oxygen depletion occurs this film breaks down very quickly and the base metal corrodes in a very similar way that 316 does.

Don't try re-inventing the wheel. Bronze is the metal of first choice for any underwater use. The only reason it is avoided is high cost.

Edit: I have just read the page you were sent on Titanium.

This is a direct copy from it

Titanium alloys, like other metals, are subject to corrosion in certain environments. The primary forms of corrosion that have been observed on these alloys include general corrosion, crevice corrosion, anodic pitting, hydrogen damage, and SCC.
 
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burgundyben said:
I keep telling him bronze is best....
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Okay Okay, got the message.

Now then you are an Engineer so answer me this one:- If silicon bronze has about 90% of the strength of 316 and mebbe about half of high tensile? then how many bolts and of what diameter should they be in order not to all shear off and turn the back of the boat into a colander when 3 tons of boat lands off a largish wave at say 50 knots? :-]
 
Scarron said:
Okay Okay, got the message.

Now then you are an Engineer so answer me this one:- If silicon bronze has about 90% of the strength of 316 and mebbe about half of high tensile? then how many bolts and of what diameter should they be in order not to all shear off and turn the back of the boat into a colander when 3 tons of boat lands off a largish wave at say 50 knots? :-]
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Big ones.
 
Just to update this thread, last year when I replaced those bolts I didn't remove the exhaust outlets themselves, somewhat foolishly about 7 years ago I fitted them with polyurethane, there's no place for polyurethane on a wooden boat, I had always been scratching my head as to how to remove them without damaging the transom wood. A while ago while working on an old car I used a blowlamp in an attempt to set fire to a rubber suspension bush that was reluctant to come out, as it happened, minimal heat let the bush pop out, brainwave!! Having drawn the bolts I set a blow lamp on the exhausts, within about 30 seconds I withdrew them by hand, barely warm. Result!

Among other things, this shows the pitfalls of relying heavily on a survey, in November the boat was surveyed for insurance and said to be in fine shape, apparently a testament to its owner (the daft **** that he is).

I guess they are 304/A2...

Back of flange

20130307_204041_zpsff9ae4f7.jpg


Halfway along, under a Mikalor W4 clamp, between rubber hose and pipe.

20130307_204006_zps74edbc1a.jpg


For reference.

20130307_204025_zpsf2af7af0.jpg
 
Scarron said:
Surely not, They really ought to bronze.

This doesn't look too bad. http://www.metals4u.co.uk/Bronze-Tube-and-Hollow-Bar-SAE-660/4-OD--x-3.12-ID-SAE-660-Tube/94/2232/detail.asp. Would a 500 mm length be enough, you can borrow my big lathe, soon braze /weld a flange on the end.
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Yes, it was doable, discussed with BD marine at length yesterday, either way, I was running into higher cost, so opted for A4, was over your way this morning dropping the old ones off at BD.

If she sells this year I'll home cast some in bronze, preferably without setting fire to the manshed and its contents.

Anyway, I'm sulking this morning.
 
The bottom photo is an even better example than your bolt!

Are there no off-the-shelf bronze versions of these? Vetus also do the type that is a clamp ring over the rubber hose, in sizes up to 90 mm. Is that type no use for you?
 
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