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USnavy


Nov 20, 2008, 11:22 PM
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The dangers of Stress Corrosion Cracking (pull test video)
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FYI. My video: http://www.youtube.com/watch?v=z0bmZGcQOu8


(This post was edited by USnavy on Oct 30, 2011, 10:49 PM)


suilenroc


Nov 20, 2008, 11:29 PM
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wow... Shocked


fatoomchk


Nov 21, 2008, 2:45 AM
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clicky.


johnwesely


Nov 21, 2008, 4:02 AM
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Those were really sketchy hangers to begin with. Don't they make marine grade hangers?


atlnq9


Nov 21, 2008, 10:49 AM
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Re: [USnavy] The dangers of Stress Cracking Corrosion (pull test video) [In reply to]
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It is stress corrosion cracking not stress cracking corrosion

when I get some time here this weekend I'll explain the metallurgy behind it


hafilax


Nov 21, 2008, 11:16 AM
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Re: [atlnq9] The dangers of Stress Cracking Corrosion (pull test video) [In reply to]
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I'm assuming that it's tensile strength not "tinsel strength", as well.


USnavy


Nov 21, 2008, 11:30 AM
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Re: [johnwesely] The dangers of Stress Cracking Corrosion (pull test video) [In reply to]
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johnwesely wrote:
Those were really sketchy hangers to begin with. Don't they make marine grade hangers?
I have about 75 of these all pulled off from the same area. They are all the same age. Most of them fail at 35 - 40 kN.


bigo


Nov 21, 2008, 12:22 PM
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Why are you quoting the final failure load as the strength rather than the peak load? I looks like both hangers held over 10KN before the hanger failed at the crack. Because your puller applies constant displacement rather than constant load you are able to continue loading past the point of initial failure. The low number of 4.XX Kn is effectively the residual strength of the hanger post failure.

thanks for posting - cool stuff.


jdefazio


Nov 21, 2008, 12:44 PM
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Re: [bigo] The dangers of Stress Cracking Corrosion (pull test video) [In reply to]
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bigo wrote:
Why are you quoting the final failure load as the strength rather than the peak load? I looks like both hangers held over 10KN before the hanger failed at the crack. Because your puller applies constant displacement rather than constant load you are able to continue loading past the point of initial failure. The low number of 4.XX Kn is effectively the residual strength of the hanger post failure.

thanks for posting - cool stuff.

He is (in the video text) quoting the peak load before failure. The digital display is force in lbs.


JimTitt


Nov 21, 2008, 12:47 PM
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Re: [USnavy] The dangers of Stress Cracking Corrosion (pull test video) [In reply to]
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Before this degenerates into yet another thread of mis-information on SCC:-

Most of the cracks which are visible (really close-up macro photo´s of the surface would help!) look to be crevice corrosion, certainly the corrosion in the last bolt near the bolt hole are typical of this. It would also be suprising if the failure cracks were SCC as there wouldn´t normally be any tensile stress in this area.
Of course a cracked hanger is a cracked hanger, bad news either way.

Crevice corrosion and it´s related problem of pitting are common in coated stainless steel products and really anyone using this sort of coated hanger is asking for trouble. In order to get good adhesion of the coating it is nescessary to destroy the oxide coating with some kind of etch primer, thus removing the stainless steel´s protective barrier. Once the coating is damaged the crevice corrosion can begin.

After a number of requests for coloured bolts we looked into alternatives, one with some promise is titanium coating which gives yellow to brown shades and is becoming widespread in the architectural field. Unfortunately the process involves spraying a titanium plasma in a vacuum chamber and it is unlikely that the average climber is going to want to pay the inevitable cost.

I would not agree with the statement that SCC can occur up to 15 miles from the coast as it is completely misleading. It can occur anywhere the right conditions exist such as in industrial areas or, more relevant to climbers, where water run-off contains sufficient chloride salts.

There are three lines of defense against stress corrosion cracking, crevice corrosion and pitting.
The first and most effective is to ensure a smooth surface with all the ferrous compounds removed, allowing the chromium to form a complete oxide layer. Normally this is done by passivating to remove the ferrous compounds then polishing.
Second is to keep the surfaces clean, exposure to regular rainfall does wonders.
Third is to use a material with higher resistance, for crevice corrosion and pitting the PRN of the material is a good guide, against SCC you have to start looking at the nickel content of the alloy and then it´s physical state, the duplex stainless steels such as 1.4462/2205 having extremely high resistance to all of these kinds of corrosion, even at high temperatures.
To learn more about stainless steel and corrosion you can start here: http://www.ssina.com/index2.html or look at some of the useful info from Atlas Steel.

