- Why does it happen, how to prevent it
Appendix Part 1 - Alternatives to 316
Nitronics, 2205 rigging fittings etc
Are any of the other currently available alloy alternatives viable? no. they are all too cheap
. How many small sailing dingies are there? Versus how many yachts cruising in the tropics?
So that right there is the problem. 10 million small sail boats for who 316 is brilliant and affordable, cheap
fun toys versus the very small number of cruising yachtsmen with very expensive masts. Couple of thousand at the most. 300 do the puddle jump. And out of these how many would be interested in never being dismasted? 20%?
Very small numbers.
So basically there is no chance of ever getting never fail alloy fittings and rope
All the other alloys available are exactly the same as 316 or worse – they are the cheapest they can possibly be. All of them. Competition to stay in business has ensured this.
You will be fooled by Crapitalists. They are out to get you
, and they will succeed.
Nitronic 50's et al have has worse SCC than 316. So that is that alloy done.
2205 was designed to be cheap. As nitrogen costs nothing and molybdenum cost heaps.
If you put in some nitrogen, then the PREN rises considerably. (the Pitting Resistance Equivalence Number = the corrosion
resistance to salt
ie the PREN formula is basically PREN = %Cr + 3.3*%Mo + 16*%N
so Nitrogen has by far the biggest effect, and Nitrogen costs absolutely nothing, and since 2205 has TWICE the strength of 316, manufacturers only have to use HALF AS MUCH steel in their products. So this gives them an incredible commercial
advantage. If you look at the price
chart further down, you can see the impact of the rising price of alloying elements like molybdenum. Astounding price rises. 3 cents extra of moly costs more than the 316 alloy in total (but due to the huge volume increases of 2205, molly prices are just now falling rapidly for the first time) .
This explains the huge rise in popularity of the lightly alloyed 2205 duplexes. There is huge price pressure to change over to lean alloys from high volume users.
So 2205 is a cheaper replacement for 304, for sinks, knives and forks. it has a PREN (Pitting Resistance Equivalent Number (PREN)) of <40, and the Europeans say 40 is the minimum PREN number alloys are allowed to have to be called corrosion
resistant to sea water
So even though 2205 Duplex rigging fittings are starting to appear, remember that 2205 was designed from the outset to be cheap and so it has exactly the same problems as 316. Designed to be cheap only, not designed to be used in sea water
, or for SCC resistance, or take load cycles in chlorine and salt
. Cheaper than 304 was the only design criteria.
2205 does NOT meet european building codes for steel structures exposed to chlorine, and so it can not be used by law, not even for STATIC, stationary buildings and structures. So don't even think of using it for dynamic loads.
It can not by law be called salt water
It can not legally be used for lifting or rigging, as any alloy that suffers intergranular corrosion can not be given engineering data suitable for designing a true safety
factor. (engineering societies will not publish such).
Yet some yacht rigging companies are now starting to use 2205, but only
because they can now make the part cheaper than 316.
This is the temperature 2205 starts to fail at. Same as 316. Just cheap rubbish.
AOK, and real good.
This is what 3 extra cents is all about.
SCC accelerated testing.
Look at the difference 3 extra cents of moly makes. This is the ballpark all yacht rigging should be in. Note that 316 failed at 5% of the yield strength. It completely dissolves in hot water.
This is why so many people from cold countries never have a problem, but then they go sailing in the tropics and get dismasted. 316 is utter rubbish. Useful for non-stressed parts
only. Your rigging fittings will definitely get hot in the midday sun. 316 is a very very old, very primitive and very stupid alloy. It breaks apart in hot water, what more can I say?
Remember “Susceptibility to Intergranular Corrosion eliminates any
safe stress level"? Any engineer
should say a bigger safety
factor for dynamics, so some percent of 170 MPa, but realistically, if you tried to talk me into using 316 with 50 MPa as data suitable for design for dynamic loads in 35000 ppm chlorine
, after seeing that it breaks at 5% of it's yield
in hot water i'd still think you were soft in the head
, as 316 literally dissolves and falls apart in hot water while under stress. What more can I say?
