Rust, chemically known as iron oxide (Fe₂O₃), forms when iron or its alloys are exposed to oxygen and moisture, leading to the gradual degradation of the metal. Dealing with this corrosion requires chemical intervention that either removes the oxide layer completely (dissolution) or transforms it into a stable, non-reactive compound (conversion). Both approaches rely on specific chemical agents to halt the degradation process and prepare the metal surface for protection.
Chemicals That Dissolve Rust (Acidic Solutions)
Rust dissolvers typically use acidic compounds to react with iron oxide, converting the basic compound into a water-soluble salt that can be rinsed away. This process is fast and effective for removing heavy rust layers and restoring metal to its bare state.
Phosphoric acid is a common component in rust removal gels and liquids. It acts as a mild acid that reacts with iron oxide to form soluble iron phosphate, which is easily removed. Oxalic acid, a weaker organic acid, is frequently used for lighter rust stains, particularly on surfaces like wood or stone where harsher acids might cause damage. It converts the iron oxide into iron oxalate, a colorless, water-soluble compound that can be rinsed away after soaking.
For extremely heavy corrosion, hydrochloric acid (muriatic acid) offers a potent solution. This strong mineral acid rapidly dissolves iron oxide into iron chloride, but its aggressive nature means it can quickly etch and damage the underlying metal. Chelating agents, such as ethylenediaminetetraacetic acid (EDTA), offer a non-acidic approach. These agents work by bonding directly to the iron ions within the rust structure, forming a stable, water-soluble complex that lifts the rust from the surface.
How Rust Converters Chemically Stabilize Corrosion
Rust converters chemically alter the existing rust rather than dissolving it. This approach is useful when complete mechanical removal is impractical, such as in tight crevices or on large structures. The objective is to transform the reddish-brown iron oxide into a stable, black, inert layer that prevents further corrosion.
A primary ingredient in many converters is phosphoric acid. When formulated for conversion, it reacts with iron oxide (Fe₂O₃) to create a layer of black iron phosphate (Fe₃(PO₄)₂). This new iron phosphate layer is tightly bonded to the surface, resists moisture and oxygen, and serves as an excellent foundation for paint or other protective coatings.
Tannic acid, a naturally occurring organic compound, is also commonly used in rust converters. It reacts with iron oxide to form iron tannate, a dark, bluish-black compound that is stable and chemically inert. The iron tannate layer seals the surface, starving the underlying metal of the oxygen and moisture required for continued rusting. Commercial rust converters often combine these active ingredients with organic polymers, which act as a binder to create a tough, protective film adhering to the metal surface.
Safe Handling and Application Methods
Working with rust-eating chemicals requires strict adherence to safety protocols to prevent personal injury and damage to the metal being treated. Personal protective equipment (PPE) is mandatory and must include chemical-resistant gloves, safety goggles or a face shield, and proper ventilation to avoid inhaling corrosive fumes.
Before application, it is important to remove any loose, flaky rust, dirt, or grease using a wire brush or degreaser, as this allows the chemical to penetrate and react directly with the dense, adherent rust. Acidic solutions, particularly the stronger mineral acids, require careful application and a specific contact time to prevent pitting or etching of the base metal.
Following the chemical treatment, thorough rinsing with water is necessary to remove the dissolved rust and any residual acid. For strong acidic treatments, a neutralization step, often involving a mild alkaline solution like baking soda and water, is advised to stop the chemical reaction completely and prevent flash rusting. Finally, concentrated chemicals must be stored securely away from children and incompatible materials, and any spent solutions or waste must be disposed of according to local environmental regulations to avoid contamination of storm drains or waterways.