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A welder showed me a way of promoting rust on steel by brushing on muriatic acid and then a hydrogen peroxide / water solution. What's going on, chemically speaking?

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    $\begingroup$ You know that all substrates need to be equation? Also steps are usually written separately. $\endgroup$ – Mithoron Dec 28 '20 at 19:50
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    $\begingroup$ First muriatic acid is an old name from the 18th century. Forget about it. Today it is hydrochloric acid. Second, an equation must show the same atoms on both sides of the horizontal arrow. So Iron must appear in the left-hand side of the equation. $\endgroup$ – Maurice Dec 28 '20 at 20:09
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    $\begingroup$ @AlLelopath Stack Exchange sites are not forums, and no, Chemistry.SE is not strictly for professionals. Some comments may appear harsh, but they get straight to the point. Please don't take them personally, they are solely targeted at your question, not you. I think this is a good question, but the main reason for downvotes, I suspect, is the chemical reaction scheme which does not really make sense. I would probably exclude it at this point, and go with textual description. $\endgroup$ – andselisk Dec 28 '20 at 21:36
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    $\begingroup$ I downvoted because both the title and “chemical equation” are nonsensical and there is an informative article on rust in wikipedia. As for what the welder said, I do not doubt it because the hydrochloric acid just cleans off oxides and such and then the hydrogen peroxide solution provides the two other necessities for the rusting of iron in steel: oxygen and moisture/water. I will remove my downvote if the title is fixed and the “chemical equation” is deleted. $\endgroup$ – Ed V Dec 28 '20 at 21:52
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    $\begingroup$ And rust is not Fe2 O3. $\endgroup$ – blacksmith37 Dec 28 '20 at 21:59
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I understand the point of your question, here is a discussion of the associated science.

The process remains a classic electrochemical process where here H2O2 is a replacement for oxygen and the require $\ce{H+}$ is provided by the $\ce{HCl}$ which is also a source of chloride ions noted to promote corrosion as well as forming salts which are excellent electrolytes.

Some references, starting with a good depiction of the underlying electrochemistry, to quote:

Iron (or steel) rusting is an example of corrosion -- an electrochemical process involving an anode (a piece of metal that readily gives up electrons), an electrolyte (a liquid that helps electrons move) and a cathode (a piece of metal that readily accepts electrons). When a piece of metal corrodes, the electrolyte helps provide oxygen to the anode. As oxygen combines with the metal, electrons are liberated. When they flow through the electrolyte to the cathode, the metal of the anode disappears, swept away by the electrical flow or converted into metal cations in a form such as rust.

And some comments from ScienceDirect on the referenced chloride properties:

The chloride ions present in the water promote local corrosion such as crevice corrosion and pitting, and the higher the chloride content, the more corrosive the water.

I hope this helps.

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  • $\begingroup$ This confuses me in that it says the HCL provides H+ which contrasts the comments above in which I'm told the HCL is just for cleaning. $\endgroup$ – Al Lelopath Dec 29 '20 at 16:34

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