Cyclic load cycles pump
chlorine in and out of any crevice corrosion cracks. This is why a cyclic load makes alloys fail at astoundingly low loads
, many many factors under the yield strengths. To stop this cyclic loading phenomena, your alloy MUST be immune from crevice corrosion.
316 has 3 cents of moly in it, and is absolute rubbish of a metal. but if you add an extra 3 cents, a miracle happens. This new alloy is immune from crevice corrosion, and so then a cascade of quality happens. Since crevice corrosion can't start, then Stress Corrosion Cracking never happens. Since you don't need chrome oxide on the surface either, then bolts and chainplates don't need oxygen, and so stay perfectly strong forever, even if embedded in butyl or silicon etc. So 3 cent extra mollies have 20 times the usable strength of 316
under load cycles in salt. You get astounding extra value out of those extra 3 cents. 2000% extra value.
Bizarre the difference 3 tiny extra cents makes.
There is one minor drawback. If the temperature goes above 40 degrees C, then all the 316 dribble can start again, albeit very very slowly. So a rigging fitting sitting in the hot tropic sun may get above the initiation temperature.
You'll certainly be perfectly ok, but I think it's a bit silly to go to all this trouble and still have even the slightest possibility. Especially when these old alloys are twice the price of 316, and modern alloys that have twice the strength are half the price.
In general, austenite and ferrite are formed equally. If you add one alloying element that makes one phase perfectly corrosion proof, but leaves the other rusting, ie 316 then no good. So the trick here is to add alloying elements that dissolve into each phase and make them equally rust proof.
However some of these alloys still suffer from the 316 as cheap as they can possibly be syndrome.
Ordering exotics in small amounts is more expensive, but some of them are actually cheaper if ordered in bulk. So if a manufacturer actually ordered bulk, then some of the never fail alloys are actually cheaper than 316. With twice the strength, this means half the cost of 316 rigging fittings.
However, remember that capitalists always push it right up against the line. So a lot of these have a PREN of 40 due to european law requiring salt water
corrosion proof alloys, but they still suffer from MANY other as cheap as possible induced failures in many areas of performance. (cycles under load etc)
But we are almost there.
I say Zeron is out and 2507 is out. Even though some metallurgists say these are ok, others say they are not
, as a hot metal fitting in the tropical sun will definitely go over it's crevice corrosion temperature in chlorine. 40 degrees C.
Remember Part 9 and Part 3, solubility and precipitating out of phase, manufacturing and fabrication induced errors severely degrading properties leading to parts
way under spec unless endless dance steps are performed perfectly all to save 3 cents. In the end it will still be delicate. Not robust. So even though many cheap bastards will disagree and say 2507 is overkill, I say it is still delicate dribble suffering from crapitalisms $9.95 syndrome.
can kill this delicate alloy instantly. Read up on all the endless tests that have to be performed if you use this alloy and demand it actually be true to it's word, and actually meet it's marketing specs.
ASTM 923, G48, G150, E562, ISO 8249, ISO 3651 testing for carbide precipitation, etc etc etc. The list goes on forever
and deals with surface hardness, work
hardening, phase separation, acid test surface corrosion equivalence to true pren 40 specs etc, and the reason the list goes on forever is because high volume users are trying to save three cents again, and so they are really pushing the envelope on cheapness. And they have pushed it far too far. So it has the minimum legally required PREN of 40, but it then fails in one hundred other areas.
Basically all of these expensive tests are designed to reject this alloy
as corrupted, as the part will definitely get intergranular corrosion, and so we are right back at the 316 failure mode, again due to trying to save 3 cents of molybdenum.
There are astounding numbers of mistakes
you can make that can kill the 2507 part and bring it WAY below it's virgin specs. All the dribble you have to go through just to ensure that you've polished your turd up to an acceptable standard. To be equal
to the corrosion resistance of all the other PREN = 40 alloys when given the acid test. Right up against the line, as LOW a quality as they can legally get away with. The reason all these tests exist is of course capitalism trying to push as hard as they possibly can and do the cheapest job they can get away with.
