mixing metals: Bad, good, Irrelevant?


Original Post
kenr · · Unknown Hometown · Joined Oct 2010 · Points: 10,599

in another thread, Jim Titt wrote:
> If you add a galvanised hanger to a stainless bolt
> and there _is_ galvanic corrosion the zinc plating is sacrificed
> to protect the hanger but does absolutely nothing to protect the
> stainless bolt which anyway is not being corroded.

I've often heard from respected (American?) sources that "mixing metals" with bolts + hangers + anchor hardware is Bad. The explanation sometimes associated was "galvanic corrosion".

Here I'm learning that it's more complicated. So really Stainless Steel does not have galvanic corrosion, so sounds like "mixing metals" is actually not Bad for the stainless parts.

I guess Stainless must still be subject to _other_ kinds of corrosion (like from sodium ions?), otherwise nobody would pay for titanium.

Which gets me to plain old Carbon steel, which I am understanding _is_ subject to galvanic corrosion (as well as other kinds), so using it together with Zinc is Good for it (tho bad for the zinc), so that's why for outdoor use we prefer zinc-Plated also called "galvanized" steel.
And I'd guess that putting a pure zinc washer between a non-stainless bolt and non-stainless hanger would be Good for bolt+hanger (but I haven't found where to purchase pure zinc washers).

Which gets me to my real "actionable" question: What about installing a
Stainless hanger on a Plated-steel bolt?
Irrelevant for corrosion rate of the bolt?
or Bad for the bolt?

Ken

Brady3 · · Unknown Hometown · Joined Apr 2014 · Points: 15

http://www.ssina.com/corrosion/galvanic.html

With a stainless hanger and plated bolt the zinc would corrode away relatively quickly, potentially faster than if they were switched (plated hanger and stainless bolt).  Once the zinc is gone the carbon steel may corrode faster than otherwise depending on the rock ("Research has shown that galvanic corrosion is not a concern between stainless and carbon steel in concrete." from the link above).

So, yes galvanic corrosion is a problem with mixing metals.  But it will actually cause the stainless to corrode more slowly (more noble than zinc or carbon steel), so the part that should fail first would be the plated part.  Surface area also plays a part in the rate of reaction, a larger surface area of the anode (the zinc or carbon steel) will slow the reaction but not prevent it.

I work with electrolytic reactions, which are not the same as galvanic reactions (loosely opposite of each other), so I am not an expert.

Gregger Man · · Broomfield, CO · Joined Aug 2004 · Points: 1,249

There are many cases where folks upgraded the hardware on a route in the late '90s by adding stainless SMC hangers to plated wedge bolts. I've replaced many of them, and although it is impossible to say whether the plated bolts would have rusted as quickly or not if they had not been in contact with stainless hangers, they have all been pretty friggin' rusty. I consider it to be an unwise practice, especially since the plated bolt is on the losing end of the voltage gradient and can't really be inspected. 

Jim Titt · · Germany · Joined Nov 2009 · Points: 490
kenr wrote: in another thread, Jim Titt wrote: > If you add a galvanised hanger to a stainless bolt > and there _is_ galvanic corrosion the zinc plating is sacrificed > to protect the hanger but does absolutely nothing to protect the > stainless bolt which anyway is not being corroded. I've often heard from respected (American?) sources that "mixing metals" with bolts + hangers + anchor hardware is Bad. The explanation sometimes associated was "galvanic corrosion". Here I'm learning that it's more complicated. So really Stainless Steel does not have galvanic corrosion, so sounds like "mixing metals" is actually not Bad for the stainless parts. I guess Stainless must still be subject to _other_ kinds of corrosion (like from sodium ions?), otherwise nobody would pay for titanium. Which gets me to plain old Carbon steel, which I am understanding _is_ subject to galvanic corrosion (as well as other kinds), so using it together with Zinc is Good for it (tho bad for the zinc), so that's why for outdoor use we prefer zinc-Plated also called "galvanized" steel. And I'd guess that putting a pure zinc washer between a non-stainless bolt and non-stainless hanger would be Good for bolt+hanger (but I haven't found where to purchase pure zinc washers). Which gets me to my real "actionable" question: What about installing a Stainless hanger on a Plated-steel bolt? Irrelevant for corrosion rate of the bolt? or Bad for the bolt? Ken

As I said, the stainless bolt isn´t interested in having a zinc-plated part attatched to it, it won´t corrode but may rob the zinc from the other part. The problem arises when the zinc coating dissapears through leaching or is damaged and the base steel will then attack the stainless with catastrophal results.