Anyway, it is 9.30 Friday evening here, it´s snowing and I´ve been bending stainless all day. I´m off for a beer! (Or two).


bigo


Nov 21, 2008, 2:19 PM
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Re: [jdefazio] The dangers of Stress Cracking Corrosion (pull test video) [In reply to]
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In reply to:
He is (in the video text) quoting the peak load before failure. The digital display is force in lbs.

oh right, thanks ...Blush


scottek67


Nov 22, 2008, 3:03 AM
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Re: [johnwesely] The dangers of Stress Cracking Corrosion (pull test video) [In reply to]
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I have to say I love these pull tests! marine grade hangers are stainless steel. the 10 year old hangers rated for 22 kN failed at 37-40 kN. that is fantastic! I feel safer already! the last 3 hangers in the video failed because they were fractured. apparently cracked from SCC. not sure if I believe it though. I work with stainless and am familiar with SCC in certain enviroments. stainless does corrode but cannot rust! cheaper stainless typically contains 0.03% carbon. the higher the carbon content the greater the strength. because there is rust on those hangers makes me believe they were installed with a non-stainless bolt in which electrolysis would cause serious corrosion. I'll keep cheering on the destruction as long as I don't have to pay for it!


USnavy


Nov 22, 2008, 10:34 AM
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scottek67 wrote:
I have to say I love these pull tests! marine grade hangers are stainless steel. the 10 year old hangers rated for 22 kN failed at 37-40 kN. that is fantastic! I feel safer already! the last 3 hangers in the video failed because they were fractured. apparently cracked from SCC. not sure if I believe it though. I work with stainless and am familiar with SCC in certain enviroments. stainless does corrode but cannot rust! cheaper stainless typically contains 0.03% carbon. the higher the carbon content the greater the strength. because there is rust on those hangers makes me believe they were installed with a non-stainless bolt in which electrolysis would cause serious corrosion. I'll keep cheering on the destruction as long as I don't have to pay for it!

Nope, they were installed with stainless steel 5/16" bolts. I have about 50 of these hangers and 20 bolts and they all look like this. Many of the hangers on the routes on the island look like that too (although not as bad as the ones in the video).

I do not really think its rust. It’s more or less the powder coating turning brown. The actual steel isn’t all that browned. It is to a point but most of the brown color resides on the actual powder coating.

Also only some of the old hangers failed at 40 kN. Many are failing below their rating because of SCC. Every one of the 50 hangers I have been placed in a marine environment.


USnavy


Nov 22, 2008, 5:55 PM
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Re: [JimTitt] The dangers of Stress Cracking Corrosion (pull test video) [In reply to]
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JimTitt wrote:
There are three lines of defense against stress corrosion cracking, crevice corrosion and pitting.

Second is to keep the surfaces clean, exposure to regular rainfall does wonders.

Well those bolts in the video were in Hawaii where we have more annual rainfall then any state. Actually it’s thought that the heavy rain expedited SCC. If there was no rain here one would believe that SCC would be non-existent or extremely slow.


JimTitt


Nov 23, 2008, 1:44 AM
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It´s hard to see how rainfall can accelerate corrosion in stainless steel and extensive testing has shown the opposite to be true.

The quote below is from a series of tests in Scandinavia:-
"It can also be established that the conditions inside the Söderledstunnel are very
corrosive causing by definition significant corrosion not only on grade 4310 (AISI
301), which is expected, but also on 4404 and 4436 (316L and 316 high Mo
respectively) as well. The reason is a combination of de-icing salt brought into the
tunnel with the traffic and lack of cleaning from rainfall.
The conditions underneath the Öresund Bridge are similar to those inside the tunnel,
i.e. the molybdenum-alloyed grade 4404 has suffered significant pitting and also the
duplex grade 2304, while the roadside conditions can be described as almost
harmless. This emphasises the importance of cleaning, which can be either from
rainfall or the result of a manual maintenance. And it also emphasises that airborne
chlorides or chloride-containing mist may cause staining, but not really any severe
corrosion."

Or from another study of stainless steel roadside furniture :-
"- The component should be placed so regular cleaning can occur, either by
manual maintenance work or natural rainfall.
- Design should minimize presence of crevices where dirt and above all salt can
be collected."

Or from Stainless steel World:-
"Atmospheric environments are most commonly divided into four categories: rural, urban, industrial and marine. The environments vary depending on the severity from a corrosive point of view. The importance of keeping the surface clean by regular washing to avoid staining and dust cannot be stressed enough."

