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USnavy
Dec 27, 2009, 11:51 AM
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Background info: We are in a marine environment (Hawaii). We are in the final stages of a six year retrobolting project where we are replacing all the old 3/8" stainless steel bolts with titanium alloy glue-in staple bolts. The routes being retrobolted had either Metolius 304 SS hangers or Fixe 4 mm 304 SS hangers. Among the Metolius hangers, some are powder coated, some are not. All the hangers on all the retrobolted routes are about ten years old.
I pull test about 30% of the hangers I retrieve off the routes we retrobolt. One thing I have noticed is ONLY the Metolius SS powder coated hangers fail below their rating. Although the Fixe 4mm SS hangers and the non-powder coated Metolius hangers are suffering from some minor uniform corrosion and discoloration, they are not suffering from SCC, pitting, crevice, or any other severe corrosion issues. However the powder coated Metolius hangers are suffering from almost every corrosion issue relevant with many showing serious cases of Stress Corrosion Cracking. Consequently some fail with as little as a 2 kN load.
So why is it that ONLY the Metolius powder coated hangers suffer from any form of serious corrosion? All of our uncoated 304 stainless steel hangers fail at 90 - 175% of their rating despite they were placed on the same routes, on the same bolts, and are equally as old as the powder coated hangers. Not all the powder coated hangers fail from SCC below its rating but many do.
Other then powder coated hangers, the only other piece of SS material I have seen show any form of serious corrosion here are stud bolts (we used mostly 5-piece Rawl's). So what is unique about powder coating that affects the material’s ability to resist warm chloride corrosion?
Powder coated hangers:
Non-powder coated hangers:
(This post was edited by USnavy on Dec 27, 2009, 11:57 AM)
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trenchdigger
Dec 27, 2009, 1:57 PM
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I have been under the impression that powder coating was primarily for the purpose of coloring/hiding the hangers rather than preventing corrosion. As soon as the coating gets chipped, it will tend to entrap moisture and accelerate the corrosion process.
I'm also curious about your testing process. Are you fixing the hanger to a bolt prior to pulling?
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jaablink
Dec 27, 2009, 2:51 PM
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ask jimtitt
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kobaz
Dec 27, 2009, 3:31 PM
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How did the bolts themselves fare? And do you have some pictures of the old bolts, and the new ones.
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JimTitt
Dec 27, 2009, 3:32 PM
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You don´t need to ask me, I already told US Navy the answer on Nov 21st, 2008.
"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. "
But maybe this was too simple! Once the powder coating is damaged water can get underneath it and will be trapped, excluding the oxygen which forms the protective layer on the stainless steel. Crevice corrosion is then inevitable.
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philbox
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Dec 27, 2009, 9:11 PM
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I just hid a couple of off topic posts. I hope adatesman doesn't mind me stepping in to the lab forum and doing this during the holiday period when he could or could not be around to do the job.
Just to remind folks to keep to the topics at hand after all we make no bones about the modzing here in the lab.
Thanks all, the regular program will now resume.
(This post was edited by philbox on Dec 27, 2009, 9:12 PM)
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USnavy
Dec 28, 2009, 5:01 AM
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trenchdigger wrote: I'm also curious about your testing process. Are you fixing the hanger to a bolt prior to pulling?
Yes, I fix it to a grade eight 3/8" bolt then pull it to destruction in my jig.
kobaz wrote: How did the bolts themselves fare? And do you have some pictures of the old bolts, and the new ones.
The guy that installed the bolts used 3/8" x 3.5" 5-piece Rawls with a stainless steel bolt and carbon steel sleeves. Accordingly there is serious galvanic corrosion and the sleeves are completely rusted to hell. The actual 5/16" bolt itself is in equivalent condition to the 4 mm hangers I pictured above. With the acceptation of stud bolts, we have had no bolt failures and none of the 5/16” bolts within the Rawl bolt assembly show any signs of serious corrosion. The cones however seem to be corroded as it takes upwards of 120 ft. /lbs of torque to remove the bolt. It is likely the cones used are not stainless steel. In most cases the torque required to get the bolt to turn is so extreme the bolt breaks at the thread within a few turns. I have yet to remove any 1/2" Rawls as those are fairing up quite well.
JimTitt wrote: You don´t need to ask me, I already told US Navy the answer on Nov 21st, 2008. "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. " But maybe this was too simple! Once the powder coating is damaged water can get underneath it and will be trapped, excluding the oxygen which forms the protective layer on the stainless steel. Crevice corrosion is then inevitable.
Alright, thanks.