The zinc is applied to steel parts to protect the steel itself if the coating becomes damaged, 99% of the time the plated parts you see are actually zinc chromate treated (commonly called passivating in the galvanising industry) after plating to protect the zinc from the environment (leaching from rain etc) and there will be no ion interchange anyway to the stainless part as the chromate is effectively electrically neutral. The bolt streaks in the second photo on the other thread are from hot-dipped bolts which were not passivated and the water  run-off has stripped the galvanising completely then allowing the steel itself to rust. In limestone areas this is more of a problem than other rock types but it occurs no matter what.

It all goes wrong if the coatings are damaged or not completed which is why the European Standard requires all parts of rock anchors to be of the same material and this is also a UIAA requirement.

John Byrnes · · Fort Collins, CO · Joined Dec 2007 · Points: 451
kenr wrote: I guess Stainless must still be subject to _other_ kinds of corrosion (like from sodium ions?), otherwise nobody would pay for titanium. 

Small nit-pick: it's not NaCl but CaCl2 and MgCl2 that cause Stress Corrosion Cracking in stainless, with CaCl2 being worst.   

The sea contains all the necessary ingredients, so sea cliffs are the worst in regards to SCC.  However, it's been shown that limestone, CaCO3, dissolves in rainwater (and eg. forms stalactites) and the Ca recombines with Cl ions in the runoff, and ends up on the bolts when the runoff dries.  So inland cliffs are not immune, but they generally take longer to crack, or may never due to other environmental conditions (Temps, Rain, exposure, etc.).

Which gets me to my real "actionable" question: What about installing a Stainless hanger on a Plated-steel bolt? Irrelevant for corrosion rate of the bolt? or Bad for the bolt? Ken

I think several guys above have answered that with a clear "Don't do it."    The Access Fund is asking us to not use Plated steel at all.  Go all stainless. 

Pnelson · · Unknown Hometown · Joined Jan 2015 · Points: 95

I always thought mixing metals was AWESOME.

Francis Haden · · Unknown Hometown · Joined Dec 2016 · Points: 27

Matching metal grades is one of the 'standard rules' and I would add that matching from the same supplier is also of benefit since grade is one consideration but material quality is another. Previous threads have discussed that some bolt suppliers go to greater extents than others in ensuring the quality of their stainless products.

Another aspect of metal matching is the state of the stainless steel itself; active or passive and that even in metal matched circumstances, corrosion can still initate. 

An active stainless steel is essentially a passive stainless steel with its chromium oxide layer removed due to any number of corrosion mechanisms or by physical damage. Expansion bolts can be passivated and polished yet the act of installation; tightening the locking nut, scratching the metal surface on a hanger, hammering the bolt into the hole, can all leave metallic deposits and/or damage the chromium oxide layer thereby allowing the damaged area to revert to its active state. Alternatively a 316SS bolt holding down a 316SS hanger may experience differential oxidation around the bolt / hanger interface whereby the hanger remains passive, but the bolt, due to machining and work hardening may revert to its active state and corrosion of the bolt head.

Corrosion is also affected by area ratio (typically a concern in sheet metal held down by different material grade fasteners). If the larger item of stainless steel is of higher reactivity than the fastner eg 316 SS hanger combined with a 304 SS bolt then this could encourage corrosion in the way that reversing the metal grades might not.

These issues, whether they are significant as individual processes (or not) just reinforce the advantages that a single piece, non welded, glue-in bolt have over a multi component expansion bolt.

snowdenroad · · Unknown Hometown · Joined Jun 2007 · Points: 40

I climb at an area with powder coated Metolius hangers on plated powers 5 piece bolts, some of which I placed 15 years ago.  Does the coating perhaps inhibit galvanic corrosion?   

kenr · · Unknown Hometown · Joined Oct 2010 · Points: 10,599
Francis Haden wrote:

If the larger item of stainless steel is of higher reactivity than the fastner eg 316 SS hanger combined with a 304 SS bolt then this could encourage corrosion in the way that reversing the metal grades might not.

This and the careful analysis around it seems to contradict other careful analysis on this thread. And I know I'm not competent to sort it out.

So I'll ask for some application of analysis to what I see is the currently "live"
actionable mixed-metal questions on the USA scene:

* attaching Plated-Steel external anchor hardware (e.g. quick-links + carabiners + mussy hooks) to the "foundational" 304 Stainless wedge bolts and hangers. Because PS anchor hardware is cheaper and still being sold by respected American distributors, and bleeding of the PS itself is not seen as much of an issue in quasi-desert environments.

