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WD-40's "help page" on "How To Loosen Rusty Bolts, Screws And Nuts" states:

Hydrogen peroxide can dissolve rust. Applying hydrogen peroxide to the fasteners and letting it sit is also an effective method to remove rust and loosen nuts and bolts.

I'm no chemist, but most of what I've found in the last couple of hours tells me that hydrogen peroxide is more likely to cause (create) rust than "dissolve" (remove?) it, for example, see LibreText: "Hydrogen peroxide is considerably stronger as an oxidizing agent than as a reducing agent".

Can someone who understands these things please let me in on what happens when I mix rust and hydrogen peroxide? Is it true that *"applying hydrogen peroxide […] is also an effective method to remove rust […]? I find this claim dubious.

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    $\begingroup$ I think its action is mostly mechanical - it decomposes and bubbles disrupt layers of rust. $\endgroup$
    – Mithoron
    Commented Oct 5, 2021 at 23:06
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    $\begingroup$ Also, it's mostly water and that helps a lot in terms of getting stuff into solution. $\endgroup$
    – Todd Minehardt
    Commented Oct 6, 2021 at 0:12
  • $\begingroup$ @Mithoron Are you saying that "there is no reaction between rust and hydrogen peroxide" (as far as you aware)? $\endgroup$
    – jaimet
    Commented Oct 6, 2021 at 6:42
  • $\begingroup$ @ToddMinehardt But rust is insoluble in water (at least that's what a few posts on stackexhange would have me believe!) $\endgroup$
    – jaimet
    Commented Oct 6, 2021 at 6:46
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    $\begingroup$ Rather than dissolve read disrupt. That is not a chemistry book. The statement makes sense, context given. Let me add that iron is a catalyst for hydrogen peroxide decomposition, so the latter should be vigorous at the right place. $\endgroup$
    – Alchimista
    Commented Oct 6, 2021 at 6:57

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There is usually a darker-colored layer of rust below the outer layer. The outer layer is easy to remove and has more iron(III) hydroxide in it. When the layer that holds the rusty bolts is treated with hydrogen peroxide, it also turns into an easily removable powder. It does not dissolve, it is just reacting and washed off. The reaction converts some of the iron(II) compounds to iron(III). Both are practically insoluble.

hydrogen peroxide is more likely to cause (create) rust than "dissolve" (remove?) it

If you start with metallic iron, then this is true, but given that the formation of thick layers of rust has already happened over a long time, adding hydrogen peroxide for a brief time may help to loosen it. There are better options for doing this, however.

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