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Soldier pile retaining wall.

We're putting 25 foot long w8x48 beams into 16 foot deep holes and filling with concrete.

So the steel beam is encased in concrete.

I would like to increase their longevity by attaching a sacrificial (zinc ?) anode but I don't know where to attach it.

Can we weld them right on the outside surface of the aboveground exposed beam ? Or does the sacrificial anode need to be buried in the soil and then welded to the beam with a copper wire ?

Or should we weld it to the beam under the ground, it will get encased in concrete and we will never be able to inspect or replace it - but would it be more effective encased in the concrete ?

EDIT: Lots of doubt expressed about the ability to use cathodic protection at all in an application like this that is not submerged in water but I draw your attention to the very common (universal ?) practice of attaching sacrificial anodes to steel propane tanks that are buried underground ... why is this different ?

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  • Sacrificial entities need to be in contact with the same fluid medium as the oxidizing or oxidized part. You'd have to wet to the rust which in turn would damage your concrete and beam further, so bad move. Look for ways to keep moisture out, preventing rust and replace beams when they get too rusted.
    – Abel
    May 17, 2023 at 2:41
  • @Abel : See comment below ... how do sacrificial anodes work for buried propane tanks ? That is commonly done and the bag of zinc is "submerged" in the same soil as the tank ... but of course some amount of the tank is unburied under the valve riser ... so why can't we do the same thing with a steel beam ?
    – user227963
    May 17, 2023 at 3:30
  • Rebar corrosion protection is done with galvanizing or epoxy type coatings on the rebar. May 17, 2023 at 15:36

2 Answers 2

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Concrete itself protects steel from corrosion, which is why there are specifications of 2-3" concrete cover for steel reinforcing, depending on environmental exposure (3" for coastal.) See https://www.concrete.org/tools/frequentlyaskedquestions.aspx?faqid=668 and https://www.civillead.com/concrete-cover/

So I think you're barking up the wrong tree, here.

It's the 9 feet sticking out you should be concerned with corroding, and for that, they make Corten steel. Or you could just buy Corten sheet pilings to be driven in (with a pile driver, of course) and forget the concrete and holes.

Also, you should keep the steel 2-3" from the soil on the bottom and all sides of the hole (for instance by pouring a pad at the bottom of the hole before setting the steel in the hole.)

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  • Steel rusts inside concrete. It rusts differently, and perhaps more slowly, but it definitely corrodes - possible badly, depending on moisture and soil. If you can protect the entire piece of steel with a sacrificial anode, then you don't care about above ground vs. below ground - the entire beam is one piece. I have this already on my buried propane tank and it is a common practice - I just wonder what the details are for a beam encased in concrete vs. a tank buried in sand ...
    – user227963
    May 18, 2023 at 1:10
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Sacrificial anodes only work if both the anode and the metal it is protecting are submerged in electrolyte wit a current path between them.

That works for boats. It works for water heaters. It does NOT work for cars, since they aren't immersed (one hopes). I am not at all convinced it would work for metal buried in concrete.

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    @JimStewart Thank you. As I mention elsewhere, it is common practice to bury propane tanks with bags of magnesium attached to the tank with wires. So the tank and the anodes are "submerged" in the ground together. Isn't this what I am suggesting with my steel beams ? It seems like the exact same thing ... so I would think I "submerge" my sacrificial anodes in the earth next to the holes the beams are in and then connect with a wire ?
    – user227963
    May 18, 2023 at 1:14
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    Google "cathodic protection" and read some articles. What is the contex of the steel post in concrete are you trying to protect? May 18, 2023 at 15:14
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    Unless this soil is extremely wet I doubt sacrificial protection is warranted. But this is a question that should be directed to professionals in the field of cathodic protection. May 19, 2023 at 1:32
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    Agreed. This sounds like the kind of thing you can't afford to get wrong, hence the kind of thing you CAN afford to get an expert opinion on.
    – keshlam
    May 19, 2023 at 2:50
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    I think the fact that anodes and concrete isn't common knowledge (but anodes with tanks and boats is) should be an indication that you're barking up the wrong tree. Secrets are not how the construction industry works.
    – longneck
    May 19, 2023 at 22:55

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