silverseal
New member
I have just bought a stainless steel fitting. How come the yacht's fridge magnet sticks to it? I thought all stainless was non magnetic?
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Stainless steel
- Thread starter silverseal
- Start date
jkim1
New member
There are many different types of stainless steet. It sounds like you have purchased something made of 304. If you go down to your local library and find a chemical engineering book which covers corrosion you will be enlightened. Most chandlers sell 304 which is 10%cheaper than 306. It is generally thought that A4 nuts and bolts are the best for marine use. A2 dont last too long at all. Even 306 should I believe be 306L which is passivated, its all a nightmare, and very difficult to check, but if its magnetic dont use it (thats what I was always told) as its the cheapest type of SS. When I read the chemical book it basically said the best stuff was naval metal, an alloy of copper tin and a load of goodies, it just turns green and lasts forever, but it doesnt look as good as stainless.
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AIDY
Active member
Yes sounds like you have bought a cheaper grade stainless. with other properties mixed in with it. something like 304/6. which will have surface corrosion over time. lots of shackles and boat fittings are made out of this.
316 food grade stainless, won't be effected by a magnet and will not corrode. Although it is much dearer.
<hr width=100% size=1>/forums/images/icons/wink.gif <font color=blue> Regards Andy </font color=blue>
316 food grade stainless, won't be effected by a magnet and will not corrode. Although it is much dearer.
<hr width=100% size=1>/forums/images/icons/wink.gif <font color=blue> Regards Andy </font color=blue>
Birdseye
Well-known member
There are two common types of stainless steel - martensitic types (the 400 series) which can be hardened and tempered, contain only chrome, and are mainly used for things like knife blades on decent cutlery and for razor blades, and the austenitic type (the 300 series) which are alloyed with chrome, nickel and in some cases molybdenum.
The martensitic ones rust quite easily and are never used on boat fittings. The austenitic ones rust to varying degrees, but this is only rarely a problem above the water line even in a marine environment. Most of your boats fittings are likely to be the 302/304 non moly type which will rust a bit, but there is a world of difference between this discoloration and component failure. The 316 moly type often described as marine stainless rust very little but are a bit more expensive. By the way, German nomenclature is A2 for the 304 type and A4 for the 316 type.
The martensitic steels are naturally magnetic, but when you deform austenitic types by cold working (for example wire drawing) then they become magnetic too. There is a difference of degree, but they are definitely magnetic. And you wont effectively tell between 304 and 316 by magnet.
Non of the common stainless stells should be used below the waterline if there is no flow of oxygenated water. So do not embed them in grp or wood or use at the bottom of a muddy mooring. They need oxygen to renew the protective oxide fiilm that makes them stainless. Remove that, and they rot just as badly as mild steel.
<hr width=100% size=1>this post is a personal opinion, and you should not base your actions on it.
The martensitic ones rust quite easily and are never used on boat fittings. The austenitic ones rust to varying degrees, but this is only rarely a problem above the water line even in a marine environment. Most of your boats fittings are likely to be the 302/304 non moly type which will rust a bit, but there is a world of difference between this discoloration and component failure. The 316 moly type often described as marine stainless rust very little but are a bit more expensive. By the way, German nomenclature is A2 for the 304 type and A4 for the 316 type.
The martensitic steels are naturally magnetic, but when you deform austenitic types by cold working (for example wire drawing) then they become magnetic too. There is a difference of degree, but they are definitely magnetic. And you wont effectively tell between 304 and 316 by magnet.
Non of the common stainless stells should be used below the waterline if there is no flow of oxygenated water. So do not embed them in grp or wood or use at the bottom of a muddy mooring. They need oxygen to renew the protective oxide fiilm that makes them stainless. Remove that, and they rot just as badly as mild steel.
<hr width=100% size=1>this post is a personal opinion, and you should not base your actions on it.
Anchorite
Member
Stainless steel is an iron-based alloy containing at least 10% chromium. The chromium forms a corrosion protective oxide skin on the surface.
The 300 series is austenitic: they contain chrome and nickel (often stamped on the article, e.g. 'teaspoon stainless' is 18.8, chrome/nickel).
For all practical purposes 304, 304L, 316 and 316L are non-magnetic. 306 does not exist in the AISI series. The 400 ferritic series (no nickel in the alloy) are magnetic and generally cheaper with higher mechanical properties but lower corrosion resistance in a marine atmosphere (where 316L is the best of the 4 above).
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The 300 series is austenitic: they contain chrome and nickel (often stamped on the article, e.g. 'teaspoon stainless' is 18.8, chrome/nickel).
For all practical purposes 304, 304L, 316 and 316L are non-magnetic. 306 does not exist in the AISI series. The 400 ferritic series (no nickel in the alloy) are magnetic and generally cheaper with higher mechanical properties but lower corrosion resistance in a marine atmosphere (where 316L is the best of the 4 above).
