When it comes to chemical rust removers – there are two main methods: spray-on/gel or soak. To get an idea of the acidity level of each of these solutions, here is a list of their pH values – the closer to 0, the more acidic a solution is (water has a pH of near 7).
Acetic acid (vinegar) 2.4
Citric acid 2.2
Lemon juice 2.0
Oxalic acid 1.3
Phosphoric acid (naval jelly) 1.0
Muriatic acid (HCl) 0.1
The amount of time it takes to dissolve or neutralize rust depends on the amount of rust, and the concentration of the solution used. Objects should be periodically removed, and the gunk scrapped off. Note that very few tools are uniformly rusted, so rust will be removed at a differential rate. Treated steel can rust very quickly after exposure to air – rinse, dry, and treat immediately.
The simplest form of chemical rust remover is household vinegar, which is usually about 5% acetic acid. Acetic acid is also known as ethanoic acid, with the formula CH3CO2H. Vinegar will leave a grey finish that is a type of rust crystal that adds a small degree of rust resistance. Iron (III) oxide reacted with Acetic acid will produce Ferric acetate and water.
Fe2O3 + 6CH3COOH → 2(CH3COO)3Fe + 3H2O
Sometimes baking soda is used after the vinegar to neutralize the solution. Note that vinegar is mildly corrosive to metals, including iron, magnesium and zinc (acetic acid + iron = iron acetate + hydrogen). It also works extremely well for cleaning and sharpening files.
2. Citric acid
Citric acid is a very weak acid, but marginally stronger than vinegar. A 10% solution is adequate, so 100g of citric acid with 900ml of warm water (which works faster than cold water). Once the object has been placed in the citric acid, bubbles will start to form which means the citric acid is working. Citric acid + rust yields iron oxide, carbon monoxide, water and hydrogen.
C6H8O7 + Fe2O3 → 2 FeO + 6 CO + 2 H2O + 2 H2
NOTE that citric acid etches iron and steel – if you leave things too long microscopic etched pits can form. Lemon juice is slightly more acidic than vinegar, and has a similar effect on rust.
Molasses made from cane sugar is acidic (beet molasses has a pH of 8.0). There are a number of ratios of molasses to water in the literature – 1:4, 1:6 or 1:9. Molasses such as Crosby’s Fancy molasses has a pH of 4.5-6.0 (with a 1:1 dilution). The water and molasses mixture, when exposed to air ferments, producing acetic acid, amongst other substances. The downside to molasses is that it is incredibly slow – it may take a week or so. Molasses won’t remove as much steel if you leave it in the bath too long. It seems to be the most gentle.
Evapo-Rust is a commercial product that works by chelation, bonding to iron. An iron chelator is a chemical that forms a soluble, complex molecule with certain metal ions, inactivating the ions so that they cannot normally react with other elements or ions to produce precipitates. This process does sometimes leave a black film on the iron, the longer the item is in the solution, the darker the film becomes (this black film is supposedly carbon).
Evaporust is everything one could want in a rust remover – non-toxic, no fumes, biodegradable. Amazing stuff. Could be expensive for large items.
5. Oxalic acid
Oxalic acid is an organic compound with the formula H2C2O4. In common use it is found in a product called Barkeeper’s Friend. Barkeeper’s Friend can also be used in a paste form by shaking some onto a damp rag, rubbing it all over the surface and leaving it for 10-15 minutes. This is supposedly extremely good at removing rust from chrome plated surface.
6. Restore Rust Remover
A new product, Restore Rust Remover Gel, is from the UK. A gel is great for applications where a wooden handle can’t be easily removed in order to submerse the entire tool. The product contains both etidronic acid, and trisodium nitrilotriacetate. Both are chelating agents, pH neutral.
7. Naval Jelly
This is a strangely named product is actually just phosphoric acid (H3PO4). Naval jelly may have been one of the first commercially available rust removers – advertised in Popular Mechanics in the 1960s, and was available in 55-gallon drums. The phosphoric acid will convert iron oxide Fe2O3 to iron phosphate FePO4.
Fe₂O₃ + 2H₃PO₄ yields 2FePO₄ + 3H₂O
Other chemical methods
It is possible to use chemicals like hydrofluoric acid (HF), hydrochloric (Muriatic) acid – but used incorrectly (like in the wrong concentrations) both are likely to etch the iron. These chemicals have relatively harsh fumes associated with them, even in low concentrations. I would avoid the more dangerous acids in rust removal – they aren’t necessary. What about Coke? Is it at all possible to dissolve rust in Coca Cola? Possibly – it contains high levels of phosphoric acid, Coke has a pH of about 2.7.
If the rust is really ingrained, then you can build a DIY electrolysis system. Electrolysis is a method of removing the iron oxide by passing an electrical charge from a battery through the rusty metal to stimulate an exchange of ions while the tool is submerged in an electrolyte solution. For items such as saw blades it may be easier to scour the surface with synthetic steel wool/fine 400-800 grit wet-and-dry sandpaper and mineral spirits – not really chemical so to speak, but involves the use of a chemical as a lubricant.
Stay tuned for the experiments!