If you need more I can supply a few dozen links.

Personally I still think the corrosion is pitting which has extended into crevice corrosion, both from the appearance of the hangers and the lack of any tensile stress in the application. If you can get a micrograph of the affected area it will be easier to tell for sure.

In an area such as Hawaii which is volcanic the run-off water is going to play a significant role, corrosion being associated with neutral or acidic solutions containing halides amongst others.

Airborne hydrogen sulphide is another culprit so don´t bolt down wind of anything active!

As you may have discovered, corrosion in stainless steel is an extremely complicated subject, even after 25 years in the marine industry I still need to call on the services of a specialist consultant occasionally.

Jim


USnavy


Nov 23, 2008, 4:05 AM
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Well those statements are quite conflicting based off the bolts we have here. The climbing site is way too far away from the ocean for there to be any physical contact of salt water from waves. There are no active volcano’s on the island so that does not apply. There are not any body’s of water containing salt or industrial parks in the mountains above the climbing site so that does not apply. So the only way salt can get onto the hangers that I can think of is from mist in the air being carried from the ocean up the mountain onto the hangers.

Well these hangers receive very regular rainfall which covers the "cleaning" part of your quote. They are never in contact with actual salt water so the only way the salt can get on the hangers is through the air and your quote specifically says "chlorides or chloride-containing mist may cause staining, but not really any severe
corrosion".

So where does the problem lie?


JimTitt


Nov 24, 2008, 1:38 AM
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As I have pointed out before, water run-off is one probable culprit.

Halides (which include common salt), which cause corrosion in stainless steel are produced by the reaction between halogen and other elements, and is considered to be how the sea became salty. This reaction is associated with volcanic activity and volcanic rocks contain relatively large amounts of these compounds.
So much that for example, Kala Namak, a black volcanic salt is mined in India and sold to gullible gourmets worldwide.

There is a study of halide content of volcanic rock under http://www.journalarchive.jst.go.jp/...amp;from=jnlabstract which includes references to studies done in Hawaii and gives some of the results that were obtained, although we are talking heavy going here!

Of course it could have been a bad batch of material, contamination or the incorrect use of acid etch before coating amongst other things that caused the corrosion. The only real way to know is send them for laboratory analysis but this will probably cost more than all the hangers in Hawaii!

The solution for bolt-ins is to use ones made from 1.4529/1.4565 (these are the European grades), so-called HCR anchors which are designed for corrosive environments. Hangers would normally be made of 1.4462 duplex but would have to be specially made.
Glue-ins are available in 1.4462 and are superior to bolt-ins with an expected life in sea water of over 100yrs (the material manufacturers guarantee).


chossmonkey


Nov 24, 2008, 4:29 AM
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johnwesely wrote:
Those were really sketchy hangers to begin with. Don't they make marine grade hangers?
They weren't to begin with. They were Metolius Enviro hangers made of 304 SS.

Marine grade hangers (316 SS) are fairly new within the last few years. Most SS hangers and bolts are 304 SS.


USnavy


Nov 24, 2008, 8:32 AM
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JimTitt wrote:
As I have pointed out before, water run-off is one probable culprit.

Halides (which include common salt), which cause corrosion in stainless steel are produced by the reaction between halogen and other elements, and is considered to be how the sea became salty. This reaction is associated with volcanic activity and volcanic rocks contain relatively large amounts of these compounds.
So much that for example, Kala Namak, a black volcanic salt is mined in India and sold to gullible gourmets worldwide.

There is a study of halide content of volcanic rock under http://www.journalarchive.jst.go.jp/...amp;from=jnlabstract which includes references to studies done in Hawaii and gives some of the results that were obtained, although we are talking heavy going here!

Of course it could have been a bad batch of material, contamination or the incorrect use of acid etch before coating amongst other things that caused the corrosion. The only real way to know is send them for laboratory analysis but this will probably cost more than all the hangers in Hawaii!

The solution for bolt-ins is to use ones made from 1.4529/1.4565 (these are the European grades), so-called HCR anchors which are designed for corrosive environments. Hangers would normally be made of 1.4462 duplex but would have to be specially made.
Glue-ins are available in 1.4462 and are superior to bolt-ins with an expected life in sea water of over 100yrs (the material manufacturers guarantee).

