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Unusually, this question arises in a DIY context rather than a chemistry one.

I'm doing some work in my garden with reinforced concrete (a retaining wall against a high bank of soil). As some will know, it usually doesn't matter if the steel used in reinforced concrete gets rusty, because it will still have its inner bulk of steel for strength, still be gripped when concrete pours around it, and won't corrode further provided it is deep enough inside the body of concrete.

There's one exception. If the steel is used to join two sections of concrete below ground level that are poured separately, without a "water stopper", water will inevitably enter the point where the two sections of concrete meet, and in favourable conditions can slowly corrode the steel there. It's a low risk, and a long term issue, but can happen.

That's the situation I'm in, and I didn't realise the issue early enough to take precautions. Having already poured one section with its steel last year, I'm trying to retrospectively prevent possible issues before pouring the second part.

To counter any corrosion issues, I plan to use a zinc based galvanising coat to retrospectively protect the steel in the region of the join between the two lots of concrete. But there's a problem. Galvanisation relies on electrical conductivity to create the sacrificial aspect, and rust doesn't conduct. Physically stripping rust back to steel is also hard on these bars, as they are designed with an irregular surface, to enhance grip.

So I'm wondering if there is a solution via chemistry - some easily accessible way to revert a rust coat back to metallic form, or remove rust to expose metal. Most rust removal products as far as I suspect, attempt to passivate or stabilise rust, not actually return a surface to metallic conductive form.

Is there anything I can easily do to return the rust layer to an electrically conductive/metallic layer? If not, any ideas how else I might chemically reduce the scope for future corrosion over the years?

Update: noting for clarity, the join is below groundwater level, with some degree of hydrostatic pressure behind it (it's a 1m retaining wall where a soil slope in the garden was cut away, and I can't add drainage below the joint because the bottom slab is already set and the joint will be 30cm below ground level even on the lower side, hence below groundwater level). Also the concrete mix is self-consolidating/self-compacting (SCC) so air bubbles as mentioned in one answer are not an issue - both mixes are luckily, specifically designed to preclude them, and the formulation sets pre-compacted without vibrating.

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If you have a plastic bowl or container large enough to accommodate your item, fill it with water with with some salt (e.g., kitchen salt). Then you need Aluminum foil lightly wrapping your rusty piece of iron which altogether are immersed in this said solution (luke warm is enough). One important detail is that the foil is in mechanical contact to the iron, while the solution is able to be in contact with the foil and the cast iron simultaneously.

Over the course of the next hours, Aluminum will be "eaten away" -- or, more chemically speaking, Aluminum will be oxidized, and the rusty iron will be reduced. To facilitate the process, it does not harm to scratch the Aluminum surface with a brusch on the surface exposed toward your iron pieces.

To direct the work of this oxy-reduction reaction toward your rusty iron pieces, it is important that the container for this setup itself is not made of metal. Depending on the size of the item "to recover" things like a glass / china bowl, or a plastic bucket are suitable. After half a day or so, remove your item, rinse it with water and a brush; dry it and protect it against new corrosion. As already mentioned by you, this may be done by galvanizing, or by galvanic anodes.

An alternative to using a salt solution is the addition of vinegar to the water. The key point of both (salt or vinegar) is that they increase the electric conductivity of the solution, hence accelerating the process. If using vinegar, however, do not forget to protect your hands with gloves and eyes (goggle) against the acid.

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"it usually doesn't matter if the steel used in reinforced concrete gets rusty" isn't exactly accurate. It is good if the rebar used in concrete is slightly rusty, for a better bond. But loose rust results in a poor bond. And if the steel "gets" rusty (rustier), the corrosion produces ferric oxide/hydroxides which have a larger volume than the original iron, and the expansive stresses, especially in combination with freeze-thaw conditions, result in spalling of the concrete over the corrosion.

Cold galvanizing with zinc-rich paint sounds like it might give you some extra protection of the yet-to-be-coated rebar. However, it seems like overkill, considering that you admit it's a "low risk". If you want to improve the corrosion resistance of the yet-to-be-coated rebar, how about coating it with cement paste just before casting the new concrete? Knock off any loose rust with a wire brush. Then take a heavy duty paint brush and coat the exposed rebar with a coating of cement paste (watery; w/c ~ 0.55) to soak into all the tiny crevices on the rebar surface. Also on the surface of the old concrete, to help provide a better bond and leakproofness. (There are latex additives for concrete patches for improving bond to old concrete, but this may also be overkill. Just clean off the old concrete and paste it with some of the cement paste before casting.) Then cast the fresh concrete. This will prevent (or minimize) air bubbles being trapped at the rebar surface, which help initiate corrosion.

And since you are concerned about possible corrosion at the junction of the two pours of concrete because of water ingress, you could provide a weep hole at the bottom of the concrete (above ground, but not distractingly visible).

Commercially and industrially, returning rebar rust to reinforcing status is never done. The distressed concrete is removed and replaced.

Another thing: you will have two large concrete electrodes (dry, not very active), connected by rebars. If the two concrete blocks are very similar, the voltage between them will be very small. But do not make a big change, e.g., by adding a corrosion-inhibiting admixture to the new concrete, otherwise you may set up a large galvanic cell which will favor corrosion in one or the other blocks.

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  • $\begingroup$ Thanks James. As it happens, much of your 2nd paragraph is probably taken care of, because by lucky chance, the pours are both self-consolidating/self compacting (SCC) mixes - a formulation that simply doesn't need vibration because there aren't air bubbles when it sets. So vibration + paste may well not add anything, nor needed. However weep holes aren't practical as the join is below groundwater level. Your last para about galvanitic cell creation is a biggie, it sounds like. The concretes are identical formulation. Do you want to update your answer in light of the Infos? It would help :) $\endgroup$ – Stilez May 28 '19 at 5:39
  • $\begingroup$ I visualized a vertical joint; a horizontal one is more troublesome. Essentially you have a dam. It may be all right. At least you won't have freeze-thaw problems. SCC concrete is usually rich, so will be alkaline as long as the water flow does not remove Ca(OH)2. Instead of making a straight butt joint, you could overlap concrete in back of the joint so water would have to travel farther to get to the rebar. This also reduces dependence on the rebar connection. It doesn't sound like galvanic interaction will be serious - just a theoretical speculation. $\endgroup$ – James Gaidis May 28 '19 at 13:28
  • $\begingroup$ Insanely rich. 400-480.kg per cubic m, from memory. I'm more worried about spelling and expansion issues in years to come, than the basic loss of a strong connection. $\endgroup$ – Stilez May 28 '19 at 14:56

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