Lamellar corrosion is the name. It occures in extruded and rolled shapes.

A-36 plate from that era often contained many layers of non-metallic inclusions depending on melting practice and could be at the cut end surface.

Corrosion will preferentially work along these inclusions and result in long layers of attack.
As the material corrodes the corrosion products take up more volume than the original steel.
This is what causes the swelling and pealing of the layers.
Even without excessive inclusions you will get this as the surface corrodes and separates
it will accelerate corrosion in the crevice below it. And this keeps repeating.

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P.E. Metallurgy, consulting work welcomed

So what would be the best practice to address this problem? Is Zinc-based coating the preferred solution? Thanks.

Use a zinc based coating on the bare steel but you'll want an epoxy coating on top to build thickness and durability. The zinc coating protects the metal if the outer epoxy coating gets damaged or has voids.

Will you be able to dry this tunnel out for painting?

Your coating supplier can give you options.
Often these are three layer systems.
You blast back to rough white metal, prime (Zn rich), build coat (also Zn), top sealer/protectant.
Clean, dry, and the correct blast profile are critical.

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P.E. Metallurgy, consulting work welcomed

It could be corrosion by water containing oxygen of a plate (oriented material) of which the lining went off =)

Hi, Chumpes. I am not following what you are saying. Do you mind elaborating?

hello Damianyu
I mean that it is normal to experience corrosion because carbon steel material will corrode when exposed to free water that dissolves oxygen : the painting system has to remain in good condition (repair of the painting may be required time to time) otherwise CS will corrode.
in addition, we can see on your pictures that the corroded zone delaminates following a dominant orientation : this shows that your material is a plate that was rolled, and the rolling operation oriented the microstructure and led to this layered aspect of the corroded zone.

in brief, CS needs a consistent painting system otherwise it will corrode when in contact with air and water.

...and a caution with coating systems. Once cured, some coating systems do not readily accept 'touch up' applications.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik