Dissimilar Metals & Galvanic Corrosion-Let's get this straight.

Let's come to a clear understanding of when (or if) it's ok to use dissimilar metals.  To clarify, I am talking about issues that arise with galvanic corrosion.  

This is a good review of what occurs during galvanic corrosion:

So, in the case of a SS hanger and a zinc plated carbon steel bolt, the the bolt would act as the sacrificial anode.   That's bad.  So is the reverse scenario of a SS bolt and a plated hanger.

But, what about zinc plated quick links with a SS bolt?  My view is it depends.   The sacrificial anode in this case is the quick link in contact with the SS bolt.   If anything, it acts in a beneficial manor and should REDUCE corrosion of the SS bolt.

In my experience I have found the following with plated quick links:
-They corrode predictably and from the outside.  
-The quick link wears faster from mechanical stresses than from chemical corrosion.

So IMO it should not matter using a carbon steel quick link with a SS bolt/hanger unless the routes sees very little traffic.  

However, I do wonder about long term mechanical wearing for the hanger/glue in due to the quick link.  In this case, a softer SS quick link may reduce the wearing on the bolt/hanger?   I would love to find a cheap, sturdy, easily replaceable plastic insert for a glue in and/or quick link that would stop the metal on metal wearing.  

Ok, I've thrown in my 2 cents.  Now tell me what you know.

Galvanic corrosion is WAY overblown. Rust is the problem in most of the U.S.

I agree with Greg. Way overblown. Most of the hundreds of bolts i  have replaced have been mixed metal. Ss hanger carbon bolt. On those that have carbon steel hanger the bolt has been no less rusty.

I have seen carbon steel quick links and chain on a stainless hanger wear on both. The oxidation seems to act as a grinding compound on the SS hanger.

So i am very opposed to attaching chain to a small cross section bolt such as Bolt products twist bolt or wave bolt with a carbon steel quicklink. 

As a 'Liability notice' I'm all for NOT mixing metal and using uniform metal bolts, hangers, chain etc. Found this article explaining the effects of corrosion on the strengths of metal. Not a climbing article but you can draw the parallels and such from it. 

https://www.corrosionpedia.com/effect-of-corrosion-on-a-materials-tensile-strength-and-ductility/2/5976

Also the classic electronegativity scale of metals

https://www.engineeringtoolbox.com/galvanic-piping-corrosion-d_906.html
 

Galvanic corrosion is a myth started by Big Stainless to push zinc-plated steel out of business

I had a really bad case of galvanic corrosion once. It laid me out for weeks and I lost 80 pounds. Wouldn't wish that on the people who report posts here.

For any long-term / “permanent” anchor, the bolt and hanger should be made from the same corrosion resistant alloy (i.e., 300 series stainless steel for salt free environments & titanium for salty environments).

Components that are easy to inspect for metal loss and replace (quick links, chain, etc.) should be either the same material or should be anodic to the bolt & hanger.  

Salt water environments are slightly more complicated.  Don’t use materials that are susceptible to “chloride stress corrosion cracking” in those environments.

I personally wouldn’t be concerned about differences in hardness.  I cannot recall having ever seen a SS hanger that has been significantly mechanically abraded.  

Keep in mind that galvanic corrosion will only be significant when everything is fully immersed in water for extended periods of time.  “Crevasse corrosion” tends to be the predominant degradation mechanism for most CS bolts and hangars, used for climbing.  CS quick links and chains, used for climbing, are predominantly affected by “general atmospheric corrosion”.

So mild thread drift, any issue with the aluminum carabiners being left on bolts/other non-aluminum metal anchors to make lowering easier?

I'm no metallurgist but note that zinc-plated bolts are themselves "mixed metal." And also note that stainless steel and bolt-grade carbon steel are pretty close on the anodic scale = very little potential for galvanic corrosion. One would not expect much galvanic corrosion activity ever, and recall that GC only occurs when the materials are wet with an electrolyte-bearing solution  (like salt water or salty mist)(inland rain water, not so much). Galvanic corrosion came into vogue among bolt-corrosion forum geeks due to some actual failures in the Pacific NW involving aluminum hangers. Heed the warning! Don't use aluminum hangers. Otherwise, forget about it. The main reason to use SS quicklinks is that you have more years when you can remove them with a wrench before they rust and you have to cut them off. But please - just use all stainless bolts and hangers. Or titanium on the coasts. It's really that simple.

Matt Castelli wrote: So mild thread drift, any issue with the aluminum carabiners being left on bolts/other non-aluminum metal anchors to make lowering easier?

Aluminum is surprisingly reactive, however readily builds a protective oxide layer, giving the false impression that it’s inert. If an aluminum biner is left on a steel hanger and there is regular movement (wind) that disturbs the oxide layer, the aluminum can corrode quickly.

J Achey wrote: I'm no metallurgist but note that zinc-plated bolts are themselves "mixed metal." And also note that stainless steel and bolt-grade carbon steel are pretty close on the anodic scale = very little potential for galvanic corrosion. One would not expect much galvanic corrosion activity ever, and recall that GC only occurs when the materials are wet with an electrolyte-bearing solution  (like salt water or salty mist)(inland rain water, not so much). Galvanic corrosion came into vogue among bolt-corrosion forum geeks due to some actual failures in the Pacific NW involving aluminum hangers. Heed the warning! Don't use aluminum hangers. Otherwise, forget about it. The main reason to use SS quicklinks is that you have more years when you can remove them with a wrench before they rust and you have to cut them off. But please - just use all stainless bolts and hangers. Or titanium on the coasts. It's really that simple.

