Stainless Steel and Corrosion?

[macd]

Any metal on a yacht should be 317 marine grade stainless steel, as said if you are puttng ss into Aluminium put Duralac on the thread.

Yeah, and gold-plated wiring throughout. A frankly ridiculous counsel of perfection (not that 317 is perfect, ). I doubt that most boats have any 317 on board at all (but may have bronze which is better in many applications).

I'm assuming your "Any metal" insistence was an oversight rather than advocating SS spars, hulls and wiring, but with you it's hard to tell.

 

[halcyon]

Going to the depths of my memory, 1960's doing metallurgy at Tech. A square of stainless steel with a rubber band around it in a water solution, in time the rubber band cut a slot in the stainless steel due to electrolytic corrosion.

Brian

 

[vyv_cox]

Going to the depths of my memory, 1960's doing metallurgy at Tech. A square of stainless steel with a rubber band around it in a water solution, in time the rubber band cut a slot in the stainless steel due to electrolytic corrosion.

Brian

Not electrolytic, there is no impressed current. This is crevice corrosion, quite common under the flutes of cutless rubber bearings, rope cutters and the like. Quite a few examples on my website under Metallurgy.

 

[vyv_cox]

Old thread I know, but some useful info you may find helpful:

2. Especially austenitic stainless grades like 304 and 316, which is why they are never used for keel bolts, needs to be a higher spec grade like Duplex 2205 or Super Duplex 2507 to be fully resistant to slat water, an environment without oxygen or stress corrosion cracking.

The vast majority of production boats are manufactured with 316 keel bolts. If installed properly with a decent sealant between the keel and hull the bolts never see seawater. Hence Moody and early Beneteau boats had carbon steel bolts, which survived perfectly well for many years. Some time ago a poster showed us his 30 year old Beneteau bolts which, despite having rusty heads from bilge water, were otherwise bright and shiny once removed.

Duplex 2205 (23% chromium) has rather better corrosion resistance than 304 and 316 although its higher molybdenum content gives it better pitting resistance. Its advantage is that its structure is a mixture of austenite and ferrite, which therefore allows it to be strengthened by heat treatment, unlike 304 and 316 which are austenitic. The alloy is used where greater strength is required, such as a few stainless steel anchor shanks, but corrosion resistance is similar to that of the 316 flukes.

2507 contains 25% chromium, which gives it good corrosion resistance, particularly to stress corrosion cracking. It can also be heat treated thanks to its ferrite content. However, it is very expensive. Perhaps you can tell us which builder uses this expensive stuff for keel bolts that should never see sea-water?

 

[vyv_cox]

Yeah, and gold-plated wiring throughout. A frankly ridiculous counsel of perfection (not that 317 is perfect, ). I doubt that most boats have any 317 on board at all (but may have bronze which is better in many applications).

I'm assuming your "Any metal" insistence was an oversight rather than advocating SS spars, hulls and wiring, but with you it's hard to tell.

Apparently 317 is very much better than 316 in boiling hydrochloric acid. Maybe relevant to Kelly's Eye but not to many others of us.

 

[halcyon]

Not electrolytic, there is no impressed current. This is crevice corrosion, quite common under the flutes of cutless rubber bearings, rope cutters and the like. Quite a few examples on my website under Metallurgy.

Now this is the best part of 60 years ago, but it related to the surface under the rubber band and the surface not under the rubber and having a different surface, this produced a galvanic process. It was a long time ago, may have had a name change, may now be what you say, but it was the movement of material from under the band to the plate surface outside.

Brian

 

[pvb]

Now this is the best part of 60 years ago, but it related to the surface under the rubber band and the surface not under the rubber and having a different surface, this produced a galvanic process. It was a long time ago, may have had a name change, may now be what you say, but it was the movement of material from under the band to the plate surface outside.

That experiment is to demonstrate crevice corrosion due to oxygen concentration differential. Nothing to do with electrolytic or galvanic action. See this link...

https://faculty.kfupm.edu.sa/ME/hussaini/Corrosion Engineering/model-06.htm

 

[AngusMcDoon]

2507 contains 25% chromium, which gives it good corrosion resistance, particularly to stress corrosion cracking. It can also be heat treated thanks to its ferrite content. However, it is very expensive. Perhaps you can tell us which builder uses this expensive stuff for keel bolts that should never see sea-water?

The hinge pins on Dragonfly trimarans are super duplex stainless steel. They are eye wateringly expensive.

 

[vyv_cox]

Now this is the best part of 60 years ago, but it related to the surface under the rubber band and the surface not under the rubber and having a different surface, this produced a galvanic process. It was a long time ago, may have had a name change, may now be what you say, but it was the movement of material from under the band to the plate surface outside.

Brian

Electrons are always transferred in any corrosion event. In the case of active and passive modes of stainless steel electrons pass from one to the other. See http://coxeng.co.uk/metallurgy/crevice-corrosion/ although I have not indicated the electron path in the diagram. Perhaps now is the time to do it.

 

[macd]

Apparently 317 is very much better than 316 in boiling hydrochloric acid. Maybe relevant to Kelly's Eye but not to many others of us.

Not sure about the boiling part, but I've sometimes wondered if he's been posting on acid.