* placing a new 304 Stainless hanger on an old Plated Steel bolt (or an old bolt which it is unknown if it is Plated or Stainless) - (see Gregger Man's post above). Because the hanger can be seen to be rusty while the (more dangerous) corrosion at the bolt-rock interface cannot. Because replacing the underlying bolt takes much more work. Because various American authorities say "Stainless is right, Plated is bad." 

* USA web distributor now selling 316 Stainless external anchor hardware at a remarkably attractive price. But there's almost no 316 bolts and hangers installed in USA so far. Therefore I assume that many of these well-designed inexpensive units are getting attached to 304 Stainless bolts -- and to Plated-Steel bolts.

How about some further analysis of those questions?

Ken

Jim Titt · · Germany · Joined Nov 2009 · Points: 490

Galvaanic corrosion is something of a black art, there is an entire industry trying to stop stuff rotting away to more or less success.

Size does make a difference (surface areas exposed to the electrolyte) but with respect to 304 on 316 it´s kinda irrelevant, all the tables give the galvanic potential as overlapping i.e they have the same potential and will not make a galvanic cell. There is a difference between passive or active stainless steel but for our application this appears to be irrelevant, the two grades and states have been succesfully mixed together for decades.

Adding stainless to a plated bolt is potentially bad.

Adding plated hangers to stainless bolts is definately bad.

As all components are readily available in stainless steel or plated there is no reason to mix them and as has been pointed out several times NOBODY in any position of authority or influence in the climbing world recommends you do so. If you can´t afford to do the job at least reasonably then either improve your fund-raising skills or stop when your money runs out. It saves other people  work and expense in the future.

Rich Farnham · · Unknown Hometown · Joined Aug 2002 · Points: 278
Jim Titt wrote:

...As all components are readily available in stainless steel or plated there is no reason to mix them and as has been pointed out several times NOBODY in any position of authority or influence in the climbing world recommends you do so. If you can´t afford to do the job at least reasonably then either improve your fund-raising skills or stop when your money runs out. It saves other people  work and expense in the future.

Jim - I get your point, but would like to ask you more about one of Kenr's scenarios:

"attaching Plated-Steel external anchor hardware (e.g. quick-links + carabiners + mussy hooks) to the "foundational" 304 Stainless wedge bolts and hangers. Because PS anchor hardware is cheaper and still being sold by respected American distributors, and bleeding of the PS itself is not seen as much of an issue in quasi-desert environments."

Given that resources aren't infinite, and there are a LOT of bolts/anchors that need upgrading, is this really a dangerous situation?  Stainless screw-links, etc. are significantly more expensive.  For anchors that see a lot of use, I like the idea of using plated hardware for the external, easily-replaced components (screw link and ring, and/or mussy hook).  It saves money that can be used to upgrade other anchors, and doesn't seem to be dangerous.  From what I've read, the setup I describe doesn't really have the contact surface area to be an issue, and the interface is out in the air where the exposure to moisture is brief (compared to at a hanger/bolt interface).

Like Kenr, I'm trying to keep up with the topic but am not an expert and would appreciate the input of people like yourself that are.  Thanks!

Mike Slavens · · Houston, TX · Joined Jan 2009 · Points: 35
Rich Farnham wrote:

"attaching Plated-Steel external anchor hardware (e.g. quick-links + carabiners + mussy hooks) to the "foundational" 304 Stainless wedge bolts and hangers. Because PS anchor hardware is cheaper and still being sold by respected American distributors, and bleeding of the PS itself is not seen as much of an issue in quasi-desert environments."

Given that resources aren't infinite, and there are a LOT of bolts/anchors that need upgrading, is this really a dangerous situation?  Stainless screw-links, etc. are significantly more expensive.  For anchors that see a lot of use, I like the idea of using plated hardware for the external, easily-replaced components (screw link and ring, and/or mussy hook).  It saves money that can be used to upgrade other anchors, and doesn't seem to be dangerous.  From what I've read, the setup I describe doesn't really have the contact surface area to be an issue, and the interface is out in the air where the exposure to moisture is brief (compared to at a hanger/bolt interface).

