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While this question relates to an automotive application I consider that it is liable to be best asked on this site.

Summary:

  • I wish to create a low cost gel suitable for mixing with Oxalic acid to produce a material that was adequately thixotropic to remain on the underside of horizontal metal surfaces in a thin layer for maybe an hour.
    Longer would be good.

  • It would be a significant bonus if it also was suitable for use with phosphoric acid.

I'm investigating various means of automotive chemical rust-conversion and rust-removal.
There are many weird and wonderful DIY solutions (potatoes actually do work ! :-) ) but the two that seem most worthwhile are

  • Phosphoric acid as a rust converter.
    This is a very common and well respected means of converting rust to insoluble and stable iron-phosphate.
    Concentrations in the order of 25-50% seem typical.

  • Oxalic acid as a rust remover.
    This seems to be slightly less well known but is a technically sound method of converting rust to water soluble iron-oxalate which can then be removed by hosing or waterblasting.
    Supplied as a water soluble powder mixed with maybe 3 to 10 parts by volume with water (may have much more water).

I have tried a commercial rust removal gel whose formula is intractably secret (I tried) but which works superbly to convert serious rust to a soluble state which could easily be removed with a waterblaster. The product is excessively costly for the area that I wish to deal with.

Oxalic acid (solid powder dissolved in water to suit) also works well BUT, as it is about as viscous as water, has the nasty habit of dripping off rather than remaining on the surface for the desired periods of tens of minutes or more. Especially so on vehicle undersides :-).

I would like to produce an Oxalic acid gel carrier to solve the above requirement. Ideas encountered for possibly suitable gels include agar, gelatin, cornstarch & vinegar, seed based gels - with boiled rice "porridge" a likely candidate, and more.

I'd be interested in knowing if anyome else has experience in either DIY gels or other products that might allow me to formulate a suitably thixotropic rust remover.

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    $\begingroup$ Polyethylene glycol could be a candidate. There are commenrcially available preparations of KOH in polyethylene glycol gels for oven cleaning and I don't see why oxalic acid would not work. $\endgroup$
    – Waylander
    Commented Mar 14, 2023 at 12:30
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    $\begingroup$ Acids and alcohols can react to form esters? Do you think this suspension would be stable in the long term? $\endgroup$
    – ACR
    Commented Mar 14, 2023 at 16:22
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    $\begingroup$ Depends on storage conditions and how you define "long term". This SE answer suggests that in the absence of catalyst the mixture is stable. chemistry.stackexchange.com/questions/68662/… $\endgroup$
    – Waylander
    Commented Mar 14, 2023 at 19:04
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    $\begingroup$ @AChem Ester formationg is not a very exenthalpic process, to say the least. Unless you remove the water, it just doesn't proceed. $\endgroup$
    – Karl
    Commented Mar 14, 2023 at 20:54
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    $\begingroup$ @RussellMcMahon I'm out of my area of expertise here, but I guess that you could modify the viscosity by adding shorter chain alcohols or even water $\endgroup$
    – Waylander
    Commented Mar 15, 2023 at 7:30

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I made an oxalic acid gel with xanthan gum. I’m trying to make an alternative to barkeepers friend, like a soft cleanser, but with fewer ingredients. It sounds like what I make would work for you- blend a few teaspoons of xanthan gum in a blender with water and in a separate container, mix oxalic and hot water til dissolved. Add to blender and mix with xanthan gum mixture. The result will be a gel like consistency.

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