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mick
Member
Aaaaarrrgggghhhh!!!! Before the start of this season I repaired a damaged skeg/rudder using stainless strips and screws. Presumably these will have to be replaced. What is the best material to use?
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Anchorite
Member
Re: Aaaaaarrrrggggh (sp?)
It won't fall to pieces overnight: if it's bedded on sealant, paint it the next time it's out of the water. Most prop shafts were 316 until the relatively recent no-stainless-under-the-water movement (started, I think, by the RNLI).
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It won't fall to pieces overnight: if it's bedded on sealant, paint it the next time it's out of the water. Most prop shafts were 316 until the relatively recent no-stainless-under-the-water movement (started, I think, by the RNLI).
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Birdseye
Well-known member
In these increasingly litigous days, I cannot advise you what to do. If you need advice, then go to a professional engineer and pay him accordingly. What I can do is say what I have done myself in similar circumstances.
The bits that are exposed to oxygenated water flow are unlikely to suffer. For example, the prop shaft or the outside surface of a flat piece of stainless steel. So I have left these alone. The problem is the hidden surface and in particular the bits that are embedded in wood or grp.
For example, I have taken 10mm bolts out of the skegs of my own boat that in 10 years were eaten away down to less than 1mm dia and where the sealant used had failed. I had much the same experience with the stainless bolts that Hunter had used to secure the o/b fairing piece on my H26 (and Hunter are a good builder so I know the job was done conscientiously) They looked like pieces of Gruyere cheese.
Trouble is the rate of deterioration is unpredictable and cannot be judged from the bit you can see.
Alternatives? Well I thought of the appropriate bronze or even exotic nickel alloys since I knew someone in the aerospace metals business. In the end I decided to use 316 stailess steel, use it thick and strong and inspect regularly. I take care to ensure that it is thoroughly bedded into marine silicone sealant to ensure no water ingress. I avoid underwater self tappers and use decent sized bolts.
This has worked. I still get corrosion, for example on the wear surface of the stainless rudder shoes, but the last time I drew my skeg bolts the sealant had worked and no corrosion was evident.
<hr width=100% size=1>this post is a personal opinion, and you should not base your actions on it.
The bits that are exposed to oxygenated water flow are unlikely to suffer. For example, the prop shaft or the outside surface of a flat piece of stainless steel. So I have left these alone. The problem is the hidden surface and in particular the bits that are embedded in wood or grp.
For example, I have taken 10mm bolts out of the skegs of my own boat that in 10 years were eaten away down to less than 1mm dia and where the sealant used had failed. I had much the same experience with the stainless bolts that Hunter had used to secure the o/b fairing piece on my H26 (and Hunter are a good builder so I know the job was done conscientiously) They looked like pieces of Gruyere cheese.
Trouble is the rate of deterioration is unpredictable and cannot be judged from the bit you can see.
Alternatives? Well I thought of the appropriate bronze or even exotic nickel alloys since I knew someone in the aerospace metals business. In the end I decided to use 316 stailess steel, use it thick and strong and inspect regularly. I take care to ensure that it is thoroughly bedded into marine silicone sealant to ensure no water ingress. I avoid underwater self tappers and use decent sized bolts.
This has worked. I still get corrosion, for example on the wear surface of the stainless rudder shoes, but the last time I drew my skeg bolts the sealant had worked and no corrosion was evident.
<hr width=100% size=1>this post is a personal opinion, and you should not base your actions on it.
silverseal
New member
I want to thank all who participated in the thread... I have learned a lot, and certainly did not know that stainless was not to be used for propeller shafts.. I will obviously have to can the idea than stainless is non magnetic
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MainlySteam
New member
<<<and certainly did not know that stainless was not to be used for propeller shafts>>>
I think that you have been led astray there. In fact most (if not all) propeller shafts in modern boats are made of stainless steel. As the steel is often a high performance one (from strength point of view) they often go under trade names, Aquamet being just one, and are ground finished - that may be what confused the previous poster. They are, however, stainless steels, often, I am sure of surprise to some, basically 304 stainless as far as the alloy is concerned.
John
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I think that you have been led astray there. In fact most (if not all) propeller shafts in modern boats are made of stainless steel. As the steel is often a high performance one (from strength point of view) they often go under trade names, Aquamet being just one, and are ground finished - that may be what confused the previous poster. They are, however, stainless steels, often, I am sure of surprise to some, basically 304 stainless as far as the alloy is concerned.
John
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Birdseye
Well-known member
not sure where the "and certainly did not know that stainless was not to be used for propeller shafts" came from. I thought I said I dont worry about the shaft. Its in oxygenated water, and as you say almost everyone uses it.
Yes the bars are usually surface ground because there arent many people making drawn stainless at these sizes. They are not special grades of stainless, usually ordinary 316. And they're not high strength - simply annealed and then ground at about 35TT - so they are actually pretty soft. They dont need to be strong so much as straighter than normal steel stockholder material.
