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Plated steel bolts: Corrosion on the embedded portion of the bolt

David Reeve · · Unknown Hometown · Joined Jul 2020 · Points: 0

Important detail is missing in this discussion.

OK the bolts are plated, but is the fixed hanger stainless? Those pics from Joshua Tree look very much like a stainless on plated steel combination.

The presence of a large block of SS electrically connected to a plated steel bolt will drive the surface of the bolt within the hole strongly anodic. The plating will go much quicker than usual, and then we'll see attack of the steel beneath. I expect plated steel bolts matched with plated steel hangers to last much longer.

TIMMY G · · Unknown Hometown · Joined Apr 2020 · Points: 15
David Reeve wrote:

Important detail is missing in this discussion.

OK the bolts are plated, but is the fixed hanger stainless? Those pics from Joshua Tree look very much like a stainless on plated steel combination.

The presence of a large block of SS electrically connected to a plated steel bolt will drive the surface of the bolt within the hole strongly anodic. The plating will go much quicker than usual, and then we'll see attack of the steel beneath. I expect plated steel bolts matched with plated steel hangers to last much longer.

Is the bi metallic corrosion notable in the Joshua Tree photos? It seems the corrosion is coming from the tip of the bolt not the hanger section. I can see Shiney metal near the head of the bolt. If anything I think there was chemical corrosion from the rock eating the sleeve of the bolt

Sam Whatley · · Huntsville, AL · Joined Feb 2018 · Points: 1,006

I can only speak for the bolts that I posted above. They were Plated bolts with stainless hangars, and yes I agree with David. This is definitely another large factor in the ones I pulled.

Really appreciate all of the contributions and conversations here from everyone. Thanks!

David Reeve · · Unknown Hometown · Joined Jul 2020 · Points: 0
TIMMY G wrote:

Is the bi metallic corrosion notable in the Joshua Tree photos? It seems the corrosion is coming from the tip of the bolt not the hanger section. I can see Shiney metal near the head of the bolt. If anything I think there was chemical corrosion from the rock eating the sleeve of the bolt

The galvanic effect is electrical. Any steel in electrical contact with stainless steel will tend to lose metallic iron into solution. The electrons so generated flow through the metallic link back to the stainless steel end where the complementary cathodic reaction is to add them to hydrogen ions to produce atomic hydrogen. 

Brian in SLC · · Sandy, UT · Joined Oct 2003 · Points: 22,793

"into solution".  There likely isn't much solution especially a solution that's highly conductive (like being submerged in seawater).  Pretty dry location.

Any severe corrosion would be visible where the dissimilar metals are in intimate contact I'd think.  Not like the J Tree examples corroded the furthest distance from the contact of the stainless hanger.

I've always wondered if the galvanic cell potential could be measured...based on bolt diameter and hanger size...etc.

Anyhow... interesting.

TIMMY G · · Unknown Hometown · Joined Apr 2020 · Points: 15
David Reeve wrote:

Important detail is missing in this discussion.

OK the bolts are plated, but is the fixed hanger stainless? Those pics from Joshua Tree look very much like a stainless on plated steel combination.

The presence of a large block of SS electrically connected to a plated steel bolt will drive the surface of the bolt within the hole strongly anodic. The plating will go much quicker than usual, and then we'll see attack of the steel beneath. I expect plated steel bolts matched with plated steel hangers to last much longer.

So after rereading this, does this imply that it changes the polarity of the plated steel and allows the composition of the rock to eat the plated steel easier??? My brain was thinking it was the 2 metals eating each other not a shift of electrons screwing around with the polarity letting rust set in else where.... 

That is soooooo damn cool! But sketchy

David Reeve · · Unknown Hometown · Joined Jul 2020 · Points: 0
TIMMY G wrote:

So after rereading this, does this imply that it changes the polarity of the plated steel and allows the composition of the rock to eat the plated steel easier??? My brain was thinking it was the 2 metals eating each other not a shift of electrons screwing around with the polarity letting rust set in else where.... 

That is soooooo damn cool! But sketchy

Think about how screwing a block of zinc to the steel hull of a boat protects all steel electrically connected to that block by introducing a sacrificial anode that corrodes preferentially to the steel. The protection so offered extends well beyond the zinc block. In the case of connecting stainless steel to plain steel you are doing the opposite. You are rendering all the length of the bolt, even that down the depth of the hole, anodic, and thus lowering the energy barrier to moving iron atoms into solution wherever a moisture pathway exists.

