Oxalic acid
The acid in most rust removers and wood bleaches. It works by holding iron in solution rather than by dissolving a deposit, and it corrodes the steel the rust was sitting on.
What not to do
Oxalic acid is unsafe on Stainless steel
Unsafe
Do not do this. The risk is established, and it is described below.
Precautions
- Wear gloves
- Wear eye protection
- Rinse thoroughly afterwards
Evidence
Supported
Sources
Right to Know Hazardous Substance Fact Sheet: Oxalic Acid (RTK 1445)
New Jersey Department of Health, 2010 · Government or regulator
Relied on for: That oxalic acid corrodes steel. (states this directly)
Oxalic acid is unsafe on Polished marble
Unsafe
Do not do this. The risk is established, and it is described below.
Evidence
Supported
Sources
Care & Cleaning of Natural Stone
Natural Stone Institute · Trade or professional body
Relied on for: That calcareous stone is sensitive to acidic solutions and that lemon, vinegar and other acids may dull or etch it. (states this directly)
Precautions
- Wear gloves
- Wear eye protection
- Never mix with other cleaning products
- Rinse thoroughly afterwards
- Keep away from children and pets
- Test on an inconspicuous area first
In detail
Oxalic acid is a solid organic acid sold as a white powder or crystal, and the New Jersey Department of Health's fact sheet names its three domestic uses in one sentence: rust remover, radiator cleaner and ink stain remover. Grouping rust and ink under one substance is the useful part, because nothing about the two marks suggests a shared chemistry. Both are iron problems — rust is hydrated iron oxide, and traditional inks are iron-based — and oxalic acid attacks both the same way, by binding iron ions into a soluble complex and carrying them off rather than by scouring anything away. That is chelation as much as acidity, which is why it clears a stain a stronger acid would leave behind. Two things follow that the packaging rarely says. It corrodes steel, so the metal underneath a rust patch is not exempt from the treatment. And it is corrosive to skin and eyes, absorbed through the skin, and recorded by the same regulator as capable of damaging the kidneys — this is not a mild household acid in the way citric acid is, despite being sold beside it.
Properties
- Chemical name
- Ethanedioic acid
- pH
- Strongly acidic
What works
Oxalic acid is effective against Rust Supported
It binds iron into a soluble complex rather than scouring the oxide off, which is why it clears a stain a stronger acid can leave behind.
Oxalic acid is effective against Ink Supported
Iron-gall and similar inks are iron compounds, so the same complexing that lifts rust lifts them.
Why
- Oxalic acid is explained by Chelation
- Oxalic acid is explained by pH in cleaning
- Oxalic acid is explained by Corrosion and passivation
Sources for this page
Sources
Right to Know Hazardous Substance Fact Sheet: Oxalic Acid (RTK 1445)
New Jersey Department of Health, 2010 · Government or regulator
Relied on for: That oxalic acid is used as a rust remover, radiator cleaner and ink stain remover, that it is corrosive, and that it corrodes steel. (states this directly)