White cast iron is a specific type of cast iron that is known for its high hardness, wear resistance, and excellent castability. Unlike other types of cast iron, white cast iron contains little to no graphite, resulting in a white appearance.

While white cast iron is highly desirable for its mechanical properties, its susceptibility to corrosion, including rust, is a significant drawback.

To understand the rusting behavior of white cast iron, it is important to first explore the factors that contribute to the corrosion process. Rust, scientifically known as iron oxide (Fe2O3), is formed when iron reacts with oxygen and water.

This reaction is accelerated in the presence of certain conditions, such as high humidity, saltwater, or acidic environments. Rusting is a common phenomenon for iron and iron alloys, including cast iron, which is primarily composed of iron, carbon, and other alloying elements.

In the case of white cast iron, its unique microstructure plays a crucial role in its susceptibility to rust. White cast iron is characterized by the presence of hard, brittle carbide particles, primarily cementite (Fe3C), embedded in a matrix of pearlite.

 Cementite is a compound of iron and carbon that is extremely brittle and does not possess corrosion white cast iron.

When white cast iron is exposed to an environment containing moisture and oxygen, the following corrosion process occurs:

1. Initial stages: Moisture from the environment comes into contact with the surface of the white cast iron. The iron reacts with oxygen to form iron oxide (FeO), commonly known as ferrous oxide or wüstite. This initial oxide layer is typically thin and non-protective.

2. Further rusting: Over time, the thin ferrous oxide layer undergoes additional reactions with oxygen and moisture, resulting in the formation of hydrated iron oxides, commonly referred to as rust. This process involves the conversion of ferrous oxide to ferric oxide (Fe2O3), which is a more common form of rust.

3. Pitting corrosion: Due to the presence of hard carbide particles in white cast iron, corrosion tends to occur preferentially around these particles. This localized corrosion, known as pitting corrosion, can lead to the formation of small pits on the surface of the material, further accelerating the rusting process.

4. Continuous rust formation: Once the initial rust layer forms, it acts as a porous barrier that allows oxygen and moisture to penetrate deeper into the white cast iron. This leads to the continuous formation of rust, which can result in the degradation of the material over time.

The susceptibility of white cast iron to rust can be attributed to its high carbon content and the presence of brittle carbides. The absence of graphite, which acts as a lubricant and provides corrosion resistance in other forms of cast iron, leaves white cast iron more prone to corrosion. Additionally, the hardness and brittleness of white cast iron make it difficult to apply protective coatings or perform surface treatments to mitigate corrosion.

To protect white cast iron from rusting, several preventive measures can be taken:

1. Coatings: Applying protective coatings, such as paints, varnishes, or epoxy coatings, can act as a barrier between the white cast iron and the environment, preventing direct contact with moisture and oxygen.

2. Galvanizing: Electroplating the surface of white cast iron with zinc can provide a sacrificial layer that corrodes before the iron, protecting the underlying material.

3. Alloy modifications: Adjusting the composition of white cast iron by adding alloying elements, such as nickel, chromium, or molybdenum, can enhance its corrosion resistance. These alloying elements can form passive layers that protect the iron from further oxidation.