Well we have these same hangers on different climbing areas that are not in the mountains and do not have any runoff going onto them and they suffer from the same problem. One specific area is a big rock about 40 feet tall in the middle of nowhere. There is nothing on top the rock and there is no water runoff from anywhere. It suffers from SCC to a higher extent than that of our main climbing area (where these hangers came from). We believe the higher rate of corrosion is due to the fact that its only 300 feet from the ocean.

Here is a pic of said area:



Regarding the water runoff on these hangers; there is only about 200 feet of land above the climbing site. The wall is 800 feet up a 1000 foot tall mountain. It is extremely steep above the wall and there are no streams up there. Any water that was to pour down from above the wall would come from rain. What could possibly be up there that would cause contaminated run off? The rock above the wall and the rock that makes up the wall is basalt.

Another thing that discounts water runoff is the fact that the SCC is not limited to any one area. It is found at all climbing areas on the island including the one pictured above. Its ultra unlikely it’s a bad batch because we have many hundreds of hangers of different models and makes and they all suffer SCC to some extent. The Enviro hangers are the most prone to SCC but they are not the only ones prone to SCC. From what we found, the closer the climbing area is to the ocean the faster the hangers corrode which points back to the mist in the air causing the problem.


(This post was edited by USnavy on Nov 24, 2008, 8:33 AM)


dingus


Nov 24, 2008, 8:50 AM
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The SCC problems with stainless steel on Cayman Brac and areas such as Thailand are well-documented.

I don't buy the 'runoff' theory.

DMT


adatesman


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chossmonkey


Nov 24, 2008, 1:04 PM
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adatesman wrote:
Some good info and discussion coming out of this... Could we get some pics of the hangers?
Threaten him with the banz!!!!!!




As far as Thailand, I thought the issue was water and chemicals in the rock forming some nasty cocktail with the sea air?


JimTitt


Nov 25, 2008, 4:50 AM
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Before anyone jumps to the wrong conclusion I should point out that I am trying to show that corrosion in stainless steel is a very complicated subject and there are many more factors to be considered than one would think.
Without laboratory tests one is merely guessing the type of corrosion and its causes and so any cure is also only going to be a guess.
The University of Hawaii Dept of Mechanical Engineering have an extensive corrosion laboratory and test sites all over the islands. I´m sure they would only be too happy to look at your problem hangers and perform a few electron microscope scans, especially if you point out that members of the U.S. forces are risking death (the U.S. Army is a sponsor)!!
Their website is http://hawaiicorrosionlab.org/


dingus


Nov 25, 2008, 6:07 AM
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JimTitt wrote:
Before anyone jumps to the wrong conclusion I should point out that I am trying to show that corrosion in stainless steel is a very complicated subject and there are many more factors to be considered than one would think.
Without laboratory tests one is merely guessing the type of corrosion and its causes and so any cure is also only going to be a guess.

That makes good sense.

The only follow up comment I'd offer is this.... those bolt hangers that pulled apart so easily?

Fixing them won't wait on some study, sorry. We climbers often have to make our best guesses and move on. We experiment (too often) with our asses. Our tests are akin to military live-fire exercises.

So while yeah, I accept your assertion that SCC and stainless steel corrosion is a complex subject?

Those bolt hangers STILL gotta be replaced, nawmean?

So.... you are very familiar with this topic... what's YOUR BEST GUESS as to an appropriate longer term solution for bolting in salt spray environments?

Cheers
DMT


USnavy


Nov 25, 2008, 11:08 AM
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dingus wrote:
JimTitt wrote:
Before anyone jumps to the wrong conclusion I should point out that I am trying to show that corrosion in stainless steel is a very complicated subject and there are many more factors to be considered than one would think.
Without laboratory tests one is merely guessing the type of corrosion and its causes and so any cure is also only going to be a guess.

That makes good sense.

The only follow up comment I'd offer is this.... those bolt hangers that pulled apart so easily?

Fixing them won't wait on some study, sorry. We climbers often have to make our best guesses and move on. We experiment (too often) with our asses. Our tests are akin to military live-fire exercises.

So while yeah, I accept your assertion that SCC and stainless steel corrosion is a complex subject?

Those bolt hangers STILL gotta be replaced, nawmean?

So.... you are very familiar with this topic... what's YOUR BEST GUESS as to an appropriate longer term solution for bolting in salt spray environments?

Cheers
DMT

The hangers in those videos were replaced a long time ago. We found a solution many years ago to this problem. Titanium staple glue-in U bolts are not prone to SCC and have a life span of 10x that of conventional stainless steel hangers in marine environments. We have switched out to using these types of bolts here. There are still some hangers around but they are all younger than 10 years old.

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