Stainless Steel

[Little Dorrit]

I have to screw some threaded bar into a windlass made of 316 stainless. This is not underwater or on the outside but will be sealed and run through the deck but obviously, it's a boat so could be damp. While my instinct is to get 316 threaded bar 304 is more easily available. Is it ok to have 316 next to 304 stainless steel adjacent in a marine environment? Would there be any electrolysis?

 

[geem]

Having bought some 316 s/s tube recently that is growing surface rust I can't imagine what 304 will look like after a while.
A marked contrast to the original 316 that the boat was built with that is perfect

 

[vyv_cox]

All the 300 series are very similar in the galvanic series. Slight risk of crevice corrosion easily overcome with a good sealant.

 

[jwfrary]

Well polished stainless will shine for far longer

 

[geem]

Well polished stainless will shine for far longer

Not the 316 from Metals4U. 316. Super shiny. Going rusty.
1980 original 316 s/s polished by me is totally rust free

 

[Gary Fox]

Not the 316 from Metals4U. 316. Super shiny. Going rusty.
1980 original 316 s/s polished by me is totally rust free

So it sounds as though a company can use the label '316' , then supply steel which goes rusty.
I thought 316 was a specific alloy, with the ingredients defined.
If not, the number is meaningless, and we don't know what we are buying.

However, if 316 defines a specific combination of metals, then Metals4U must surely be fraudulently selling substandard goods.
Maybe email them a photo of the rust, and mention consumer protection laws?

 

[Moodysailor]

Remember that stainless steel is only stainless as long as there is air present. If you are threading it into something, and feeding that through a deck, be careful of crevace corrosion.

For the above reasons, I would go with 316L, or higher grade (there is a difference between 316 & 316L).

 

[Stemar]

No expert, but I'd also be concerned about galling. Some sort of anti-seize compound would prevent both corrosion and galling, meaning you'll be in with a chance when the time comes to disassemble.

Stainless Steel Galling and Its Causes | RG Group

 

[PCUK]

Having bought some 316 s/s tube recently that is growing surface rust I can't imagine what 304 will look like after a while.
A marked contrast to the original 316 that the boat was built with that is perfect

Have you checked to see if it is magnetic? 316 should not be. 304 may be slightly!

 

[vyv_cox]

Remember that stainless steel is only stainless as long as there is air present. If you are threading it into something, and feeding that through a deck, be careful of crevace corrosion.

For the above reasons, I would go with 316L, or higher grade (there is a difference between 316 & 316L).

This is a widely held misconception. The corrosion resistance of stainless steel is provided by a thin layer of chromium oxide, which forms on freshly cut metal in microseconds. It is very difficult to destroy, being in the metal's lowest energy state. When a crevice is immersed in water the narrow tip is exposed to less oxygen than exists at the outer end, which creates a cell. Electron transfer creates the corrosion. Lots more about this on my website Crevice corrosion

 

[Moodysailor]

This is a widely held misconception. The corrosion resistance of stainless steel is provided by a thin layer of chromium oxide, which forms on freshly cut metal in microseconds. It is very difficult to destroy, being in the metal's lowest energy state. When a crevice is immersed in water the narrow tip is exposed to less oxygen than exists at the outer end, which creates a cell. Electron transfer creates the corrosion. Lots more about this on my website Crevice corrosion

Thanks for the link

I am a little confused as you apparently corroborate my statement, in that the corrosion occurs where there is a low oxygen state - or did I miss something?

Either way, it seems we are making the same advice, just getting stuck on terminology. I freely admit that I am not a metallurgist, so stand to be corrected on my definitions - but I think we are getting at the same thing

 

[vyv_cox]

Thanks for the link

I am a little confused as you apparently corroborate my statement, in that the corrosion occurs where there is a low oxygen state - or did I miss something?

Either way, it seems we are making the same advice, just getting stuck on terminology. I freely admit that I am not a metallurgist, so stand to be corrected on my definitions - but I think we are getting at the same thing

I often read that stainless steel corrodes when it arrives in an air-free environment such as underwater (!). In fact the conditions needed for crevice corrosion are quite complex. Just clarifying.

 

[Moodysailor]

I often read that stainless steel corrodes when it arrives in an air-free environment such as underwater (!). In fact the conditions needed for crevice corrosion are quite complex. Just clarifying.

There is plenty of oxygen in moving water, I am aware of that and was careful in my statement to specify an example where crevice corrosion could be an issue.

Thanks for further clarification, but there was no mention of water in my post

 

[Little Dorrit]

Am concluding from the above that using 304 for non exposed deck fittings will be ok.

 

[Moodysailor]

Am concluding from the above that using 304 for non exposed deck fittings will be ok.

Personally I would go with 316L or higher, it's certainly better than ferrous metals though. 304 is not typically used in marine applications, and is susceptible to corrosion more than 316L so if you use it I would advise to have some kind of inspection regime. It's important to try and understand the limitations of the material you select.

A useful link: Stainless steel - we use a lot of 'super-duplex' in the products we produce, that stuff is very good.

 

[AngusMcDoon]

We use a lot of 'super-duplex' in the products we produce, that stuff is very good.

But too expensive for everyday leisure marine use. Dragonfly trimarans beam hinge pins are super duplex stainless steel, & are silly money.

 

[Moodysailor]

But too expensive for everyday leisure marine use. Dragonfly trimarans beam hinge pins are super duplex stainless steel, & are silly money.

SS is going through the roof anyway at the moment due to global demand. If it were me, I'd keep searching and jump on duplex or super-duplex when the price is right.

 

[vyv_cox]

Am concluding from the above that using 304 for non exposed deck fittings will be ok.

Usual advice is 316 underwater, 304 on deck. 316 contains 2% molybdenum for resistance to pitting, and therefore crevice corrosion, same mechanism. We all have plenty of 304 on deck, stanchions, hood frames, rigging wire, etc. It will occasionally need a rub over with polish but overall is perfectly satisfactory.

 

[vyv_cox]

There is plenty of oxygen in moving water,

My point exactly. Many people seem to think the opposite.

 

[geem]

Usual advice is 316 underwater, 304 on deck. 316 contains 2% molybdenum for resistance to pitting, and therefore crevice corrosion, same mechanism. We all have plenty of 304 on deck, stanchions, hood frames, rigging wire, etc. It will occasionally need a rub over with polish but overall is perfectly satisfactory.

Yep, I am not used to seeing rust stains on what is supposed to be 316. I think my original 316 pushpit/pullpit were far in excess of the 316 standard. They never need a polish. May be once every few years. The Metals 4U stuff is crap by comparison. Only installed for a few months and brown rust stain on it