ArthurWood
New member
Westmarine sell it
all about rusty stainless steel 316
- Thread starter tcm
- Start date
G
Guest
Guest
lots of wrong info in this series of posts, will try and correct from what i remember from a career in special steel manufacture.
there are 3 common types of stainless steels - austenitic (anything beginning with a 3), martensitic and ferritic (begin with a 4). hopefully, the only ones you will meet in boat hardware are the austenitics.
of the austenitics, the common ones are 304 and 316 produced as vyv says in high and lower carbon variants. the equivalen din designation is A2 and A4. both these steels have very low magnetism though this does increase a bit if the austenitic structure is cold worked by, for example, cold forging or wire drawing. you cannot tell the difference between 304 and 316 by its magnetic permiability outside a physics lab.
there is no difference in physical strength either. typically, both are about 35 tons tensile as annealed and happily work harden in wire drawing up to and over 100 tonnes tensile. usually drawn through stearate (soap) unless you get very fine in which case it is drawn through a diamond or glass.
as far as corrosion is concerned, 316 has moly added to improve resistance in saline conditions and in an ideal world, all the stainless on your boat should be 316. in real life, much of it is 304, but that should not normally be a problem. you will get a bit more surface staining with rust, but usually that is only superficial.
the key to keeping stainless happy on your boat is to give it lots of air, either in flowing sea water or on deck. best not use stainless in areas that get stagnant water, or burried in wood or glass fibre. avoid abrasion such as you I get on my boat with stainless rudder bearing.
incidentally, the stainless in most decent knife blades is martensitic as is your razor blade. this is magnetic and you can harden and temper it, but it would rot like mild steel on a boat.
there are 3 common types of stainless steels - austenitic (anything beginning with a 3), martensitic and ferritic (begin with a 4). hopefully, the only ones you will meet in boat hardware are the austenitics.
of the austenitics, the common ones are 304 and 316 produced as vyv says in high and lower carbon variants. the equivalen din designation is A2 and A4. both these steels have very low magnetism though this does increase a bit if the austenitic structure is cold worked by, for example, cold forging or wire drawing. you cannot tell the difference between 304 and 316 by its magnetic permiability outside a physics lab.
there is no difference in physical strength either. typically, both are about 35 tons tensile as annealed and happily work harden in wire drawing up to and over 100 tonnes tensile. usually drawn through stearate (soap) unless you get very fine in which case it is drawn through a diamond or glass.
as far as corrosion is concerned, 316 has moly added to improve resistance in saline conditions and in an ideal world, all the stainless on your boat should be 316. in real life, much of it is 304, but that should not normally be a problem. you will get a bit more surface staining with rust, but usually that is only superficial.
the key to keeping stainless happy on your boat is to give it lots of air, either in flowing sea water or on deck. best not use stainless in areas that get stagnant water, or burried in wood or glass fibre. avoid abrasion such as you I get on my boat with stainless rudder bearing.
incidentally, the stainless in most decent knife blades is martensitic as is your razor blade. this is magnetic and you can harden and temper it, but it would rot like mild steel on a boat.
chippie
New member
I have used 304(I think) stainless screws buried in epoxy and wood in my boat.What problems should I expect?
richardandtracy
New member
Re: More info: s/s 316 v 304, & telling them apart
304 shouldn't be magnetic. It's fully austenitic & therefore non-magnetic.
Your cutlery is probably a ferritic/ martensitic SS, which can be heat treated to harden it. This is likely to be of the 400 series stainless steels - possibly 412 as cutlery is a recommended use for 412. The 400 series are magnetic and rust like crazy in marine environments
Regards
Richard
304 shouldn't be magnetic. It's fully austenitic & therefore non-magnetic.
Your cutlery is probably a ferritic/ martensitic SS, which can be heat treated to harden it. This is likely to be of the 400 series stainless steels - possibly 412 as cutlery is a recommended use for 412. The 400 series are magnetic and rust like crazy in marine environments
Regards
Richard
tcm
...
Re: i was wrong bout that
yes as vyv have pointed out; both 304 and 316 non-magnetic when annealed. sorry!
yes as vyv have pointed out; both 304 and 316 non-magnetic when annealed. sorry!
