The effect of phosphoric acid on the corrosion of steel

Phosphoric acid (H₃PO₄) is a widely used inorganic acid in both industrial and laboratory settings. It plays a dual role in the context of steel corrosion: while it is a corrosive agent, it is also commonly used in surface treatment processes to reduce future corrosion. This apparent contradiction makes phosphoric acid a particularly interesting chemical when studying the corrosion behavior of steel.

Corrosive Nature of Phosphoric Acid

Like most acids, phosphoric acid can corrode steel by reacting with the iron (Fe) to form soluble iron phosphate compounds. When steel comes into contact with concentrated or diluted phosphoric acid under uncontrolled conditions, a redox reaction occurs, leading to the release of hydrogen gas and the formation of iron(III) phosphate or other iron salts, depending on the concentration and temperature. The overall corrosion process weakens the steel, reduces its mechanical integrity, and results in material loss.

The general corrosion reaction can be represented as:

Fe + H₃PO₄ → FePO₄ + H₂↑

Factors such as acid concentration, exposure time, temperature, and the presence of dissolved oxygen or other ions significantly influence the rate of corrosion.

Protective Role in Controlled Applications

Interestingly, phosphoric acid is also used in steel treatment processes like phosphating, which aims to reduce future corrosion. In this controlled process, steel is treated with dilute phosphoric acid solutions that react with the surface to form a passive layer of iron phosphate. This layer is relatively insoluble and acts as a protective coating, enhancing corrosion resistance and improving the adhesion of paints and coatings.

This transformation turns a corrosive process into a protective one, especially when combined with other inhibitors or sealants. The phosphated layer also reduces the electrochemical activity on the metal surface, thus lowering the rate of further oxidation.

Use in Rust Removal

Another beneficial application of phosphoric acid is in rust conversion. When applied to rusted steel surfaces, phosphoric acid reacts with iron oxides (rust) to form a stable black coating of ferric phosphate. This helps to stabilize the surface and prevent further corrosion, making it a valuable step in the restoration of corroded metals.

Industrial Implications

In industries such as construction, automotive, and shipbuilding, understanding the dual nature of phosphoric acid is crucial. While accidental spills or prolonged exposure to concentrated phosphoric acid can damage steel equipment, its deliberate application under controlled conditions can significantly extend the life of steel structures. Safety protocols, appropriate concentrations, and neutralization steps are essential in handling this acid.

Conclusion

Phosphoric acid can both corrode and protect steel, depending on how it is used. In uncontrolled environments, it promotes corrosion by reacting with the iron in steel. However, in controlled applications like phosphating and rust conversion, it enhances corrosion resistance. This dual behavior highlights the importance of context in evaluating chemical effects and demonstrates the versatility of phosphoric acid in metal treatment processes.