Chemical reactions and self-drive hangers (M8 Spits)

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Chemical reactions and self-drive hangers (M8 Spits)

Postby Jeff Bartlett » Sep 9, 2012 11:33 pm

OK, I've been looking for a reference for this for a while and I give up. I know one of you rigging geeks fine cavers can help me out!

It has been my understanding that the primary factor which causes self-drive anchors -- Spits, M8s, Petzl Cheville Autoforeuse and probably several other names -- to wear out is a chemical reaction between the aluminum alloy hanger and the hardened steel bolt/sleeve. Petzl's own page for this product points to a dead link about corrosion in climbing hardware that would probably be helpful here if it still existed. I have two questions:

(1) Does anyone have a reference which discusses this corrosive process?
(2) Can it be entirely prevented by using the Raumer stainless steel M8 bolts and hangers, which work with the Cheville Autoforeuse sleeves, or is there still a reaction between hardened steel and stainless steel? Please note: these hangers are not available in this country, though they can be purchased from international speleo-vendors.

This is the only reference for this issue I can presently find, from Alpine Caving Techniques, which simply acknowledges the problem and makes a recommendation for avoiding the aluminum hangers (which, of course, are in caves everywhere all over the globe and are sold directly by Petzl for use with the sleeves).

Alpine Caving Techniques wrote:Most hanger plates are manufactured from a light aluminum alloy (zicral), for obvious considerations of weight. These hangers should never be left for long in a moist environment. They will corrode as a result of an electrochemical reaction between two metals (with water acting as the electrolyte): the steel in the anchor, and the aluminum in the hanger. The aluminum oxidizes into alumina, a white powder that forms a translucent gel in the presence of water (this cannot occur under water due to the lack of oxygen). After a long period of disuse the hanger can corrode to the point of passing right over the head of the screw (and extracting) the moment It is loaded. So in particularly humid caves. always use stainless steel hangers.


What is this process? Does the use of stainless M8 hangers (again, not available in this country, at least not the Raumer version) prevent the problem, or simply mitigate it?
"Although it pains me to say it, in this case Jeff is right. Plan accordingly." --Andy Armstrong
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Re: Chemical reactions and self-drive hangers (M8 Spits)

Postby PeterFJohnson » Sep 10, 2012 12:39 am

Jeff,

You might find this old article from the NSS news relevant. Even though it deals with aluminum carabiners and steel hangers, it discusses the corrosion process you describe in question #1. :

http://theeyegame.com/speleo/carab/agecarab.htm

The article seems to conclude that stainless steel will not prevent the reaction that occurs between aluminum alloys and steel. Whether it mitigates it is unclear. I am looking at this quote in particular:

"No coating can prevent corrosion of carabiners in many cave applications. Any nicked or scratched surface, such as that which invariably results when a carabiner is loaded on a steel bolt hanger, will allow corrosion to begin. The commonly recognized type of galvanic cell then exists between the steel hanger (even if it is stainless) and the carabiner. The situation is aggravated by the large cathodic surface of the hanger concentrating the current flow through the small anodic area of the scratched anodize on the carabiner."

Don't know whether or not that helps, but as you probably know aluminum alloys will undergo this corrosion even if they aren't in direct contact with steel.
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Re: Chemical reactions and self-drive hangers (M8 Spits)

Postby Martin Sluka » Sep 10, 2012 12:59 am

Image Original photo Frantisek Musil
After a research the conclusion was: the total corrosion is result of impurities in material of carabiner which created local electrochemical cells.
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Re: Chemical reactions and self-drive hangers (M8 Spits)

Postby Scott McCrea » Sep 10, 2012 7:42 am

Just in case you haven't seen these already...
http://www.caves.org/section/vertical/nh/47/bolting.pdf
http://theeyegame.com/speleo/Pubs/anchors/Anchors.htm
http://www.safeclimbing.org/education/bomberbolts.htm

As for your specific question, I dunno. I just use all stainless for permanent anchors and cheap steel for temporary.
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Re: Chemical reactions and self-drive hangers (M8 Spits)

Postby Jeff Bartlett » Sep 10, 2012 10:15 am

Martin Sluka wrote:Image Original photo Frantisek Musil
After a research the conclusion was: the total corrosion is result of impurities in material of carabiner which created local electrochemical cells.

WOW! Martin, that's incredible!!
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Re: Chemical reactions and self-drive hangers (M8 Spits)

Postby gindling » Sep 10, 2012 10:27 am

I remember a few years ago there being an article in the NSS News about this. But no, the use of SS does not change the outcome, nor does it slow the process. The ion exchange is between the aluminum and the carbon ( i believe ) that is present in all steels. I have seen SS bolts with aluminum hangers covered in this white powder and gel many times.I've seen people open up their gear bags that they put away moist a week or so ago, after washing or just the crazy humidity out east, and their gear will be covered with the stuff, mainly the biners and the aluminum gear.
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Re: Chemical reactions and self-drive hangers (M8 Spits)

Postby Jeff Bartlett » Sep 10, 2012 10:31 am

I think I should clarify a bit. We know the following combinations cause in-cave corrosion to the aluminum components:

(1) Self-drive sleeve AKA Petzl Cheville Autoforeuse (hardened steel) + Petzl Vrillee combo stud (is this steel or aluminum?) & hanger (aluminum), or similar stud/hanger combo utilizing aluminum
(2) Anything involving a steel hanger that has an aluminum carabiner permanently affixed to it

But what about this combination?

