Rust removal by chelation
What is rust and how it is removed
Rust on steel is a form of oxidation. It occurs when iron or steel comes in contact with both moisture and oxygen of the air. Without moisture, rusting will not occur. What makes rusting problematic is the consistency of the rust. It occurs as a flaky material that easily breaks off the surface of the steel exposing fresh metal to the elements. Eventually, the steel will corrode and continue to flake off rust until the metal gets weak and fails. Removing rust and prevention is therefore a necessary excersise to prevent expensive damage to structures and equipment.
Typically, rust would be removed in a rather corrosive media such as hydrochloric acid, sulfuric, or phosphoric acid. Although phosphoric acid has the added benefit of forming a protective iron phosphate to prevent rerusting of the cleaned steel, all these acid present disposal challenges and are a hazard to store. In this site I will discuss rust removal by chelating agents. Chelating comes from the Greek word Chele which means claw. Chelating agents are organic molecules that form a structure like a claw and hold metal atoms within their molecules so they dissolve in solution.
Why Not Use Mineral Acids?
The most common and tradional method to remove rust and scale was to simply dunk the metal part or object in a bath of a strong mineral acid. The acids used where usually, hydrochloric, sulfuric, and phosphoric. All three are highly corrosive and require considerable care in handling, In addition, hydrochloric acid is volatile so fumes are a real problem. Both hydrochloric and sulfuric acids leave the steel surface in a chemically "active" state. This is a problem because the higher activity of the metal means it can flash rust very quickly. If left unintended, especially hydrochloric acid, the metal will rust much more severly that it origonally was. On the other hand, phosphoric acid is a little more tame and does form a rust preventative iron phosphate coating on steel. This iron phosphate is very good for applying primer and proceeding with painting. Furthermore, phosphoric acid does not have the fume problems hydrochloric acid presents.
Aside from acute health and safety concerns, one must also consider the enviromental implications of acids. They are almost always harmful to the environment and to water supplies. The issue becomes more compounded when these acids contain metal salts generated by removing the rust and scale from metal. Fortunately, there are more organic and safer methods to manage rust. Some of these materials may even sound familiar from food products!
Oxalic acid or ethanedioc acid is an organic carbon based acid that occurs naturally in plants like rhubarb spinach and the leaves used to make black tea. It is one of the strongest of the organic acids and is a very good chelating reagent for metals including iron.Despite its natural occurance, in concentrated form it is corrosive and poisonous for ingestion and personal protection is necessary.
A warm solution of oxalic acid in water of 5% or concentration will actively remove rust. Oxalic acid is also a strong reducing agent and the red ferric oxide present in rust will be reduced to the ferrous state which forms a yellow ferrosoxalate complex. Being a chelating agent, it only attacks the rust and not the metal and for this reason it is superior to hydrochloric and sulfuric acids which react and dissolve the metal also.
Another crystalline organic acid that actively dissolves rust without attacking the metal is citric acid. It occurs natural in many fruits and has the added benefit of not being appreciably toxic. In fact the supersoar candies sold in stores are lace with crystals of citric acid! None the less, eating citric acid is not recommended as it might damage your tooth enamel. Like oxalic acid a fairly dilute solution can be used. When rusty parts are left overnight, the rust will be gone. Citric acid is superior to oxalic acid in that the iron salts formed are soluble where as the oxalates formed are not very soluble. Oxalic acid can leave a green deposit on steel that has been treated for rust. The citric acid forms iron citrate in solution which is reduced in direct sunlight to a colorless compound.
For this reason clothing with rust stains can be treated with salt mixed with fresh squeezed lemon juice and left to dry in the sun. The rust stain will first turn green forming ferric citrate but then the UV rays from the sun cause the citric acid to reduce the iron to ferrous citirate which is white. There still is a crusty solid left behind so washing with the laundry is necessary.
Rust Removal With Citric Acid
Molasses the nonacid alternative
If you are not in any hurry and you really want the safest solution to remove rust, try a 10% solution of molasses in water. Molasses is not an acid but a sugar or carbohydrate which forms a complex with iron. It can take several days or even weaks but there is no toxicity or environmental concerns. Check out the video below.