G
Guest
Guest
never had experience of stainless in epoxy, so cant comment on your specific case.
if you are worried, you should seek proper tech advice from an expert (which I am not!)
<P ID="edit"><FONT SIZE=-1>Edited by howard_easton on 23/09/2002 23:24 (server time).</FONT></P>
if you are worried, you should seek proper tech advice from an expert (which I am not!)
<P ID="edit"><FONT SIZE=-1>Edited by howard_easton on 23/09/2002 23:24 (server time).</FONT></P>
G
Guest
Guest
chippie - apologies if the above reply seemed unhelpful, but I really cant advise on your situation. not expert enough!
my comments on stainless fasteners in wood and glassfibre in the original post referred to using stainless below the waterline. as an example, my boat has 12 10mm bolts through the skegs holding the stainless rudders fixings. these looked 100% on the outside where they had flowing water going past the heads and nuts, but when I drew them I found that they were all really badly eaten away in the middle and had to be replaced. they were 10 years old.
water had been able to creep down the interface between bolt and grp.
my comments on stainless fasteners in wood and glassfibre in the original post referred to using stainless below the waterline. as an example, my boat has 12 10mm bolts through the skegs holding the stainless rudders fixings. these looked 100% on the outside where they had flowing water going past the heads and nuts, but when I drew them I found that they were all really badly eaten away in the middle and had to be replaced. they were 10 years old.
water had been able to creep down the interface between bolt and grp.
chippie
New member
No problem Howard,Having thought about it further I am not too worried as the screws were only used to fasten plywood before the epoxy went off so there are no strength issues involved.
The thing i find interesting is that corrosion is caused by oxidisation in most cases but from what I have read here, the oxidisation in fact is part of the protection.
Thankyou for replying.
The thing i find interesting is that corrosion is caused by oxidisation in most cases but from what I have read here, the oxidisation in fact is part of the protection.
Thankyou for replying.
vyv_cox
Well-known member
The driving force is differential oxidation. In the situation of a crevice, for example a nut and washer or a threaded bolt. The surfaces that are fully exposed to air have a substantial and protective oxide film. Surfaces deep inside the crevice have far less exposure to air and the protective film is either not established or thin. A galvanic cell is created between the two when exposed to a conducting liquid.
In non-flowing situations underwater the supply of oxygen may be completely absent. The potential for galvanic corrosion may then be high, if other surfaces of the same metal have an oxide film. This has been a major problem for standby pumps on offshore platforms - not even duplex stainless steel, that has far better corrosion resistance, has been up to this job.
In non-flowing situations underwater the supply of oxygen may be completely absent. The potential for galvanic corrosion may then be high, if other surfaces of the same metal have an oxide film. This has been a major problem for standby pumps on offshore platforms - not even duplex stainless steel, that has far better corrosion resistance, has been up to this job.
tyger
New member
Re:What is Duplex Stainless Steel? NM
?
?
charles_reed
Active member
Vyv,
It's probably worth mentioning the differences beween 316 A4 and A2, the latter should be the one of choice for below-waterline fittings.
It's probably worth mentioning the differences beween 316 A4 and A2, the latter should be the one of choice for below-waterline fittings.
tcm
...
Re: clarifying: a4 and a2
yes, let's have this clarified - another post says A2 is the name given to 304 stainless fasteners (nuts etc) and a4 means that the nut/bolt is 316. Is this correct?
yes, let's have this clarified - another post says A2 is the name given to 304 stainless fasteners (nuts etc) and a4 means that the nut/bolt is 316. Is this correct?
vyv_cox
Well-known member
AISI 304 = A2, AISI 316 = A4. It is common for marine bolting to be A4 but the nuts to be A2. I'm not aware of any reason why 304 would be better than 316 underwater but it is a fact that many drive shafts are made of AISI 304. Crevice corrosion is of far greater importance than pitting corrosion and there is no particular difference between the two alloys.