(3) Self-drive sleeve AKA Petzl Cheville Autoforeuse (hardened steel) + Raumer M8 bolts (stainless steel) + Raumer M8 hangers (stainless steel)

While we're all in agreement that good, long stainless expansions bolts are far superior to self-drives in all applications, is there any reaction to be had in combination (3) that would cause the bolts to wear out prematurely, due to corrosion, as in combination (1)? What I'm curious about is whether or not the stainless bolt and hanger accelerate the corrosion in the same way that aluminum components would, and what kind of lifespan a self-drive with NO hanger in place actually has.



The implication I gain from your link at http://theeyegame.com/speleo/Pubs/anchors/Anchors.htm is that the answer is yes, though no time frame is given. I'll quote some relevant portions of it here. Maddeningly, the writer just says "yes, it happens" without giving any idea of how much faster a sleeve + stainless hanger will corrode than a sleeve all by itself with no hanger. Quoting:

The casings are made from steel but have a coating (i.e. plated steel) to prevent corrosion. Of course, they still do corrode, the plating is damaged when installation occurs. Generally an Aluminium alloy hanger is fitted to the casing by a high tensile 8 mm diameter steel (Grade 8.8) bolt ... Leaving the hanger in situ enhances the corrosion potential of the anchor; Aluminium and steel in close proximity in a wet environment leads to electrochemical corrosion.

Anyway, the fact is that these self-drilling bolts gradually decay and the integrity and safety of the anchor begins to diminish. Many of the spits in Tasmania have were installed in the heady days of the 70’s or early 80’s and so many of these have been installed for one to two decades. Some have had hangers left in them (to assist in relocation), these are more likely to be in a worst state due to electrochemical corrosion (see below). I have not heard of any failing (yet), but from experience overseas, this will gradually begin to occur.


They go on to discuss the issues with dissimilar metals:

When two different metals (or grades of the same metal) are in contact, especially when moisture is involved there is a potential for electrochemical corrosion (i.e. galvanic coupling). A stainless steel expansion bolt might be fitted with components made from different grades of stainless steel. Aluminium alloy hangers are fitted with a high tensile steel bolt. Often components made of steel (e.g. bolt casing) are plated with another material (e.g. Cadmium or Zinc (i.e. galvanised)) to prevent/slow corrosion. So, any particular anchor can have a variety of metals in intimate contact. Ideally all components in an anchor will be made of the same material.

Stainless steel does still corrode, it just does it at a much slower rate than normal mild steel. In sea-water, where a mild steel will corrode at a rate of about a millimetre every six years, an austentitic stainless steel will corrode about a millimetre every 200 years. This corrosion can be greatly accelerated by galvanic coupling when two dif ferent grades remain in contact. Hellyer (1988) reports that in Thailand, on seeping limestone sea cliffs, (where climbing is popular), six year old stainless bolts have already begun to show visible signs of corrosion. There have been several failures causing several serious injuries.
"Although it pains me to say it, in this case Jeff is right. Plan accordingly." --Andy Armstrong
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Re: Chemical reactions and self-drive hangers (M8 Spits)

Postby hunter » Sep 10, 2012 12:49 pm

Jeff,
This is one of the better carabiner based discussions I have seen:
http://rockclimbingcompany.blogspot.com/2009/09/corrosion-in-climbing-carabiners.html

It isn't perfect for your question but should serve as a good starting point if you want to dig deeper, not that a lot of the more relevant info is in the middle.

One very IMPORTANT note on this subject. The combination you suggest results in all stainless steel showing. This means people won't see when the anchor is corroding, for long term placements this is dangerous and it would be better to buy carbon steel hangers/bolts so you have roughly the same metals and they corrode at the same rate (Fixe sells M8 plated carbon steel hangers and M8 carbon steel bolts are readily obtained).

So your main question is, will stainless and carbon steel co-exist better than aluminum and carbon steel with respect to galvanic corossion? Just from my experience inspecting and replacing bolts (in basalt, tuff, limestone and a couple of others) I believe the answer is that this combination has very little galvanic corrosion, generally the carbon steel bolt is thoroughly rusted along it's length and the contact point between the stainless and carbon steel is not worse than the rest. The caveat to this is around salt water or sulfur (guads). The carbon steel in these areas is massively rusted and the head of the bolt usually breaks off so I don't know how the rest looks.

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Re: Chemical reactions and self-drive hangers (M8 Spits)

Postby shibumi » Sep 18, 2012 7:36 pm

The problem is that there's a lot of variables. The chemical environment can change even within a cave so it's not a simple equation.

IN GENERAL though, the closer the stainless steels match the less corrosion there will be across the board all other things being equal.

The stainless bolts and hardware put in for Emily's rescue in Lech in the Great White Way look as good as the day they were put in. I have a picture of them taken in 2007 when I inspected them while we were doing rescue preplanning and you could have told me they were put in yesterday. I have SS and hardware bolts in a gypsum passage in a cave in Kentucky that I put in in '95 that have zero corrosion, yet I have bolts I put in that same cave during that same era that look much older.

My basic takehome though is that the only time I am using self-driving bolts and aluminum hangers/biners is in my small push bolting kit. Trade routes or places where the bolts cannot be inspected before use should be exclusively matching stainless whenever possible.

I do have some older carbon steel bolts on stainless hangers in that same cave that are of the same era and some of them look fine and some of them are a little corroded, but not so bad that I won't still use them. Hardened steel should last longer than mild steel under the same environments.
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