<hr width=100% size=1>this post is a personal opinion, and you should not base your actions on it.
Yes the bars are usually surface ground because there arent many people making drawn stainless at these sizes. They are not special grades of stainless, usually ordinary 316. And they're not high strength - simply annealed and then ground at about 35TT - so they are actually pretty soft. They dont need to be strong so much as straighter than normal steel stockholder material.
<hr width=100% size=1>this post is a personal opinion, and you should not base your actions on it.
Avocet
Well-known member
That's a bit worrying! My shaft is (I guess) stainless and the majority of it is denied oxygen because it lives in an old-fashioned stern tube packed with grease (and maybe a bit of salt water). I'll have to draw it out next season and have a look.
As an aside, my rudder stock has a flanged joint in it to make the rudder easy to remove. There are four 6mm holes in it and these take all the rudder torque. Some years ago, I abandoned a trip to the Isle of man and went into Glasson dock because I started to notice play in the tiller. On removing the bolts I found one of them had corroded right through and the other three weren't far behind. Had it been rough I might well have lost the rudder! This was identified as crevice corrosion by a metallurgist friend. The bolts were "A2" or 304 stainless - the stuff that's not suitable for use below the waterline! I replaced them with "A4" (316) and they have been fine ever since (although I replace them every other year and inspect them every year now)! The bolts were secured with "Nyloc" nuts and the corrosion had started under the nylon (where the oxygen was excluded).
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As an aside, my rudder stock has a flanged joint in it to make the rudder easy to remove. There are four 6mm holes in it and these take all the rudder torque. Some years ago, I abandoned a trip to the Isle of man and went into Glasson dock because I started to notice play in the tiller. On removing the bolts I found one of them had corroded right through and the other three weren't far behind. Had it been rough I might well have lost the rudder! This was identified as crevice corrosion by a metallurgist friend. The bolts were "A2" or 304 stainless - the stuff that's not suitable for use below the waterline! I replaced them with "A4" (316) and they have been fine ever since (although I replace them every other year and inspect them every year now)! The bolts were secured with "Nyloc" nuts and the corrosion had started under the nylon (where the oxygen was excluded).
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MainlySteam
New member
No Birdseye, it was Silverseal who said that and he had been advised that was the case by another poster so I was not linking to you.
Regarding your comments on high strength ss, certainly here, and in much of the rest of the world high strength alloys are widely used in other than cheap boats, and they are generally marketed as for marine propeller shafts. Aquamet is but one and they are all (I think all) duplex stainless steels (often listed as Duplex xxxx, SS2205 being one commonly used). As well as their higher strength and their production for shafting in terms of straightness and polish they have superior resistance to crevice corrosion over other ss in stagnant water such as may be found in the shaft log or under fouling.
A google of Aquamet will throw up both the manufacturer's site and many references to its use in marine shafts (as will a google on SS2205).
All boats do not use them (using 316 instead as you say) and as one example I am aware of one family of production power boats which do not (and their shafts were also undersized so, unless twin shafted giving redundancy, we required them to be reshafted with a high strength ss if for commercial service).
John
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Regarding your comments on high strength ss, certainly here, and in much of the rest of the world high strength alloys are widely used in other than cheap boats, and they are generally marketed as for marine propeller shafts. Aquamet is but one and they are all (I think all) duplex stainless steels (often listed as Duplex xxxx, SS2205 being one commonly used). As well as their higher strength and their production for shafting in terms of straightness and polish they have superior resistance to crevice corrosion over other ss in stagnant water such as may be found in the shaft log or under fouling.
A google of Aquamet will throw up both the manufacturer's site and many references to its use in marine shafts (as will a google on SS2205).
All boats do not use them (using 316 instead as you say) and as one example I am aware of one family of production power boats which do not (and their shafts were also undersized so, unless twin shafted giving redundancy, we required them to be reshafted with a high strength ss if for commercial service).
John
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jamieh
New member
Old post I know - but stainless being magnetic is not so strange. It was mentioned that 200 series stainless (low Nickle and low corrosion resistance) is often magnetic, which it is. But even Austenitic 316 grade can be made magnetic if cold worked, so you can't read too much into that. Here is a good overview of the stainless grades.
RichardS
N/A
That is, indeed, a seriously ancient thread revival. 
Richard
Richard
Old Bumbulum
Well-known member
aka bronze.
vyv_cox
Well-known member
Naval bronze is one of those fiddle-factor alloys like manganese bronze, not a proper bronze at all. Nowadays it is known as naval brass, CW619N. It contains 1% tin, giving it slightly better corrosion resistance than brass but not as good as the real thing. Unlike manganese bronze, which contains no tin, and in some versions, no manganese!
PCUK
Well-known member
304 is fine above water. 316 is necessary below. From years of practical experience!!!
Boo2
Well-known member
vyv_cox
Well-known member
I only know that recent Jeanneau shafts are duplex stainless steel, and have been for some years. Also rudder stocks.