I say there is no way you can reasonably investment cast, cold work, weld or machine 2507 or a Zeron 100 part without the above endless 2507 ASTM et al test defined failures occurring. Unless the labourer pouring the metal in the shop actually understands metallurgy and exactly what he is doing and why, no way is it possible. Too delicate. Any cold working above the yield strength greatly increases corrosion susceptibility as thousands of microscopic crevices have been formed. When true pren 40 is demanded
, Zeron and 2507 bolts for seawater have to be electroplated with copper to avoid corrosion problems. That right there should tell you all you need to know about the real quality.
I say it's far too difficult to ensure that your part meets the absolute MINIMUM
specifications, that you might as well go elsewhere, to something guaranteed to be robust. So that no matter what you do, the part will never fail due to fabrication mistakes, cold working, not trimming off 0.25mm, time cooling
after investment casting is as short as possible etc etc etc etc etc. Yes, you can give your machined parts heat treatments annealing and such to restore properties, but again it just adds to the endless dance steps, and more quality testing and rejection of parts. 2507 is very superior to 316. I'll probably make my anchors and other stuff out of it due to cheap cost. Yes it is very good for statics. I'll use it, But not for rigging fittings and especially not for wire rope
Do not listen to people who recommend 2507 as good enough. It is not.
An Outokumpu metallurgist recommended to me by my stainless engineering firm said on 2507; if there are crevices, such as threaded connections, crevice corrosion will begin here at approximately 20C below initiation of pitting. Basically on a hot summers day. So the threads in your turnbuckles will split.
Much like this 316 turnbuckle screw did.
Even if in the outside air, oxygen still can't get inside microscopic cracks, and so 316 bolts will just snap clean in half.
“the weakest link will be the wire rope, hence it is important to have a sound maintenance
all your 2507 wire ropes on the mast rigging with fresh water. somewhat impractical.
Also; Be Advised;
this was free advice
on 2507, and Outokumpu refuse point blank to have any liability whatsoever unless specifically contracted to study the matter.
So do not listen to the vast majority of people who recommend 2507 as good enough for rigging fittings. It is not. These people don't know any of the facts, caveats, and codicils. They have no understanding of what they are saying. They are simply repeating marketing and have no understanding.
The reason we are spending so much time on this is that 2507 in bulk is cheaper than 316
, and so there will be incredible pressure to call this stuff good enough
. All the capitalists will howl like wounded animals
, YES! it is! they'll say, trying to hypnotise you, the victim. They will hold their tiny little vampire meetings and decide they CAN get away with it, because the first failures won't happen for twice as long as 316. They do not care about the real truth, they care about getting your money
off you. They do not love you. I can see the future clearly. This is going to happen. Alloying element costs are very expensive already and still on the rise, and lightly alloyed 2507 is very very cheap.
So do not listen to people who recommend 2507 as good enough. Ask them what temperature steel left out in the hot midday sun gets to, and then ask them at what temperature 2507 starts to Stress Corrosion Crack in 35000 ppm salt. Ask them exactly how many ASTM tests were designed to specifically reject 2507 parts, and to see if it really is equal to other PREN 40 alloys
. Sure the virgin alloy is legal
, but what about the fabricated part? A bolt? Does your bolt have the legally required corrosion resistance equal to a true minimum PREN of 40? NO! It does not.
So watch it.
Some people think 2507 et al are the be all and end all of stainless just from reading the marketing and the highly dubious, virgin untouched condition only, twisted and perverted like lawyers and diluted by capitalists PREN number, and they are very wrong
. There is FAR more to the story. A single
PREN number does not tell the full story by far. Again, have a look at the price. You can smell the tainted fruit of crapitalism instantly. Have a look at the Outokompu bulk price sheet. Their 2507 is cheaper than 316.
as you can see, 3 cents more molybdenum is very expensive. Note Super Duplex 2507 is even cheaper than 316 (in bulk only; NOT in a shop, it is way more expensive. I have no idea why this is so).