No that is not dangerous if you are in a dry enough environment where the PS hardware has acceptable resistance to general corrosion.  The steal that would be corroding (PS) like you said is 1) out in the air not really allowing the galvanic cell to form needed for galvanic corrosion, 2) it is very inspect-able for signs of metal loss, galvanic corrosion, or general corrosion.  The contact surface area between the two metals is almost irrelevant because there just needs to be contact with each other for electrical/galvanic cell to occur.  The metal transfer happens through the electrolytic solution (water), not through the contact area of the dissimilar metals. 

However, this goes against the recommendations of all climbing orginizations (UIAA, AF, ASCA, etc.).  Mixing metals isn't a good idea because if you need any part of the anchor to be SS then you likely need everything in the anchor to be SS to prevent general corrosion.  The are exceptions to this rule of thumb but they are rare.

GLD · · Unknown Hometown · Joined Jan 2012 · Points: 89
Rich Farnham wrote:

Given that resources aren't infinite, and there are a LOT of bolts/anchors that need upgrading, is this really a dangerous situation?  Stainless screw-links, etc. are significantly more expensive.  

We aren't talking infinite expenditures. Do it right, don't create maintenance issues down the road.

eli poss · · Durango, Co · Joined May 2014 · Points: 456
Mike Slavens wrote:

No that is not dangerous if you are in a dry enough environment where the PS hardware has acceptable resistance to general corrosion.  The steal that would be corroding (PS) like you said is 1) out in the air not really allowing the galvanic cell to form needed for galvanic corrosion, 2) it is very inspect-able for signs of metal loss, galvanic corrosion, or general corrosion.  The contact surface area between the two metals is almost irrelevant because there just needs to be contact with each other for electrical/galvanic cell to occur.  The metal transfer happens through the electrolytic solution (water), not through the contact area of the dissimilar metals. 

However, this goes against the recommendations of all climbing orginizations (UIAA, AF, ASCA, etc.).  Mixing metals isn't a good idea because if you need any part of the anchor to be SS then you likely need everything in the anchor to be SS to prevent general corrosion.  The are exceptions to this rule of thumb but they are rare.

I live in a pretty dry climate so YMMV. But, in my experience, even on routes that are not super popular the fixed lowering hardware becomes critically worn loooooooong before corosion becomes an issue. I can think of one route in particular that has had lowering hardware replaced at least 3 times and the original chains have mild surface rust but no pitting or any other alarming corrosion. In this case, it makes more sense to use PS for completely external hardware.

Now, we do have an issue with bolts being coroded because the developers used plated steel and it is in limestone, but the external hardware has never seen an issue with corrosion. 

Mike Slavens · · Houston, TX · Joined Jan 2009 · Points: 35
Francis Haden wrote:

An active stainless steel is essentially a passive stainless steel with its chromium oxide layer removed due to any number of corrosion mechanisms or by physical damage. Expansion bolts can be passivated and polished yet the act of installation; tightening the locking nut, scratching the metal surface on a hanger, hammering the bolt into the hole, can all leave metallic deposits and/or damage the chromium oxide layer thereby allowing the damaged area to revert to its active state. 

The definition of stainless steel revolves around the chromium content being high enough where it will naturally form the passivization layer of Cr2O3.  In other words, if you knock of the passivization layer, the same process (oxidation) that would cause rust/corrosion also causes the passivization layer to reform thus protecting metal once again.  You don't need to re-passivate the metal if you beat it up during installation, it does that on its own.   That's the beauty of SS and that's why its called stainless.  

Greg Twombly · · Conifer, CO · Joined Nov 2007 · Points: 260

The accepted approach to corrosion in dissimilar materials is to use materials with a very similar galvanic index (table below) or an insulating layer between the materials. The worst case is carbon fiber with aluminum; either is stable in seawater, together the aluminum corrodes rapidly, the bane of carbon rigs on sailboats everywhere. This addresses the ion exchange between dissimilar materials, often with a mediating fluid like seawater. It does not address direct corrosion in its many forms (sulfidation, chloride corrosion, stress corrosion cracking etc). Though its a little off topic in this thread, stress corrosion cracking is most often chloride corrosion, but not exclusively as it is a reflection of the total energy state of the material, including mechanical, electrical, chemical, thermal, and even radiation. 

Mike Slavens · · Houston, TX · Joined Jan 2009 · Points: 35
GLD wrote:

We aren't talking infinite expenditures.

So I think this point is where the debate really needs to be happening.  The funding side, not the metallurgy.  

