Galvanic Bolt Corrosion Question

I have a question about galvanic corrosion. I see a lot of bolts with stainless hangers on carbon steel bolts. I know this is bad. My question is would a plated steel hanger (ala Fixe) on a carbon steel bolt (ala 1/2" Rawl 5-piece) be better? Worse? Would the plated steel work like a "sacrificial anode", and corrode first, saving the bolt for a while?

Please don't response with "Only use Stainless". I know that already. I am not placing any new carbon steel bolts.

Thanks

All of the Rawl 5-piece bolts I've seen that are not stainless, are also plated (usually with zinc). If you have a plated bolt, it is best to put a plated hanger on it, not stainless. It can be difficult to find non-stainless hangers though.

I'm trying to think if I've seen any pure carbon steel bolts with no plating, and I don't think I have...especially Rawls/Powers. They are either stainless, or zinc plated.

So did they ever make non-plated 5 piece bolts? What about during the 80's? I have seen some new plated steel 5-piece bolts that are much more shiny than some of the older ones. Do they dull over time?

Thanks for the great info.

Ben Sachs wrote:Do they dull over time?

Yeah, and that's what they're designed to do. The zinc plating forms an oxide, which is the barrier for any other corrosion issues.

I've pulled a fair number of stainless hangers that were attached with carbon steel/zinc plated bolts. Never seen what I'd call any enhanced corrosion due to a galvanic effect. Its pretty dry here in Utah, though. Probably need to have consistant moisture to drive any of that type of corrosion.

Some wiki info on galvanic coatings here:

en.wikipedia.org/wiki/Galva…

Note that hangers that are zinc plated are pretty hard to paint, unless you use a zinc rich primer (and/or a primer made specifically for use on zinc plating). Best to buy them prepainted. Most of the Rust-o-leum primers won't work. If you read the fine print closely, you'll see most of them say, "not for use on galvanized surfaces". Which means zinc plated. Stainless hangers take paint/primer much better.

just a side note, a route that i climbed this weekend was put up in the late 90's with carbon/plated bolts and stainless hangers. there was a substantial amount of rust on the bolt and nut. this was in CO.

slim wrote:just a side note, a route that i climbed this weekend was put up in the late 90's with carbon/plated bolts and stainless hangers. there was a substantial amount of rust on the bolt and nut. this was in CO.

i have seen this too. maybe even on the same route ;)

interestingly enough, i have to admit that i have used this combination to find that a few have corroded, and many have not. but i agree that ideally it is best if you dont mix the metals.

I like the idea of a sacrificial anode. I wonder how long a strip of magnesium ribbon twisted around a hanger would last.

that's a good question. i've been running into a similar situation at work.

Somewhat related, and worth looking at just to see what *can* happen:
cascadeclimbers.com/forum/u…

Peter Springs wrote:Somewhat related, and worth looking at just to see what *can* happen: cascadeclimbers.com/forum/u…

Pretty sure those Kong hangers were aluminum. Not galvanic corrosion, IMHO.

Saw the same hangers on a route at the City of Rocks (Twin Sisters area) a couple weekends ago. Scary, but, at least the City is much dryer.

Some info I dug up for that CC thread, related to this type of corrosion of aluminum:

"Corrosion products building up along grain boundaries exert pressure between the grains and the end result is a lifting or leafing effect. Aluminum alloys that have been extruded or otherwise worked heavily, with a microstructure of elongated, flattened grains, are particularly prone to this damage. The damage often initiates at end grains encountered in machined edges, holes or grooves and can subsequently progress through an entire section.

In this particular form of intergranular corrosion the expansive force of insoluble corrosion products tends to force the grains apart and leads to exfoliation corrosion, sometimes known as lamellar or layer corrosion. In extreme cases, the edges of the affected area are leaf like and resemble the separated pages of a wetted book that has become swollen and begun to open up."

The inside of the hole tends to stay wet and thus the corrosive process continues around the clock. ALso, the threads on the bolt are the thinnest part of the system and they are necessary for pullout strength (and it all comes down to pullout). Being thin, they wear out faster with corrosion.

BTW, I tried the sacrificial anode thing... it doesn't work. Apparently the connection has to be super good, like actually submerged in water.

Brian in SLC wrote: Pretty sure those Kong hangers were aluminum. Not galvanic corrosion, IMHO.

Could be, lot's of speculation about it. Either way, it'd still be a reaction between 2 different metal types. Certainly gives pause for thought seeing it happen so out-right.

