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I have noticed that, when a structure with bars (such as a gate or a grate) develops rust, this sometimes happens differently across the bars. In particular, it seems that the bars are alternatively clean and covered in rust.

The photos below (taken by me) illustrate this effect (I have highlighted the rusted bars with red arrows). In both cases, from inspection, all bars appeared to be made from the same material and covered in the same kind of protective paint (the rusted parts were exposed to the elements).

My theory is that perhaps the rusting process activates some sort of "circuit" between bars, such that one bar protects the next one from rusting.

Is this possible?

Rusted grate Rusted gate nearby my house

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    $\begingroup$ Usually it happens like this: a customer wants to save money and orders metal fence as cheap and as light as possible. After a while, they realize the fence is not dense enough to serve the purpose (say, their cat is pregnant again), and extra bars get welded in the gaps (the welds on the first photo are different for every second bar). Apparently, older bars are likely already corroded, but there is also a chance that the new ones hasn't been painted or are made of an alloy of a worse resistance — then the new bars would rust faster. $\endgroup$ – andselisk Oct 31 '20 at 19:20
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    $\begingroup$ @andselisk your theory is an effective application of Occam's razor :-) However, I am not entirely convinced. In both cases, the part did not seem to have been reworked after installation. In particular, notice that in the second photo, the "cleaner" bars seem to be part of the original design and difficult to install at a later time (although of course it is possible that they have been replaced later...) $\endgroup$ – GioMott Oct 31 '20 at 19:22
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    $\begingroup$ In the second case they might as well use bars made of different alloys with different corrosive resistance. They definitely used different forging technics and metal profiles to make the straight bars and trapezoidal sections. $\endgroup$ – andselisk Oct 31 '20 at 19:30
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    $\begingroup$ It is unlikely that each bar protects the next from oxidation. If each bar is the same material and was put outside (I.e. exposed to moisture and other environmental factors) then they should generally rust at the same time. Additionally, if all the material is the same, the resistance should be about equal in each bar and electron flow would not be favored in one bar over the other. $\endgroup$ – dval98 Oct 31 '20 at 21:08
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    $\begingroup$ It seems pretty clear that second sets of bars were added and had different surface conditions from the originals. $\endgroup$ – blacksmith37 Nov 1 '20 at 0:37
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Rebar for concrete is frequently stored in heaps, and rusts. It actually develops a stronger bond if it is not too rusty, because the rebar surface is rougher and better able to adhere to the hardened cement. It's interesting to see a pile of old rebar with a fresh delivery on top - silvery, not a trace of rust - on top of a pile of rebar that looks almost disgustingly rusted. And then you think, yeah, but the stuff on the bottom will have a better bond.

The key word in the observation about the fences is "sometimes".

I'm going to guess that the maker of the fence in the top picture also had a pile (or two) of iron bars, and some were rusty from being stored long or outdoors, or were a teensy bit different from some other pile of iron bars. This fence maker, knowing how his fence weathers, has to make it as uniform as possible, so he takes one bar from one pile, then a bar from the other pile... He prepares them identically, primes them identically, paints them identically... But then nature takes over and the most corrodable bar develops a spot, which spreads until it is all corroded-looking, before the more corrosion-resistant bar has even gotten started!

And then you come along and take a picture just before the other bars begin to corrode! This experiment is in the initial stages, not yet at equilibrium. Check back on the fence every year or two - I believe things will even out in time.

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  • $\begingroup$ James Gaidis: Is your argument that the starting bars where in a different state of oxidation, and in time electrochemically corroded as a continuation of the original differential in corrosion chemistry? As such, this could be considered, in part, an electrochemistry based argument with initial bar selection arising from an arbitrary design decision. $\endgroup$ – AJKOER Nov 1 '20 at 14:55
  • $\begingroup$ James Gaidis (+1): On further thought, I like your explanation. $\endgroup$ – AJKOER Nov 1 '20 at 15:11
  • $\begingroup$ @James Gaidis: your explanation seems very reasonable. My only objection is on "Check back on the fence every year or two - I believe things will even out in time": while the first fence was something I noticed passingly during a vacation, the second one is close to where I live and work and has been like that for a good 10 years. But maybe the period required to even things out is longer... $\endgroup$ – GioMott Nov 7 '20 at 10:18
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So the easiest non-chemical answer is to search for fence corrosion in google

As you will see a quick glance is showing that you probably have taken a non significant sample in your investigation since there is plenty of fences where you have a uniform corrosion. Indeed there can be also more chemical explanation related to corrosion. But before doing that we need to define corrosion in this case. We are referring to galvanic process where your metal deteriorate through oxidation and usually form it's oxide. So we need a metal and oxygen and your case also humidity is a factor. This is explained visually here here

towhere you have the formation of an oxide . The anodic site on the surface of the metal is often related to an impurity or a defect in the metal (can be a lattice defect).

