The Corrosive Power of Diluted vs Concentrated Nitric Acid on Steel

Introduction

Steel is a widely used material in industries due to its strength and durability. However, its interactions with different chemical substances can vary greatly. Two types of nitric acid, diluted and concentrated, display significantly different behaviors towards steel. This article delves into the reasons why diluted nitric acid corrodes steel whereas concentrated nitric acid does not, focusing on the role of a passive film in protecting steel.

Understanding Nitric Acid's Action on Steel

Nitric acid is a strong oxidizing and corrosive acid. When it comes into contact with steel, its behavior is influenced by its concentration and the presence of other elements in the steel itself. In the case of diluted nitric acid, the corrosion process is more pronounced compared to concentrated nitric acid.

The Role of Concentrated Nitric Acid

Concentrated nitric acid, often referred to as crown acid, possesses a unique property that sets it apart from other acids. When it comes into contact with steel, a thin, protective layer known as a passive film is formed on the surface of the steel. This passive film acts as a barrier, preventing further corrosion of the steel. The chemical process that leads to the formation of this film is known as passivation.

The Mechanism of Passivation

The formation of a passive film is a result of the reaction between the nitric acid and the steel. This process involves the creation of oxides of iron, which react with the hydroxide ions present in the nitric acid solution. The resulting oxides form a protective layer that adheres to the surface of the steel. This passive film is typically composed of oxides of iron and chromium, which are much less reactive than the underlying metal.

Comparison with Other Elements

Steel is not the only material capable of being protected by a passive film. Other elements such as aluminum (Al) and chromium (Cr) also exhibit similar behavior. When these elements are present in steel, they enhance the ability of the steel to form a passive film. Aluminum reacts with nitric acid to form a thin, protective layer of aluminum oxide on the surface, whereas chromium forms a layer of chromium oxide. These oxides are much more stable and resistant to further corrosion than the underlying metal, thus providing better protection.

The Effect of Diluted Nitric Acid

In contrast to concentrated nitric acid, diluted nitric acid lacks the ability to form a protective passive film. When diluted nitric acid comes into contact with steel, it is more likely to cause corrosion. The lower concentration of nitric acid and the absence of the necessary reactants ensure that the formation of the passive film is not sufficient to prevent corrosion.

Conclusion

The behavior of nitric acid concerning its corrosive impact on steel is primarily determined by its concentration. Concentrated nitric acid forms a protective passive film on the surface of steel, thereby preventing further corrosion, while diluted nitric acid may lead to corrosion due to the lack of these protective mechanisms. The presence of elements such as aluminum and chromium in steel further enhances its resistance to corrosion.

Keywords

Nitric Acid, Steel Corrosion, Passive Film