Another quote to back up the aluminum theory:
"QUOTE (Mark Hanna @ Aug 31 2009, 10:21 AM) Hi all-

I have had ther opportunity to view the failed hanger in question, and strongly believe it is aluminum alloy with an anodized clear coat. The product appears to be non-magnetic, very lightweight compared to steel or ss, and has exfoliating corrosion between layers that looks similar to some old Eiger biners that were part of an ancient belay rig on the diamond some years ago. Aluminum is very sacrificial to the steel bolt in the presence of water. I'll return the hanger to D this week.

MH

PS- I'm not a metallurgist, but work for one who provided a cursory inspection of the hanger and agreed lacking other testing information to the contrary.

Mark Hanna"

What about any galvanic degredation between bolt hangers and quick links? Are most quick links stainless? I am Looking to put in some quick links for cleaning anchors is there any advice or things i should know? Thanks

R. Moran wrote:What about any galvanic degredation between bolt hangers and quick links? Are most quick links stainless? I am Looking to put in some quick links for cleaning anchors is there any advice or things i should know? Thanks

I've been looking at this as well. I don't recall where I read it, probably over on Supertopo, but someone suggested a good compromise. Use stainless bolts and hangers, and plated quicklinks and lowering rings.

I'd love to do the whole thing in stainless, but I'm not made of money. The advantage of this set-up is the only potential for galvanic corrosion occurs outside the hole where it is likely to be dry and is also easy to monitor. All of the lowering happens on cheaper plated rings, and can easily be replaced.

I'd love to hear feedback on this set-up, as this is what I'm likely to start doing in anchor setups that I expect will see significant use.

Rich Farnham wrote: I've been looking at this as well. I don't recall where I read it, probably over on Supertopo, but someone suggested a good compromise. Use stainless bolts and hangers, and plated quicklinks and lowering rings. I'd love to do the whole thing in stainless, but I'm not made of money. The advantage of this set-up is the only potential for galvanic corrosion occurs outside the hole where it is likely to be dry and is also easy to monitor. All of the lowering happens on cheaper plated rings, and can easily be replaced. I'd love to hear feedback on this set-up, as this is what I'm likely to start doing in anchor setups that I expect will see significant use.

that sounds fine; as you said the only place where significant corrosion would occur is outside the hole. i've been plated quick links with carabiners at anchors and like it very much.

Seems pretty standard to use the plated links. Stainless links are available at most hardware stores but its very expensive. $2.50 vs $7.50 where I live plated v. stainless for 5/16" links. I've seen setups like this where the links were super rusty, but the bolt and hanger were fine. Slap some new links/rings on there, good to go for another 10 yrs.

I have also noticed a big difference on the strength rating between the expensive and cheap. The cheap are ~15-1700lbs v.s. the expensive @ 5000lbs. I know that we are just lowering but is there any argument for the more expensive stronger? It seems that 1500lbs would be plenty to lower off. Thanks

Make sure you are comparing the same numbers. Links often show "working load" which is typically 20% of ultimate strength. The 3/8" quicklinks I use say 2200 lbs working load, so an ultimate strength of 11,000 lbs.

Brian in SLC wrote: Note that hangers that are zinc plated are pretty hard to paint, unless you use a zinc rich primer (and/or a primer made specifically for use on zinc plating). Best to buy them prepainted. Most of the Rust-o-leum primers won't work. If you read the fine print closely, you'll see most of them say, "not for use on galvanized surfaces". Which means zinc plated. Stainless hangers take paint/primer much better.

Random bump. I've found the answer to painting plated hangers (and stainless to a lesser degree).

It turns out that "normal" spray paint like Rustoleum have chemicals that react with the zinc or galvanized coating. This reaction basically forms a layer of "soap" under the dried pain meaning it doesn't stick AT ALL.

What you want is LATEX based spray paint. Krylon makes one called H2O Latex paint (found on Amazon). The Latex paint uses different chemicals that don't form the "soap". I've had good results with this on Fixe Plated hangers as well as stainless. The latex seems to stick pretty well and chips FAR LESS than the Rustoleum.

Give it a whirl.

mattm wrote: Random bump. I've found the answer to painting plated hangers (and stainless to a lesser degree). It turns out that "normal" spray paint like Rustoleum have chemicals that react with the zinc or galvanized coating. This reaction basically forms a layer of "soap" under the dried pain meaning it doesn't stick AT ALL. What you want is LATEX based spray paint. Krylon makes one called H2O Latex paint (found on Amazon). The Latex paint uses different chemicals that don't form the "soap". I've had good results with this on Fixe Plated hangers as well as stainless. The latex seems to stick pretty well and chips FAR LESS than the Rustoleum. Give it a whirl.

It should be noted that you should NEVER paint or purchase powder coated hangers for use in a marine environment or for use where there is runoff water containing salt or chlorides.