Using a higher technology can only side step the demands of reality so far. Sometimes you just have to bite the 3 cent bullet, and admit that you need 3 cents extra of a more expensive element.
The Technology of Cheapness Versus Adding Expensive Alloying Elements.
Nitrogen additions in duplex stainless steels have been found to increase both yield strength and the ratio between the threshold stress for cracking and the yield strength. The absolute value of critical cracking stress is thus raised significantly.
However; stress higher than 50% of the yield strength has always induced stress corrosion cracking in these materials in boiling 44% MgCl2 solutions (a simulating years of service
in salt water in a few days technique).
Moreover, rapid failure can occur when the specimen is plastically strained even at 1-2% of the yield strength. (This is cyclic loads
; thrumming) Equivalent results have been observed at 200°C in an oxidising solution containing only 100 ppm NaCl and 100ppm of oxygen.
So understand that if you try and be too cheap with your expensive alloying elements, then your wire rope design data, and your performance is reduced by half.
Even though the virgin yield tensile strength data is ACE!, the actual performance in salt water with cyclic stress in the field is absolute rubbish.
Sometimes you just have to bite that 3 cent bullet.
Cyclic loads at 1-2% of yield strength at only 100 ppm NaCl. Seawater is 35000 ppm chlorine.
High technology then is more fragile, and more delicate.
To quote an egg head
The relative drop in fatigue strength when comparing results in air versus seawater is clearly linked to the intrinsic ability of the alloy to resist corrosion mechanisms i.e. crevice corrosion.
In seawater there is a direct link between the resistance to metal fatigue and the chemical composition through the PREN formula. For stainless grades having a PREN value higher than 40, no significant drop of fatigue limit is observed when tested in seawater.
2507 is NOT pren 40 after manufacturing. But 2507 + 1% extra copper is. (32550)
32550 has TWICE the fatigue limit (the yield strength floor it stabilises on) yield strength of 2507 after stress cycles. (ie wire ropes thrumming)
So 32550 has a much higher permanent metal fatigue yield strength floor limit in salt water than 2507 that it never drops below. Twice.
and so, you can use HALF the metal that you would have to use if you used 2507.
So adding 1% extra copper gives you 100% better value.
Wire Rope fabrication is the corner stone
The wire rope should be made from the same alloy as the fittings to minimise dis-similar metals galvanic corrosion over a long time span (any crevice corrosion is accelerated by large factors). I am not a supporter of 2507 due to the savage fabrication steps needed to make wire ropes. I would not take delivery
of 2507 wire ropes until every single
one of those ASTM tests were performed in front of me, to ensure that it actually meets the spec. I suggest normal fabrication methods will absolutely degrade 2507 way below spec. Especially as the top 0.25mm has to be thrown away to meet nominal specifications. On a 1mm wire rope strand, this is impossible.
We need something that is absolutely above reproach.
Absolutely beyond question. And yet still satisfies capitalists by being as cheap as possible.
Robust. Forgiving. Able to take fabrication mistreatment in its stride. And still be Safe Forever.
Forever above reproach. Never fail from stress corrosion cracking in the midday sun and salt.
A.K.A. Ferralium 255SD50, UNS S32550, Uranus 52N+, DIN 1.4507
This alloy is good. It's the modern version of the more primitive 2507 Super Duplexes. It's used on oil
rigs and navy
Basically, if your 2507 has 2% copper, as well as being immune from crevice corrosion, the alloy self heals.
The copper is electro-chemically deposited into any corrosion crevices and stops them.
So it is still a cheap 2507, nowhere near the price of all the 6% molys, or thousands of true exotics like C22, 686, titaniums etc that would absolutely be well over the job specification, has fantastic tensile strength so you only need half as much of it, but most important, any crevice corrosion or stress corrosion cracks self heal. So it never fails under cyclic loads in salt.
Super Duplexes are not all the same. So don't let yourself be led astray when you talk to vendors.
