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We installed 20 2" drop in anchors to anchor several signs in place near the entrance of our pool. We have to remove the signs about twice a year and after the end of the past season, I noticed there is a small amount of rust forming. I purchased the 304 because it was not going to be in direct contact with the water and because it was solid, not coated, as opposed to the 314, which was only coated. It seems as though the 304 was also just coated so it is beginning to rust. I need to provide some sort of plan as to how to remove the 304 anchors and replace with 316.

Drop in anchors are not made to come out of concrete as the driving pin expands the bottom into place. I had thought about drilling them out but I am hoping to use that as a last resort because I don't want to damage our new concrete and need to keep the holes in tact. I read that Nitric acid can dissolve stainless steel. Will this work in my situation? Or will that also damage the concrete?

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You are not going to dissolve 304 and leave the concrete. Use sandpaper to remove the surface rust as needed : Or mechanically ( cut/grind) the 304 to remove it.

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Try this, apply a cheap and effective spray onto the iron surface employing hypochlorous acid (prepared here by mixing chlorine bleach containing dilute aqueous NaOCl with 6% acetic acid in vinegar in excess to the NaOCl). Add a source of copper (coins), or better high surface area carbon particles in the spray, serving as a cathodic zone, in this spontaneous electrochemical (basically a battery) cell. Source: Equation (7) here.

Note: This battery cell differs mainly in chemistry from the bleach battery cell by employing iron (steel) as the anode which will be the subject of considerable anodic corrosion in place of Aluminum. The cathode is high surface area carbon particles.

Mellor on Hypochlorous Acid in 'A comprehensive treatise on inorganic and theoretical chemistry, Volume 2, however, this is NOT the electrochemical based dissolution suggested above:

Iron filings immediately decompose hypochlorous acid with a brisk effervescence produced by the evolution of chlorine; the iron is partly oxidized and in part dissolved as chloride without the formation of any chlorate. A. J. Balard commented on this: "The greater number of other metallic substances do not decompose hypochlorous acid, and I am yet entirely ignorant of the cause of the peculiar behaviour of iron." P. Grouvelle passed chlorine through water with iron hydroxidelin suspension and a bleaching liquid along with ferric chloride was produced, and he found the liquid retained its bleaching properties after boiling for a quarter of an hour; but A. J. Balard failed to confirm this statement; he could not make ferric hypochlorite either (i) by the action of hypochlorous acid on iron hydroxide, for hypochlorous acid does not dissolve ferric oxide; or (ii) by the action of calcium hypochlorite on ferric sulphate, for calcium sulphate and ferric oxide are produced. Hence, adds A. J. Balard, "ferric hypochlorite cannot exist;" the results by P. Grouvelle are due to the formation of ferric chloride and hypochlorous acid in dil. soln.; when the mixture was heated, a portion of the acid distilled off, and the reaction which occurred with the cold soln. was reversed, for ferric oxide and chlorine were formed. Ferrous oxide is oxidized to ferric oxide by hypochlorous acid.

Problematic fumes so an outdoor reaction is strongly advised. Neutralize solution products with Washing Soda at the end of the reaction.

Here is a particular comment relative to steel being corroded in place of iron, without the benefit of a good high surface area cathode in diluted NaOCl.

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