In the second photo you posted, if you look at the exposed surface of the bars there is indeed a difference in the shape of the fences that probably also causes different deposits of humidity and so in principle it is possible that the anodic site has a preferential "macro" site with the consequent texturing you see. The first case you presented looks more tricky. Indeed it can be that the bars were welded in different time or painted with different paint (and also some of them pre-treated in comparison with the others) or just restored before the others. Indeed if corrosion "starts" in one site due to its weakness what you have is that the corrosion will insist in that point before spreading to other parts and that's the idea. This principle is used also in cathodic protection were you intentionally couple a sacrificial anode with a nobler metal knowing that the corrosion will proceed till it has consumed the anode. (So indeed there is a circuit since all corrosion process involves one, but that would have happened also in the case of a more uniform corrosion)

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I would like to propose an alternative to those dismissing the advent of possible complex electrochemistry behind the phenomenon and asserting instead perhaps just poor planning and/or design decisions.

In this regard, an interesting 2011 article, 'An Fe3O4–FeO–Fe@C composite and its application as anode for lithium-ion battery', discussed here, to quote:

X-ray diffraction, Raman spectrometry, scanning electron microscopy with energy dispersive spectroscopy, transmission electron microscopy and galvanostatic cell cycling have been used to characterize the structure and electrochemical performance of the as-prepared Fe3O4-FeO-Fe@C composite. The electrode shows a stable and reversible capacity of over 600mAhg−1 at a current of 53mAg−1 for up to 60 cycles, which could be ascribed to the unique double-layer core-shell and embedded structures.

Also, per another reference discussing the same electrode, to quote:

Here, we report the synthesis, characterization and evaluation of the electrochemical properties of pristine Fe3O4 nanoparticles synthesized from a base catalyzed method mechanically mixed with electrospun Polyacrylonitrile (PAN) derived carbon fibers as simple composite material. The Fe3O4/C electrodes are characterized by galvanostatic cycling experiments using currents as high as 2000 mA g−1, revealing high capacity values and capacity retention, together with a very good capacity recovery during the rate capability experiment.

The picture appears to display a red pigment, possibly Fe3O4 created at elevated temperatures. Other bars, resemble Fe/FeO.

So speculatively in the current case, an alternate proposition that if the iron metal fence was exposed to say a brush fire, heating the iron and also depositing fine carbon, this may have resulted in a series of Fe-FeO@C and Fe3O4@C electrodes naturally developing over time (aka, yes, some possible complex electrochemistry).

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    $\begingroup$ I don't see how a composite electrode for Li-ion batteries synthesized from iron(II) sulfate and glucose has anything to do with a rusting fence. Also, why would anyone apply brush fire to every second bar? Besides, neither of the sources support this theory in any way. $\endgroup$ – andselisk Nov 1 '20 at 8:10
  • $\begingroup$ Andselisk: The point is Fe3O4 makes a good anode. However, as a provided source notes, its creation requires some heating, hence fire exposure to the whole fence. There is also an apparent interesting electrode composite arising from added carbon. Note, activated carbon, for example, naturally develops zones of + and - charge buildup. The fence is subject to wind, for electrostatic charge creation,...., bottom line, a possible outline for 'some complex electrochemistry' at work. There is, per the counter-argument, equivalently no support for the renovation of the fence explanation. $\endgroup$ – AJKOER Nov 1 '20 at 14:15
  • $\begingroup$ How is any of this related to the question which is about why every second bar is corroded differently? With a stretch, the carbon build-up could explain some zone effects, but the rest makes in my opinion no sense in a context of the question. $\endgroup$ – andselisk Nov 1 '20 at 14:46
  • $\begingroup$ Andselisk: Agree, a weakness of the argument. My allusion to charge zones, implying self-selected anode/cathodes among the bars is the only idea. However, the explanation of some design selection per the answer by James Gaidis, could possibly be a stronger reply, which stills does not refute completely an electrochemistry footprint. $\endgroup$ – AJKOER Nov 1 '20 at 15:04

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