Why Industrial Iron Castings Must Undergo Anti-corrosion Treatment?
Cast iron has a high carbon content (over 2%), with flake-shaped graphite inside and a loose structure. Moreover, iron-carbon alloys have strong electrochemical activity, making them prone to electrochemical corrosion, oxidation and rust in natural environments.
When iron castings are used in open-air, humid enclosed, mining, chemical, or vapor-acid/alkali environments, they are highly susceptible to corrosion, oxidation and rust.
Without anti-corrosion treatment, iron castings may develop surface rust that affects appearance and assembly during service. Progressive corrosion reduces structural strength, shortens service life, and increases replacement and maintenance costs.
For these reasons, anti-corrosion treatment is an essential protective process for iron castings. Through physical isolation or chemical passivation, it effectively extends service life and ensures optimal performance of iron castings.
Typical examples of industrial iron castings that benefit greatly from anti-corrosion treatment include valve bodies, pump bearing housings, pump casings, and wear-resistant mining iron castings.
Comparison of Advantages and Disadvantages of Different Anti-corrosion Processes
The mainstream anti-corrosion processes for industrial iron castings include painting, phosphating, electrophoretic coating, hot-dip galvanizing, and blackening/bluing, each with distinct advantages, disadvantages and applicable scenarios.
1. The anti-corrosion principle of conventional painting is to spray resin-based anti-rust or epoxy paint on the surface of iron castings to form a physically isolating paint film to block air and moisture. This process has the advantages of low cost and flexible construction, but it has an average anti-corrosion lifespan and cannot withstand severe corrosion.
2. Phosphating is a chemical reaction that forms a water-insoluble phosphate protective film on the surface of iron castings to enhance the adhesion of subsequent coatings. This process provides strong adhesion for paints and coatings, with better anti-corrosion performance than conventional painting, but it has weak independent anti-corrosion capability and requires supporting coating.
3. Electrophoretic coating uses an electric field to drive paint particles to uniformly adhere to all surfaces of the casting, forming a uniform, dense and insulating protective film. This process features uniform coating and is suitable for castings with complex structures, but it is relatively costly and limited for large workpieces.
4. Hot-dip galvanizing involves a metallurgical reaction between high-temperature molten zinc and the surface layer of iron castings to form a zinc-iron alloy layer and a pure zinc layer. Castings treated with this process have a long anti-corrosion lifespan and can withstand harsh outdoor environments, but it is expensive and has a slight impact on dimensions.
5. Blackening/bluing is a chemical oxidation process that forms a dense oxide film on the surface of iron castings. Castings treated this way have a good appearance and are suitable for precision small parts, but they only provide mild rust protection and are not suitable for open-air heavy-duty applications.
Each anti-corrosion process varies significantly in protection effect, cost, construction conditions and applicable working conditions. There is no absolutely best process, only the most suitable one that matches the casting type, operating environment, budget and service life requirements.
How to Quickly Select the Right Anti-corrosion Process for Iron Castings Based on Working Conditions
- Based on Operating Environment
Indoor and dry: Low exposure to corrosive substances, only basic physical protection is needed, and painting and phosphating are fully sufficient.
Open-air or mining: Rain, moisture and dust accelerate electrochemical corrosion, requiring thick coatings or zinc layers for long-term isolation.
Chemical acid/alkali: Highly corrosive media have strong penetration, conventional paint films are easily eroded, and heavy-duty anti-corrosion special coatings must be used for isolation.
- Based on Casting Type
Precision small parts: Strict dimensional tolerance requirements, no coating thickening or high-temperature deformation allowed; blackening and electrophoretic coating have thin films that do not affect precision.
Large castings: Large in size and weight, unsuitable for electrophoretic coating due to equipment limitations; painting and hot-dip galvanizing have better construction adaptability.
Based on Service Life Requirements
Short-term storage/transit: Only temporary isolation from air and moisture is needed, and simple spraying can meet temporary rust prevention needs.
Long-term outdoor use: Resistance to year-round wind, rain and salt spray is required, conventional paint films lack durability, and long-term protection relies on hot-dip galvanizing and heavy-duty anti-corrosion.
- Based on Budget and Functional Needs
Standard budget: Prioritize phosphating and conventional painting, simple processes with low cost, sufficient for standard working conditions.
Pursuit of maintenance-free long-term use: Select solutions with higher durable anti-corrosion performance to reduce later maintenance and replacement costs.
Rust Prevention Measures for Machining of Iron Castings
While anti-corrosion treatment of finished products is of great importance, immediate rust prevention during the machining process cannot be overlooked either.
Iron castings, especially ductile iron castings, develop rust stains within 5–10 minutes of contact with water. It is common that the machined surfaces get rusty even before the parts finish processing. For this reason, the following measures must be adopted during CNC machining:
1. Adopt cutting fluid specially formulated for cast iron, to avoid accelerated corrosion caused by ordinary cutting fluid.
2. Control the ambient humidity in the workshop to reduce condensation on workpiece surfaces.
3. Clean workpieces immediately after machining and apply rust preventive oil to form a temporary protective film.
4. Arrange the machining schedule reasonably to prevent workpieces from being exposed and stacked for a long time.
Only by strictly implementing rust control in the machining stage can the quality of parts be guaranteed before formal finished anti-corrosion treatment, laying a solid foundation for subsequent long-term protection processes such as painting and phosphating.
