Stainless steel corrosion.
Discussion
There was some evidence inside the transom on Playtime that the exhaust outlet bolts had been leaking a little, I pulled the bolts in order to re-seal them. These have been in place for about 9 years.
The exhaust outlet has been bonded to the anode all the time, it seems that is not enough to prevent crevice corrsion.
Certainly stainless should not be considered a fit and forget material.
The exhaust outlet has been bonded to the anode all the time, it seems that is not enough to prevent crevice corrsion.
Certainly stainless should not be considered a fit and forget material.
LimaDelta said:
Why not gold? Ultimately it comes down to availability and cost. I'm pretty sure you won't find countersunk slotted M8x60 bolts in gold, tungsten, titanium or bronze at your local chandlers.
I'm not so sure about tungsten, but certainly titanium is becoming more common, not seen it in the swindlery yet though.Cunifer is pretty much the best no-cost-limit marine metal; there are(or were) fishing boats with cunifer hulls in some bit of south america. Eye-bleedingly expensive though.
Fasteners like that are exposed to exhaust products as well as seawater, even when bedded in sealant they probably need removing for inspection every 5 years or so; I've seen identically wasted SS bolts removed from the exhaust outlets on one of Playtime's siblings, and the exhaust outlet itself had crevice corrosion burrowing through the metal where the rubber hose was clamped on creating a low-oxygen environment.
Fasteners like that are exposed to exhaust products as well as seawater, even when bedded in sealant they probably need removing for inspection every 5 years or so; I've seen identically wasted SS bolts removed from the exhaust outlets on one of Playtime's siblings, and the exhaust outlet itself had crevice corrosion burrowing through the metal where the rubber hose was clamped on creating a low-oxygen environment.
Stainless is also highly susceptible to chloride stress corrosion cracking if used in coolant pipework or sees a lot of heat and gets wet. Nasty stuff sea-water.
http://www.npl.co.uk/upload/pdf/stress.pdf
http://www.npl.co.uk/upload/pdf/stress.pdf
Edited by rhinochopig on Monday 14th May 09:16
hidetheelephants said:
Cunifer is pretty much the best no-cost-limit marine metal; there are(or were) fishing boats with cunifer hulls in some bit of south america. Eye-bleedingly expensive though.
Fasteners like that are exposed to exhaust products as well as seawater, even when bedded in sealant they probably need removing for inspection every 5 years or so; I've seen identically wasted SS bolts removed from the exhaust outlets on one of Playtime's siblings, and the exhaust outlet itself had crevice corrosion burrowing through the metal where the rubber hose was clamped on creating a low-oxygen environment.
That's interesting, I'd never heard of a cunifer hull, some of the Huntsman 31's had siamesed exhausts and I think the bifurcated (good word!) part is cunifer.Fasteners like that are exposed to exhaust products as well as seawater, even when bedded in sealant they probably need removing for inspection every 5 years or so; I've seen identically wasted SS bolts removed from the exhaust outlets on one of Playtime's siblings, and the exhaust outlet itself had crevice corrosion burrowing through the metal where the rubber hose was clamped on creating a low-oxygen environment.
I can see cast bronze outlets for next year, bit of home casting maybe!!
LimaDelta said:
Why not gold? Ultimately it comes down to availability and cost. I'm pretty sure you won't find countersunk slotted M8x60 bolts in gold, tungsten, titanium or bronze at your local chandlers.
Gold is too soft. Unless you add one carat platinum of course 
There must be some composite material where you could stamp out/mould the parts fairly cheaply then cure in an oven.
bucksmanuk said:
Inconel works well too in this kind of environment
again pant wettingly expensive
I've just had some inconel bolts delivered at work.
M8 x 35 - £14 each. They are going to sit at the bottom of a gas well though
Bit sour then?again pant wettingly expensive
I've just had some inconel bolts delivered at work.
M8 x 35 - £14 each. They are going to sit at the bottom of a gas well though
Interesting write-up about the use of CuNi in ship hulls here.
Edited by hidetheelephants on Monday 14th May 00:24
Crappy stainless. There's loads of different grades for stainless steel, some are a lot lot better than others in resisting corrosion.
http://en.wikipedia.org/wiki/Stainless_steel
http://en.wikipedia.org/wiki/Stainless_steel
Looking at the timescales you mention I would go with those not being A4 / 316.
It's all very nice going into a chandlers and picking up one of the prepacks off the shelf but not all prepacks are marine grade. They can also be any of three different tensile strength grades.
Best for something like that to order direct from a fastening company that know what they about.
It's all very nice going into a chandlers and picking up one of the prepacks off the shelf but not all prepacks are marine grade. They can also be any of three different tensile strength grades.
Best for something like that to order direct from a fastening company that know what they about.
MOTORVATOR said:
Looking at the timescales you mention I would go with those not being A4 / 316.
It's all very nice going into a chandlers and picking up one of the prepacks off the shelf but not all prepacks are marine grade. They can also be any of three different tensile strength grades.
Best for something like that to order direct from a fastening company that know what they about.
It's all very nice going into a chandlers and picking up one of the prepacks off the shelf but not all prepacks are marine grade. They can also be any of three different tensile strength grades.
Best for something like that to order direct from a fastening company that know what they about.
When changing bits on mine always order from companies direct where I know its 316/ A4, and mine only sees salt water a few times a year! doogz said:
Huntsman said:
I'm not so sure about tungsten, but certainly titanium is becoming more common, not seen it in the swindlery yet though.
IIRC Ti corrodes in the presence of Fe, i.e. steel.Titanium is strange stuff. Lockheed had all sorts of problems when building the A12 and SR71 including spot welds failing due to the titanium reacting with the residue of chlorine from when the panels were washed prior to welding.
I've worked with it before and if the feeds and speeds are just right it's easy to get an excellent finish but if you're slightly off then it's a nightmare.
Cunnifer is even worse, it has, like Inconel a very high nickel content so it's tough as old boots and a b
d to machine.
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