Cast iron generally exhibits moderate corrosion resistance, often performing better than unalloyed steel in certain environments, particularly due to the presence of graphite. However, its specific resistance varies significantly depending on the type of cast iron, the environment, and the presence of alloying elements.
Understanding Cast Iron's Corrosion Resistance
The microstructure of cast iron, specifically the form and distribution of graphite, plays a crucial role in its corrosion behavior. When the iron matrix corrodes, the inert graphite often remains, forming a porous but sometimes protective layer, a phenomenon known as graphitic corrosion. While this layer can offer some resistance to further uniform attack, it severely compromises the mechanical strength of the material.
Factors Influencing Corrosion
Several factors dictate how well cast iron resists corrosion:
- Type of Cast Iron: Different microstructures (gray, ductile, malleable, white) respond differently to corrosive agents.
- Environmental Conditions: Exposure to air, fresh water, seawater, acids, alkalis, or soil each presents unique challenges.
- Temperature: Higher temperatures generally accelerate corrosion rates.
- Presence of Oxygen: Oxygen is a key driver for many corrosion processes.
- Alloying Elements: Specific additions like silicon, nickel, chromium, or molybdenum can dramatically enhance resistance.
- Surface Finish: Rougher surfaces can trap corrosive media, potentially leading to localized attack.
Corrosion in Seawater
Cast iron components, such as pipes, valves, and fittings, are frequently used in marine environments. In seawater, cast iron typically undergoes graphitic corrosion, where the metallic matrix selectively corrodes, leaving behind a carbonaceous skeleton that retains the original shape but loses mechanical strength. The rate and severity of this corrosion vary by cast iron type:
| Type | Corrosion Rate (mm/year) | Pitting Factor |
|---|---|---|
| Gray cast iron | 0.02 | 10.9 |
| Ferritic spheroidal gray cast iron | 0.025 | 14.7 |
| Malleable cast iron | 0.045 | 4.9 |
| Pearlitic gray cast iron | 0.003 | 9.3 |
The pitting factor indicates the ratio of the deepest pit depth to the average corrosion depth, highlighting susceptibility to localized attack.
As seen in the table, pearlitic gray cast iron exhibits a remarkably low general corrosion rate in seawater, though it can still be susceptible to pitting. Ferritic spheroidal (ductile) cast iron, while having a slightly higher general corrosion rate than gray cast iron in this specific environment, shows a higher pitting factor, indicating a greater propensity for localized attack. Malleable cast iron has the highest general corrosion rate but a relatively lower pitting factor compared to gray and ductile irons.
Corrosion in Other Environments
The corrosion resistance of cast iron extends beyond marine applications, with varying performance depending on the medium:
- Atmospheric Exposure: Cast iron exhibits fair resistance to normal atmospheric conditions, especially in dry, rural areas. It forms a stable rust layer that can offer some protection. However, in industrial or marine atmospheres with high humidity and pollutants, corrosion rates increase.
- Fresh Water: Generally good resistance, making it suitable for water pipelines and components. The formation of a protective scale or rust layer often helps to reduce ongoing corrosion.
- Soil: Cast iron pipes have a long history of successful use in buried applications. Soil conditions (pH, moisture, aeration, presence of bacteria) significantly influence corrosion rates, which can range from very low to moderate.
- Acids: Most cast irons have poor resistance to strong acids. Graphitic corrosion is a common failure mechanism. However, high-silicon cast irons (e.g., 14% silicon) are an exception, offering excellent resistance to many mineral acids, including sulfuric and nitric acids, at various concentrations and temperatures due to the formation of a highly protective silica film.
- Alkaline Solutions: Cast iron generally performs well in alkaline solutions and many neutral salt solutions.
- High Temperatures: In oxidizing atmospheres, cast iron can suffer from oxidation (scaling) at elevated temperatures. Alloying elements like silicon, chromium, and aluminum improve high-temperature oxidation resistance.
Types of Corrosion Affecting Cast Iron
Beyond uniform corrosion, cast iron can be susceptible to specific types of localized attack:
- Graphitic Corrosion (Graphitization): This is the most prevalent form, particularly in aggressive environments like seawater, acids, or certain soils. The iron matrix selectively corrodes and dissolves, leaving behind the inert graphite flakes or nodules. The component maintains its original dimensions but becomes weak, porous, and brittle, akin to a "sponge."
- Pitting Corrosion: Localized attack that results in the formation of small holes or "pits" on the surface. This can be particularly dangerous as it can lead to through-wall penetration without significant overall material loss. As shown in the seawater data, different cast iron types have varying susceptibilities to pitting.
- Galvanic Corrosion: When cast iron is in electrical contact with a more noble metal (e.g., copper, stainless steel) in an electrolyte, it can act as the anode and preferentially corrode.
- Crevice Corrosion: Occurs in confined spaces or crevices where oxygen access is restricted, leading to localized differences in electrochemical potential.
Enhancing Cast Iron's Corrosion Resistance
Several strategies can be employed to improve the corrosion resistance of cast iron:
- Alloying Elements:
- Silicon: High-silicon cast irons (e.g., 14% Si) provide exceptional resistance to acids.
- Nickel: Adding nickel (e.g., Ni-Resist alloys) significantly improves resistance to seawater, weak acids, and alkalis, often forming a stable, corrosion-resistant austenitic microstructure.
- Chromium: Chromium enhances resistance to oxidation at high temperatures and improves resistance to certain corrosive liquids.
- Molybdenum and Copper: Can also be added to improve specific aspects of corrosion resistance.
- Protective Coatings:
- Paint and Epoxy Coatings: Widely used for atmospheric and immersed applications.
- Asphaltic or Bituminous Coatings: Commonly applied to cast iron pipes for buried service.
- Cement Mortar Linings: Provide excellent protection for the internal surfaces of water pipes against corrosion and tuberculation.
- Metallic Coatings: Galvanizing (zinc coating) can offer sacrificial protection.
- Cathodic Protection: For buried pipes or submerged structures, cathodic protection (using sacrificial anodes or impressed current systems) can effectively mitigate corrosion.
- Design Considerations: Avoiding crevices, ensuring proper drainage, and isolating dissimilar metals can reduce the risk of localized corrosion.