Hey Jeff, this Ian that runs the RRG FGI.  Only SS for us!  I am just trying to decide if it makes sense to use SS chain and quick inks from first a corrosion perspective and secondarily considering a return on investment.  

Kent Krauza wrote:

Aluminum is surprisingly reactive, however readily builds a protective oxide layer, giving the false impression that it’s inert. If an aluminum biner is left on a steel hanger and there is regular movement (wind) that disturbs the oxide layer, the aluminum can corrode quickly.

Interesting - so It's the biner that would corrode, not necessarily the bolt/chain/etc?  Also - does it matter if the biner is anodized?

Thanks in advance.

Greg Barnes wrote: Galvanic corrosion is WAY overblown. Rust is the problem in most of the U.S.

Yes, and Stress Corrosion Cracking (SCC) in the rest of the U.S.

Kent Krauza wrote:

Aluminum is surprisingly reactive, however readily builds a protective oxide layer, giving the false impression that it’s inert. If an aluminum biner is left on a steel hanger and there is regular movement (wind) that disturbs the oxide layer, the aluminum can corrode quickly.

Also, in any area where SCC occurs and Ti is required, be very aware that aluminum is susceptible to SCC, and it progresses more rapidly than in stainless. 

I had some leaver-biners that were looking pretty bad (still have 'em BTW) after a year on a sea-cliff.  I sent them to BD who tested them to failure.  They broke easily and in an unusual place.   They then got sent to the UIAA who professionally analyzed them, and confirmed that they broke due to SCC.  

J Achey wrote: But please - just use all stainless bolts and hangers. Or titanium on the coasts. It's really that simple.

It really IS simple.  The recommendation of the Access Fund for all of the U.S. and surrounding areas is:

1) Use stainless steel.
2) If stainless discolors or corrodes in any way, then use titanium.

climb2core wrote:

Hey Jeff, this Ian that runs the RRG FGI.  Only SS for us!  I am just trying to decide if it makes sense to use SS chain and quick inks from first a corrosion perspective and secondarily considering a return on investment.  

Hi, Ian - for the RRG I would say mostly consider ease of removal at replacement time vs initial cost. Stainless links aren't THAT expensive compared to plated, considering all the money you are already spending on stainless 5-piece or glue-ins (awesome program you guys have, by the way). And you will be able to re-use stainless QLs, whereas with plated you will likely have to cut them and buy new ones. For the chain that consideration does not apply, but there is something to be said for using SS anyway, and the cost is not totally prohibitive. Plated hardware and chain can leave some pretty ugly stains on the rock, for one thing. My 2 cents. Thanks for all you do!

I have always believed that the function of the zinc on zinc-plated steel is to corrode first, before the steel, to "protect" the steel.  Here in the northeast a zinc plated quick link takes about 1 year to start rusting. I would assume a zinc plated bolt takes about the same. Rust is the reaction of iron with oxygen, greatly catalyzed by water, or damp conditions.  

I've taken rusted 5/16-inch QL's off of rap anchors that were put in 40 years ago and would have no issue using them again. Hell...what do they have to hold...200 lbs??  Rusty QL's are, in my opinion, greatly "overrated" as a safety issue.  No so with the bolts they were attached to!!!  UGLY INSIDE !  3/8-inch bolt in a 3/8-inch hole but with 1/32 to 1/16-inch of rust all around...what is the holding power of rust on rust?  "Worst" in my opinion is a plated bolt "hiding" behind a stainless hanger, not because of "galvanic action" (which might be happening) but because the ss hanger "hides" the fact the bolt may be rusty.  

Matt Castelli wrote:

Interesting - so It's the biner that would corrode, not necessarily the bolt/chain/etc?  Also - does it matter if the biner is anodized?

Thanks in advance.

Aluminum alloys are among the least noble metals and will always be the anode when in contact with any grade of steel.  The Al biner will corrode, the steel hanger, chain, bolt etc will not.

With old "oval" aluminum biners (which one might find as "resident" at an old anchor) the first thing to corrode is the little (steel) metal pin in the gate that is the main source of the "strength" of the biner when closed...without it, or with it seriously weaked by corrosion / rust / galvanic action, the biner basically would have it's "open gate" strength...which isn't much !  Even in slightly more "modern" Al biners, (e.g. "key gate") there still is the spring-and-pin mechanism that holds the bottom of the gate to the biner.

One thing to leave a "leaver biner" on a bolt to back-off (where it's likely to be removed by the next person up the climb), another to leave one on a rap-station anchor for years.

Robert Hall wrote: With old "oval" aluminum biners (which one might find as "resident" at an old anchor) the first thing to corrode is the little (steel) metal pin in the gate that is the main source of the "strength" of the biner when closed...without it, or with it seriously weaked by corrosion / rust / galvanic action, the biner basically would have it's "open gate" strength...which isn't much !  Even in slightly more "modern" Al biners, (e.g. "key gate") there still is the spring-and-pin mechanism that holds the bottom of the gate to the biner.

One thing to leave a "leaver biner" on a bolt to back-off (where it's likely to be removed by the next person up the climb), another to leave one on a rap-station anchor for years.

Respectfully suggest that it is the aluminum in direct contact with the steel pin that is corroding, not the pin itself.

Yup, you're right Kent. Had my cathod and anode mixed up.   Net result is the same, one corroded mess, probably WAY weaker than when it was made.