(This post was edited by USnavy on Dec 28, 2009, 5:03 AM)
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Kinobi
Dec 28, 2009, 9:29 PM
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Does this mean that any powder coating in any metal significantly increase chance of corrosion? Thanks,
E
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rschap
Dec 29, 2009, 2:19 AM
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JimTitt wrote: "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. "
Hum, how exactly? I have not done any research on this yet but I was wondering where you were coming from. Stainless steel does not have a protective barrier like a galvanizing or chrome. Stainless is essentially a steel alloy where chromium, nickel and other elements are added throughout to give it corrosion resistance. Generally the “etching” process consists of sand blasting but as the chromium and nickel run throughout it wouldn’t make a difference. As far as moisture getting between the powder coating and the metal, uhm ok, but I would think the plate up against the rock would have more of an effect then that, in which case they would all rot the same. I’m thinking electrolysis do to dissimilar materials is the culprit but not knowing what kind of rock or metal in the hanger it would be imposable to say.
I was wondering if maybe the Metolious hangers are powder coated steel and not stainless. Did it have a 304 SS stamp on it or do you know some other way?
On a side note the local powder coater puts a 50 year warranty on the coating, I would think they couldn’t do that if it was “more likely” to rot.
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rschap
Dec 29, 2009, 2:26 AM
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By the way, USNavy, I think what you’re doing is great. That is one hell of a corrosive environment and I would think a 316 SS would be the least you could do but Ti U-bolt glue ins are defiantly the ticket. Good job to all of you.
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USnavy
Dec 29, 2009, 2:37 AM
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rschap wrote: JimTitt wrote: "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. " Hum, how exactly? I have not done any research on this yet but I was wondering where you were coming from. Stainless steel does not have a protective barrier like a galvanizing or chrome. Stainless is essentially a steel alloy where chromium, nickel and other elements are added throughout to give it corrosion resistance. Generally the “etching” process consists of sand blasting but as the chromium and nickel run throughout it wouldn’t make a difference. As far as moisture getting between the powder coating and the metal, uhm ok, but I would think the plate up against the rock would have more of an effect then that, in which case they would all rot the same. I’m thinking electrolysis do to dissimilar materials is the culprit but not knowing what kind of rock or metal in the hanger it would be imposable to say. I was wondering if maybe the Metolious hangers are powder coated steel and not stainless. Did it have a 304 SS stamp on it or do you know some other way? On a side note the local powder coater puts a 50 year warranty on the coating, I would think they couldn’t do that if it was “more likely” to rot.
By "protective barrier" I believe he is referring to the passivation layer formed by oxygen on the steel.
Form Wikipedia:
"High oxidation-resistance in air at ambient temperature are normally achieved with additions of a minimum of 13% (by weight) chromium, and up to 26% is used for harsh environments.[9] The chromium forms a passivation layer of chromium(III) oxide (Cr2O3) when exposed to oxygen. The layer is too thin to be visible, and the metal remains lustrous. It is impervious to water and air, protecting the metal beneath. Also, this layer quickly reforms when the surface is scratched. This phenomenon is called passivation and is seen in other metals, such as aluminum and titanium."
The powder coated hangers are stainless steel. I e-mailed Metolius and they stated the Enviro hanger has always been manufactured out of stainless steel.
Galvanic corrosion is not the culprit here. Granted galvanic corrosion is playing a major roll in the corrosion of the carbon steel sleeves, but it has nothing to do with the hanger as only the powder coated hangers are showing signs of serious corrosion. So far about 20% of the powder coated Metolious Enviro hangers we replace show signs of serious corrosion, and about 1 - 3% of the non-coated Metolious Enviro SS hangers are showing signs of serious corrosion. So far 0% of the Petzl 3.2 mm SS hangers and 0% of the Fixe 4 mm hangers we have replaced are showing serious signs of corrosion. Serious corrosion has been found only on Metolious Enviro hangers and about 9 in 10 of the hangers that show serious signs of corrosion are powder coated.
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USnavy
Dec 29, 2009, 2:56 AM
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rschap wrote: By the way, USNavy, I think what you’re doing is great. That is one hell of a corrosive environment and I would think a 316 SS would be the least you could do but Ti U-bolt glue ins are defiantly the ticket. Good job to all of you.
We have actually never tried 316 SS hangers. Our 304 Fixe hangers fair up fairly well out here. We have never had any issues with them so we continued to use them until we decided to start using titanium. Now we only use titanium bolts with the exception of a temporary bolt here and there.
However we do use 316 SS for our quicklinks, chain and all the related for top out anchors. Surprisingly enough our 316 SS chain corrodes as fast as our 304 SS chain. The chain itself never shows serious signs of serious corrosion (SCC, pitting, ect.) but the material starts to discolor fairly quickly. I have however found 316 SS quicklinks last a bit longer then 304 SS quicklinks.
I have placed some sample 316 SS bolts for study. But unfortunately I have only seen 316 threaded wedges. I do not believe Powers makes their Power-Bolt in 316. Threaded studs have always been a bad idea around here. Some of our 304 SS threaded studs shattered when I hit them with a hammer well I was replacing them.