FERRALIUM 255 is the original 2507, (Ferralium 255 - Super Alloy Alloy 255 - UNS S39255, ASTM A240, ASTM A479, ASTM A789, ASTM A790, ASTM A815,)
(i've even come across one huge scientific treatise that mistook Alloy 255, Ferralium 255, for 32550, repeatedly assigning it lower properties in corrosion tests. Basically the 32550/32520 has the lowest corrosion rate. So remember when researching, that even metallurgists confuse 255 with 32550. 255 was the original Super Duplex, but they kept EXACTLY the same trade
name as the modern improved version, and so the confusion. an easy mistake to make.) and it is not good enough (for cycles in salt), but add 2% copper and it is. So watch it. Because they are all playing so incredibly close to the line (as cheap as they can get away with). They HAVE to do this, as other crapitalists are doing this, so they must compete or they go out of business.
UNS S32550 has 40 times less material loss (erosion and corrosion rates ) than Zeron 100 or 2507 in salt water, and copper is not included in the common PREN formula (too modern an idea as yet).
When you compare the compositions, take very careful note of the tiny tiny difference between them; less than 1 % copper difference between 32550 and Zeron 100, 2507 et al, and it makes all the difference.
Basically; Make sure you have either an extra 3% of moly, or a minimum of 1.7% copper, because even though some alloy may have the absolute minimum PREN of 40, that is meaningless. Land based capitalists will always push hundreds of other necessary alloy factors well below the true safety line. Well below the spirit of the law.
Well below the intent of the PREN = 40 law. Which was SAFETY.
Stupid to go to all this trouble to avoid 316 and then be right back where you started. Dismasted.
the tiniest bit more copper extra makes a HUGE difference. Critical to get it right, as the tiniest little bit less gives you 40 times more material loss, and no self healing. So watch it. You must have the minimum amount needed.
Data suitable for design (note this is for pressure vessels, but gives a good indication for rigging and lifting.)
If any yacht rigging company chose to make a lawful, law abiding, legal
, not criminally negligent, not criminally liable, decent, beyond reproach fitting, UNS S32550 is certainly going to be in the absolute top contenders. The strength alone gives half the cost of raw materials. Plus, you'd be allowed to claim an actually true and legal safety factor. An alloy legally allowed to be used for rigging and lifting on land exposed to chlorine and salt.
You can also see that a modern chemistry alloy gets much more usable tensile strength out of the ultimate tensile strength than old metal compositions. And so, using these metals is far more efficient.
You might be thinking we only get twice the strength of 316, but you are very wrong. You get 40 times the 316 wire rope true usable strength in salt water, and twice a 6% molly wire rope usable strength at half the cost. But of course the real benefit is that it will never fail. It will never have to be replaced, or inspected, and has an infinitely long life.
Erosion and corrosion
the tiniest little bit extra of copper does astounding things.
Comfort factor – are we all alone here? Are we in the ball park? Who else uses it?
use it for their nuclear submarines, as this stuff will never fail in salt water under load cycles. Legally used, and legally allowed to be used for rigging on land based structures, indoor swimming pools, anywhere exposed to salt water or chlorine.
Have no doubt that capitalists will try to pervert, twist, argue, and convince you that 2507 is good enough. But remember, all they are saying is $9.95. The words they use may change, but the underlying story never changes. $9.95.
2507 is Cheaper in bulk than 316, so something is WRONG do you understand? VERY WRONG. All they care about is capitalism. People think capitalists tell the truth, but all capitalists ever do is lie. All they ever say is give ME your money
, don't give it to the OTHER person, their stuff is SO inferior to OUR quality.
I say 2507 + 1.7% copper extra is the ABSOLUTE MINIMUM quality needed, and the navy agrees.
And please remember, there are ten thousand FAR more expensive alloys out there that are well and truly way above the quality needed.
Adding 1.7% copper to all the Kilograms of Stainless used on your rigging fittings just doesn't cost that much extra.
To be forever safe.