If Rich is out there replacing anchors he is already doing more than 99% of the climbing community with regards to route maintenance.  If we really want him to absolutely use SS hardware then we as a community should support him, the others like him, and the organizations that support anchor replacement with MONEY or at least the SS hardware.  He is already figuratively paying out of his pocket with his time and literally paying out of his pocket for PS steel, yet we sit here and judge him (harshly sometimes) yet do nothing ourselves but complain on message boards (I know, welcome to the internet).  Not picking on you GLD because this forum is just one of many where this same comment is made.  

Yes, a SS bolt is only a dollar or two more than PS.  But that cost adds up.  Yes, its not infinite but its a lot for such a small portion of the climbing community to fund. 

Jim Titt · · Germany · Joined Nov 2009 · Points: 490
Mike Slavens wrote:

The definition of stainless steel revolves around the chromium content being high enough where it will naturally form the passivization layer of Cr2O3.  In other words, if you knock of the passivization layer, the same process (oxidation) that would cause rust/corrosion also causes the passivization layer to reform thus protecting metal once again.  You don't need to re-passivate the metal if you beat it up during installation, it does that on its own.   That's the beauty of SS and that's why its called stainless.  

Hmm, it´s a bit more complicated than that which is why passivation is a common process in the stainless industry, we do it to our hardware.  The oxide layer forms on the chromium but the oxide layer on the ferrous part is called rust and you need to remove the exposed ferrous and promote the chrome oxide to grow across. The stainless round bar that comes from the stockholder is usually active unless you specify another finish (not ground). Bending, machining or welding make it active no matter what condition it was delivered.

Jim Titt · · Germany · Joined Nov 2009 · Points: 490
Rich Farnham wrote:

Jim - I get your point, but would like to ask you more about one of Kenr's scenarios:

"attaching Plated-Steel external anchor hardware (e.g. quick-links + carabiners + mussy hooks) to the "foundational" 304 Stainless wedge bolts and hangers. Because PS anchor hardware is cheaper and still being sold by respected American distributors, and bleeding of the PS itself is not seen as much of an issue in quasi-desert environments."

Given that resources aren't infinite, and there are a LOT of bolts/anchors that need upgrading, is this really a dangerous situation?  Stainless screw-links, etc. are significantly more expensive.  For anchors that see a lot of use, I like the idea of using plated hardware for the external, easily-replaced components (screw link and ring, and/or mussy hook).  It saves money that can be used to upgrade other anchors, and doesn't seem to be dangerous.  From what I've read, the setup I describe doesn't really have the contact surface area to be an issue, and the interface is out in the air where the exposure to moisture is brief (compared to at a hanger/bolt interface).

Like Kenr, I'm trying to keep up with the topic but am not an expert and would appreciate the input of people like yourself that are.  Thanks!

As has been pointed out elsewhere the expenditure isn´t infinite either, in fact the total cost of ALL the bolts in the USA is peanuts compared to the incomes of the people using them. At any popular sport cliff the value of the cars in the parking lot would pay for all the bolts to be replaced with solid gold. To say nothing of the Actreryx jackets........

snowdenroad · · Unknown Hometown · Joined Jun 2007 · Points: 40
Greg Twombly wrote:

 or an insulating layer between the materials. 

So would the powder coating on a SS hanger provide that barrier on a plated bolt?  Aka the Metolius Enviro hangers?

GLD · · Unknown Hometown · Joined Jan 2012 · Points: 89
Mike Slavens wrote:

So I think this point is where the debate really needs to be happening.  The funding side, not the metallurgy.  

If Rich is out there replacing anchors he is already doing more than 99% of the climbing community with regards to route maintenance.  If we really want him to absolutely use SS hardware then we as a community should support him, the others like him, and the organizations that support anchor replacement with MONEY or at least the SS hardware.  He is already figuratively paying out of his pocket with his time and literally paying out of his pocket for PS steel, yet we sit here and judge him (harshly sometimes) yet do nothing ourselves but complain on message boards (I know, welcome to the internet).  Not picking on you GLD because this forum is just one of many where this same comment is made.  

Yes, a SS bolt is only a dollar or two more than PS.  But that cost adds up.  Yes, its not infinite but its a lot for such a small portion of the climbing community to fund. 

Agree the debate should actually be a funding one. I don't believe being done right and a short term cash flow issue are at odds. Don't do all of them; only do your favorite or most popular routes until you have more money. The bolts don't HAVE to be replaced people can choose not to climb on them, or they can and encounter higher risk and then guess what? At that point I bet you won't have a funding issue, people might contribute a fiver here and then.

I think using dissimilar metals is half-assing the job. If you can't full-ass it, don't do it and let someone else or wait.

Guideline #1: Don't be a jerk.

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