The Power-Bolt seems to fair up well to marine corrosion, likely do to the lack of exposed threads. Thus I am very confident our 1/2" 304 SS Power-Bolts will outlast the 1/2" 316 SS threaded wedges I placed for study.
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JimTitt
Dec 29, 2009, 10:36 AM
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As USNavy points out, the protectiver layer is chrome oxide. When you have a piece of stainless steel the surface is a mixture of the various alloying metals and so it is normally nescessary to remove at least the iron parts to allow a complete chromium oxide layer to be formed. This is normally done by passivating in an acid which removes the iron (there are a number of suitable ones). Alternatively polishing, either mechanical or electrical will achieve this.
Paint, powder coat and of coarse resins (which powder coats are) don´t adhere to the chromium oxide very well so you can either roughen the surface with abrasives or sandblasting or create a pitted surface by using some type of etch primer. All of these reduce the resistance to corrosion to a lesser or greater extent, the use of the usual phosphate compounds being somewhat controversial in the coating industry as a number of these cause more problems than they solve. If you search for information from the coating industry you will discover there is no concensus on what works and if you look at the information from end users you will find a general opinion that nothing works!
The correct way to achieve a good bond for resins and paints is to wet abrade using a suitable resin as the `wet´ part but this is totally impractical in this application.
There are other ways of colouring stainless steels, the established methods using a combination of acids and electrolysis.
Surface heating has also been used but is hard to achieve accurate colour control.
A more recent development is PVD (Physical Vapour Deposition) which uses a vacuum plasma chamber to deposit titanium onto the surface which forms titanium nitride, this is how titanium coated drills and cutting tools are produced.
The current competetive market in hangers and general lack of demand make these treatments commercially unnatractive!
No powder coater is going to give any sort of guarantee whatsoever for an object bolted up against a piece of rock, hit with a hammer, clipped with karabiners and probably flexed or bent by falls, all of which will damage the coating.
Jim
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sed
Dec 29, 2009, 3:11 PM
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Out of curiosity, I'm wondering if temperature influences oxidation rate. Where I'm going is that if surfaces are painted different colors or levels of reflection (shiny verses dull) would they be higher or lower in temperature during sun exposure and therefore rust at different rates?
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JimTitt
Dec 29, 2009, 4:38 PM
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Yep, shiny is better!
The three types of corrosion in stainless we normally worry about are temperature sensitive. In particular the critical temperatures for the start of pitting (Critical Pitting Temperature) and crevice corrosion (Critical Crevive Corrosion Temperature) are quite low for most of the alloys we use. Above these temperatures corrosion starts and the higher the temperature generally the faster the corrosion.
These two values (CPT and CCCT) are closely related to the PREN (Pitting Resistance Equivalent Number and normally one uses this as a guide to the corrosion resistance.
SCC is not sensitive to temperature in the same way but has a threshhold temperature above which it starts, normally around 60°C for most alloys. The rate of crack propagation is morerelated to the stress on the material than the temperature after this point.
Most important is keeping the material clean and preventing any stagnant water from lying in cracks and crevices which is why an uncoated hanger exposed to the weather is better than one with a damaged coating. Kitchen sinks last decades without any problems despite the amounts of hot salt water, chemicals, bleach etc poured down them but remove the fittings from them and you see where the corrosion has started, the plastic drain connections being a popular site.
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flamer
Dec 31, 2009, 12:49 AM
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USnavy wrote: Background info: We are in a marine environment (Hawaii). We are in the final stages of a six year retrobolting project where we are replacing all the old 3/8" stainless steel bolts with titanium alloy glue-in staple bolts.
Just to be clear...what you are doing is REbolting, NOT RETRObolting.
There is a very big difference and alot of folks get upset about Retrobolting.
Good on ya' for taking the time and effort to make climbs safer for everyone else.
josh
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rschap
Dec 31, 2009, 4:41 AM
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USNavy, I was curious, are you guys drilling new holes or are you reusing the old holes? The reason I ask is I was wondering what you do about the residual rust from the steel sleeves, do you somehow neutralize it? I’m doing research right now for rebolting some of the old and nasty anchors out here and while we don’t have the corrosion hazard that you guys have we do have some soft sandstone and really old steel anchors. Rust is not unheard of here but it takes a lot longer do to it being a high desert.
JimTitt, I have been searching the internet for information to back up your claim that powder coated stainless steel corrodes faster than non powder coated. The reason I’m looking for info on it is the National Park Service has one hell of a bureaucracy and a lot of paper work involved with getting permission to place permanent anchors in the monument. One of the regulations they have is all anchors have to be camouflaged as much as possible so if coated hangers are no good I need written documentation to support the claim. I have been to the ASCA web site and they say “If you want to minimize the paint hassle factor, buy rock-colored hangers such as those made by Metolius or Fixe.” off their “Reducing the visual impact of bolt hangers and other fixed gear?” page. Not to say that they know all but they are the most respected as far as proper rebolting techniques and I do plan on following there guidelines as I replace anchors or even establish new routes.
