Stainless Steel and Corrosion?

Dougal

Dougal

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Fed up with receiving marine items from yacht chandlers that have poor quality, low grade stainless fasteners!!! ESPECIALLY when one of them suggests to coat the threads.

Am I correct in thinking that stainless steel should NOT be coated or sealed in any way? I seem to recall that removing oxygen from s/s will cause corrosion.

Seems that many retailers don't bother with any kind of quality control from manufacturers.
 
Use Duralec on threads going into Aluminium. Stainless corrodes when it is damp in the absence of oxygen and threads are particularly affected, so makes sense to isolate threads from damp with a sealant. However many stainless on sale is not A4, which is the more corrosion resistant, but A2 but this is usually OK if used above water. Still worth putting a bit of sealant under the head to keep water out.
 
As I understand it and in simple terms Stainless steel is stainless because of an invisable oxide layer on the surface. If you scrape or rub off the layer it takes a finite time to re-form. If at this time the oxygen required is not available it moves from being passive (higher on galvanic the scale) to being active where its position on the galvanic scale plummets and it will become anodic to many other metals with a position in the active state close to mild steel.

This commonly occurs in places where the oxygen is not available. Not just underwater as there is normally enough dissolved oxygen to allow the oxide layer to form. For instance this 'crevice' corrosion occurs in rubber cutlass bearings on cheaper grades of stainless where the shaft has rubbed off the oxide layer, the surface then bears onto the rubber surface sealing out the oxygen. The metal then becomes dissimilar to its own intact surface a few mm away, so a local galvanic cell fed by conductive seawater is formed. This is also why stainless is not normally used for keel bolts.

Therefore if a screw is wound in that has had the coating form it will be fine, but the issue with aluminium is the dissimilar effect with poor result to the aluminium, which is very anodic to most other metals (they make fresh water anodes from aluminium).

The simple test for checking stainless is a small magnet. Lower grade stainless and non stainless will attract to the magnet.
 
superheat6k said:
This is also why stainless is not normally used for keel bolts.
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However, stainless steel is commonly used for keel bolts. The reluctance to use it dates from the past when bolts would have to pass through wood keels and it would be difficult to keep them dry, so creating the conditions for crevice corrosion. Similar problems existed with mild steel, except that it was straightforward rust rather than galvanic action and various preventative methods were used to reduce the chances of corrosion, such as galvanising and dipping in hot tar with varying success. With GRP boats and modern sealants, the chances of keel bolts getting damp are significantly reduced and stainless steel bolts (or often studs) are fine.
 
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superheat6k said:
Which is why my response said 'and in simple terms'. Of course if you can elaborate on the not so simple terms I am sure the forum would like to see your advice.
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It's not that it's in simple terms, it's just not accurate.
It's all about choosing the correct grade for the application, the magnetic/non-magnetic statement is a myth.

EDIT ~ and beware of cheepo stainless that isn't really stainless, just a cheap imitation
 
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Tranona said:
However, stainless steel is commonly used for keel bolts. The reluctance to use it dates from the past when bolts would have to pass through wood keels and it would be difficult to keep them dry, so creating the conditions for crevice corrosion. Similar problems existed with mild steel, except that it was straightforward rust rather than galvanic action and various preventative methods were used to reduce the chances of corrosion, such as galvanising and dipping in hot tar with varying success. With GRP boats and modern sealants, the chances of keel bolts getting damp are significantly reduced and stainless steel bolts (or often studs) are fine.
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Indeed I am out of touch with keel bolt requirements, and do not foresee this being an issue that will cause me any concern in the future, as I am happy staying on the dark side with my stinkpots, and 316 grade stainless steel fittings and duplex shafts. After a reasonable mileage had passed under my keels, I just found hoisting main engines just became too tedious.
 
superheat6k said:
The simple test for checking stainless is a small magnet. Lower grade stainless and non stainless will attract to the magnet.
Click to expand...

Up to a point, but it's not as simple as that. And it won't discern between A2/304 and A4/316, which is the most likely choice of alloys (both are austenitic, the largest class of non-magnetic steels).
 
macd said:
Up to a point, but it's not as simple as that. And it won't discern between A2/304 and A4/316, which is the most likely choice of alloys (both are austenitic, the largest class of non-magnetic steels).
Click to expand...

And to make matters worse A2/304 can become magnetic if it has been cold worked.
Even A4/316 can be magnetic in certain functions
 
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Some interesting data there. The fasteners in question that I'm so frustrated with, are small dia machine screws, passing through plastic or wooden mounting devices (such as BBQ mounting brackets etc), with their own retaining nut buried in a hidden crevice. The last ones I used corroded so badly after just ONE season, as to badly stain the deck. I ended up having CUT OFF the mounting block from the pushpit rail!!!! This has now happened three times with such items. I learn lessons very slowly, but will no longer use ANY supplied fasteners with items purchased. Surely, having to protect these fasteners, defeats the purpose of using stainless?!
 
Sympathies. That sounds bad, and rather suggests they weren't any sort of stainless at all. In future, using superheat's magnet test can do no harm and could weed out the worst of the dross. I can't imagine that even A2 would perform as badly as you describe.
Agree that some supplied fasteners are a joke...not unlike lots of supposedly marine equipment with non-tinned wire tails. We live and learn (but don't have to enjoy the process :ambivalence:).
 
Supplying stuff like this for the marine market is, in my opinion, criminal! They MUST know it's not fit for purpose! Is it my imagination, or are online chandlers now overflowing with poor quality (yet still overpriced) equipment? I buy stuff EVERY single year that either gets dumped or returned. Rant nearly over;-)
Yours, Mr Angry;-)
 
Old thread I know, but some useful info you may find helpful:

1. You are right, stainless does need Oxygen to work - see how stainless works
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.
3.So stainless should not be coated as mentioned BUT it should certainly be isolated from Aluminium to prevent Galvanic Corrosion and Tef Gel is much better than Duralac, as it is a PTFE lubricant, doesn't go hard and is non toxic
 
I like the sound of Tef Gel. I use Duralac maybe once every three or four years, it's horribly messy and gets everywhere, plus the last tube has invariably gone hard or leaked all over everything by the next time I want it. A small plastic tube of something less messy sounds ideal.
 
Can anyone else recommend TefGel?

I need to bolt some aluminium fittings to my epoxy-sheathed plywood coachroof and I assumed I would have to make some nylon top-hat bushes to insulate the s/s bolts from the aluminium. A simpler solution would be better, provided it is reliable in the long term.
 
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