David Reeve · · Unknown Hometown · Joined Jul 2020 · Points: 0
Brian in SLC wrote:

"into solution".  There likely isn't much solution especially a solution that's highly conductive (like being submerged in seawater).  Pretty dry location.

Any severe corrosion would be visible where the dissimilar metals are in intimate contact I'd think.  Not like the J Tree examples corroded the furthest distance from the contact of the stainless hanger.

I've always wondered if the galvanic cell potential could be measured...based on bolt diameter and hanger size...etc.

Anyhow... interesting.

You are right in that a complementary fluid pathway also must be present to connect the two metals so that ionic transport can complete the circuit as it were. This is true, not just of the galvanic couples as we are discussing but for corrosion in general. Absolutely dry surfaces do not corrode. However, it is surprising how little it takes to get corrosion started, and once started the concentration of dissolved salts increases which then works against future drying out. Maybe a flippant comparison but if there is enough moisture to power a Joshua Tree then there is enough to power corrosion.

I'm sure you'd measure a voltage with a high impedance meter. Quite what it would be depends upon the concentration of ions that have built up. The standard half-cell difference is of the order of 0.5V.

Another fact you need to consider is the relative surface area presented by the two metals. The more stainless steel you hang off the bolt the worse it will be for its limited area. I've got a lot of experience here in Queensland where there was a bad habit of fixing plated steel anchor sets with 316 SS bolts. This is the reverse situation, and we see relatively rapid deplating of the steel followed by surface corrosion of the steel itself. The phenomenon is pretty evenly distributed across the entire surface as would be expected. It is not localised to the contact point.

Brian in SLC · · Sandy, UT · Joined Oct 2003 · Points: 22,793
David Reeve wrote:

Maybe a flippant comparison but if there is enough moisture to power a Joshua Tree then there is enough to power corrosion.

Another fact you need to consider is the relative surface area presented by the two metals. The more stainless steel you hang off the bolt the worse it will be for its limited area. I've got a lot of experience here in Queensland where there was a bad habit of fixing plated steel anchor sets with 316 SS bolts. This is the reverse situation, and we see relatively rapid deplating of the steel followed by surface corrosion of the steel itself. The phenomenon is pretty evenly distributed across the entire surface as would be expected. It is not localised to the contact point.

That's funny.  Higher humidity would have a more profound effect rather than very little rainfall in a pretty dry environment I'd think.

I've always been skeptical when folks point to galvanic corrosion as the main culprit to some of the degraded anchors we see out there.

Corrosion Dr. area effects

Sure, there's a higher impact to a galvanic cell with the cathode surface area is huge compared to the anode.  But, what does that mean for a stainless climbing hanger versus a bolt?  There's some really small stainless hangers out there (surface area and mass).  Those SMC stainless hangers for instance.  Is the rate of corrosion less on those than on a bigger, say, Fixe hanger?  Interesting to think about...

Here's an SMC hanger/plated bolt combo that was pulled at the City of Rocks in Idaho.  Placed in around 1992.  Pulled in 2018.  26 years in the field.  What's also interesting is a sealant was used in an attempt to protect the bolt from moisture/corrosion (you can kinda see it around the side of the hanger.  The bottom of the hanger was covered with sealant).  Actually likely accelerated the corrosion on the bottom of the bolt.

Especially in the interior of the relatively dry West, a hanger is only exposed to very limited moisture.  Makes sense that at the bottom of a blind hole that moisture could get trapped but that's a long way from the galvanic situation and the corrosion seen on the bottom end of bolts is just standard corrosion you'd expect regardless of a galvanic cell.  I've seen it on plated hanger/bolt combos too.

20 year old plated on plated hanger/bolt combo, also from the City of Rocks:

Surface above ground looks good.  Fair amount of rust below surface.  Still...hanging in there reasonably well, though.  

Good stuff...thanks!

timothy fisher · · CHARLOTTE · Joined Nov 2017 · Points: 30

Totally agree with what Brian is saying. I have seen no significant improvement in the bolts i have replaced that have steel hangers vs 304 or 316 hangers. The plated steel bolts are going to rust depending more due to environmental factors than the type of hanger. Possible exception may be aluminum hangers? 

TIMMY G · · Unknown Hometown · Joined Apr 2020 · Points: 15

One of the guys In the lab I work in used to sail and mentioned how if you bought cheap stainless steel hose clamps they would corrode on the worm gear from the steel and stainless steel interactions... I would guess the 2 just don't play nice together :) so stainless steel for everything and everyone

Guideline #1: Don't be a jerk.

Fixed Hardware: Bolts & Anchors
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