There is quite good treatment of the subject at http://www.diveweb.com/maritech/features/uw-su99.01.htm
There is quite good treatment of the subject at http://www.diveweb.com/maritech/features/uw-su99.01.htm
vyv_cox
Well-known member
Re:What is Duplex Stainless Steel? NM
Duplex stainless steels are specialty alloys that have ferritic pools in an austenitic matrix. Their properties can be manipulated to give higher strength or corrosion resistance. A few years ago they were seen as being the answer to many seawater corrosion problems, but the reality has been that they are just as susceptible to crevice attack as an 18/8.
Duplex stainless steels are specialty alloys that have ferritic pools in an austenitic matrix. Their properties can be manipulated to give higher strength or corrosion resistance. A few years ago they were seen as being the answer to many seawater corrosion problems, but the reality has been that they are just as susceptible to crevice attack as an 18/8.
G
Guest
Guest
charles.
definitely not correct. whilst 316 or A4 might not be perfect under water, it is better than 304 or A2.
definitely not correct. whilst 316 or A4 might not be perfect under water, it is better than 304 or A2.
anthonyyearsley
New member
The three Westerlies that I have owned since 1980 have all had stainless keel bolts. It has never been suggested that these should be drawn for inspection although they are not exposed to air and may be reached by salt water. What is the experience of corrosion causing wasting of the bolts? There have always been slight signs of rust staining on the nuts in the bilge, but obviously these occasionally come into contact with salt water.
vyv_cox
Well-known member
There are two factors at work. A galvanic cell is created between the iron keel and the stainless bolts when in contact with seawater. The SS is more noble than the iron, so the keel corrodes preferentially. However, the keel is much bigger than the bolts, so the rate is slow. Unfortunately, with threads the surface area is quite high relative to the volume, so corrosion can be high locally, just where it does most harm. It is probably worth checking periodically, although this is clearly not a major problem, or keel loss would be much more common. The best precaution is to ensure that bolts are well bedded in sealant and kept tight.
JRF
New member
Can you help me on this. Sailing my 7 year old Norfolk Oyster dayboat from Bucklers Hard to Cowes this Sunday, I was somewhat surprised when the jib collapsed about half a mile out of Cowes; the reason being that the stainless steel shackle connecting the becket of the jib halyard block to the galvanized mastband flew off. Either it had come undone or it had broken. The shackle itself flew overboard.
Having bought a replacement shackle in Cowes, I lowered the mast by allowing it to pivot in its tabernacle and rest on the sheethorse. Immediately on releasing the tension on the forestay once the mast was down, the forestay came away from its shackle connecting it to the self-same mastband. This time the shackle remained in place and I recovered it. The shackle had snapped. The broken surfaces show discolouration (rust) and some pitting. I now think that the shackle had broken some time before I set out on Sunday and it was only the tension in the rigging and the size of the becket which kept the whole thing up.
Neither shackle had been seized - my fault - so I cannot prove that the jib halyard shackle failed in the same way as that for the forestay but I think it the most likely explanation.
But why? All fittings are removed each winter, washed and put away with plenty of air. And there is plenty of air up at the top of the mast in the summer!
Could the failure be due to galvanic reaction? I thought that it would have been the galvanised mastband that would have suffered first.
Both shackles, incidentally, fix to the same mastband ring.
The replacements are at least twice the thickness!
Having bought a replacement shackle in Cowes, I lowered the mast by allowing it to pivot in its tabernacle and rest on the sheethorse. Immediately on releasing the tension on the forestay once the mast was down, the forestay came away from its shackle connecting it to the self-same mastband. This time the shackle remained in place and I recovered it. The shackle had snapped. The broken surfaces show discolouration (rust) and some pitting. I now think that the shackle had broken some time before I set out on Sunday and it was only the tension in the rigging and the size of the becket which kept the whole thing up.
Neither shackle had been seized - my fault - so I cannot prove that the jib halyard shackle failed in the same way as that for the forestay but I think it the most likely explanation.
But why? All fittings are removed each winter, washed and put away with plenty of air. And there is plenty of air up at the top of the mast in the summer!
Could the failure be due to galvanic reaction? I thought that it would have been the galvanised mastband that would have suffered first.
Both shackles, incidentally, fix to the same mastband ring.
The replacements are at least twice the thickness!