Sorry if this should have been posted in a new thread but it seemed on topic to me.
Randall
Edit to fix link
(This post was edited by rschap on Dec 31, 2009, 5:06 AM)
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USnavy
Dec 31, 2009, 7:54 AM
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rschap wrote: USNavy, I was curious, are you guys drilling new holes or are you reusing the old holes? The reason I ask is I was wondering what you do about the residual rust from the steel sleeves, do you somehow neutralize it? I’m doing research right now for rebolting some of the old and nasty anchors out here and while we don’t have the corrosion hazard that you guys have we do have some soft sandstone and really old steel anchors. Rust is not unheard of here but it takes a lot longer do to it being a high desert. JimTitt, I have been searching the internet for information to back up your claim that powder coated stainless steel corrodes faster than non powder coated. The reason I’m looking for info on it is the National Park Service has one hell of a bureaucracy and a lot of paper work involved with getting permission to place permanent anchors in the monument. One of the regulations they have is all anchors have to be camouflaged as much as possible so if coated hangers are no good I need written documentation to support the claim. I have been to the ASCA web site and they say “If you want to minimize the paint hassle factor, buy rock-colored hangers such as those made by Metolius or Fixe.” off their “Reducing the visual impact of bolt hangers and other fixed gear?” page. Not to say that they know all but they are the most respected as far as proper rebolting techniques and I do plan on following there guidelines as I replace anchors or even establish new routes. Sorry if this should have been posted in a new thread but it seemed on topic to me. Randall Edit to fix link
In about 98% of the cases we drill new holes. When we reuse a hole we drill out the old 3/8" hole with a new 1/2" hole as the glue-in bolts require a larger hole. In which case any residual rust is eliminated. Furthermore sense the bolts are glue-ins, the hole is cleaned well.
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JimTitt
Dec 31, 2009, 11:53 AM
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Before everyone gets the wrong end of the stick, it is damaged powder coating which causes the problem. You then get so called under-film corrosion, usually either scab (rust) or filiform.
There is information on this in books such as `Powder Coating: failures and analysis´by Pietschmann and the powder coating manufacturers should also be able to help you more. Maybe easier is to go to http://www.powder-coater.com/ and ask on their forum.
Whether an undamaged coating would increase the corrosion resistance of a stainless object is obviously a question of the corrosive condition and the powder coatings resistance to it. This is a long and complicated topic but largely irrelevant since it is virtually impossible to install or maintain a bolt hanger with the coating intact.
I would suggest you contact the manufacturers of the hangers for an estimated lifespan of their products and how to ensure the coating does not become damaged at installation or in use.
(Personally I´d be looking at one of the pulsed electrolytic or similar methods of colouring the hangers rather than a paint film as most of these are shown to increase the corrosion resistance).
Jim
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rschap
Jan 1, 2010, 4:10 AM
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I like the side shuffle. When you make a comment like “really anyone using this sort of coated hanger is asking for trouble” to me means you think there is a big problem with this particular item. I’m not sure how I’m getting on the wrong end of the stick. I was just trying to find out if you had evidence to back your claim, especially since you’re a vendor in direct competition with the manufacturers in question, or if it was your opinion. While it seems to me that there is a possibility you are right I personally am still not convinced.
(This post was edited by rschap on Jan 1, 2010, 4:14 AM)
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USnavy
Jan 1, 2010, 10:08 AM
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Out of the 750+ retired hangers we have, 98% of those in seriously poor condition are powder coated. Two of those powder coated hangers failed on a fall. I don’t know how powder coated hangers last in the normal world, but in a marine environment the Enviro coated hangers are pure trash.
(This post was edited by USnavy on Jan 1, 2010, 10:13 AM)
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JimTitt
Jan 1, 2010, 7:41 PM
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You are right about the side shuffle, it is not normal practice for manufacturers to run down others products (at least publicly)!
However it is both my personal and professional opinion that powder coating bolt hangers is not the best solution for colouring bolt hangers without compromising the corrosion resistance, though the various manufacturers may have succeeded in producing acceptable results for most applications.
Applying a coating to a safety critical item and therefore preventing visual inspection for corrosion which may have weakened it to the point of endangering life is also not acceptable in my opinion and for this reason I do not supply any coated products.
Jim
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atlnq9
Feb 5, 2010, 1:31 AM
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I agree with Jim, paint is not good because it prevents part inspection. Something which may be worth a look at is a dyed metalic zinc paint instead of whatever type of powder coating they use. It will provide moderate galvanic protection. You would still have to big issue of not being able to easily inspect them.
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