Control valve & on off valve ,solenoid-valve manufacturer

Search
Close this search box.

How To Increase Cast Iron Valves’ Resistance to Corrosion?

How To Increase Cast Iron Valves' Resistance to Corrosion

The standards for valve corrosion resistance in operational environments have gotten progressively more stringent in recent years. For instance, valves used in sewage systems, power plants using seawater cooling, and chemical systems have all shown some improvement in corrosion resistance. Therefore, it is essential to research the corrosion resistance of typical valve materials, particularly for seawater valves with a large diameter above DN1000. Increase. The corrosion resistance of frequently used cast iron materials for large-diameter valves is covered in this article.

Cast iron corrodes chemically or electrochemically most frequently, and the corrosion state might be homogenous, localized, stressed, or intergranular. Due to the hazard of localized, stressed, and intergranular corrosion, engineering demands that the corrosion be uniform. Cast iron corrosion is influenced by various parameters, including chemical makeup, metallographic structure, surface properties, other factors, and the corrosive medium’s makeup, activity, and temperature. Cast iron corrodes quite gradually in the environment.

As time passes, a protective film will eventually form on the casting’s surface, changing the alloy’s state from active to inert and considerably slowing the pace at which cast iron corrodes in the atmosphere. Due to the precipitation of special carbides and other compounds along the grain boundaries in alloy cast iron with obvious grain boundaries, the alloying elements present in the solid solution are diminished. As a result, intergranular corrosion quickly occurs along the grain boundaries, leading to holes or decreased strength of the cast iron. Pure iron, ferrite, and austenitic steels have excellent corrosion-resistant stability when other conditions are the same because the ideal structure, according to the theory of electrochemical corrosion, should be a single-phase and uniform metallographic structure. To improve corrosion resistance, however, only the homogenization of the metallographic structure may be considered since it is impossible to obtain cast iron with a single metallographic structure in advance.

In general weak media, nurture cast iron (gray cast iron after alloying) offers stronger corrosion resistance than regular cast iron, according to specific information. The stability of inoculated cast iron in sulfuric acid at average temperature is equivalent to that of low alloy cast iron. Cast iron with low nickel alloy has a 1.5 times higher stability in acetic acid (0.5%–5.0%). Therefore, it can serve as a partial replacement for low-alloy cast iron.

Corrosion amount of material under the same test conditions

How to increase cast iron's resistance to corrosion?

The preferable option is to select nurtured cast iron (Table 1) and the proper alloying to address it to increase cast iron’s capacity to withstand seawater corrosion. For instance, the metallographic structure of HT250Ni2 (Table 2) shows pearlite > 98%, fine flake, 75%–90% flake graphite, length 30–120m, binary phosphorus eutectic 1%, single pearlite, no carbide precipitation according to the rules of GB 9439, N grade gray cast iron has a tensile strength of N — (n), effectively preventing grain boundary corrosion. Additionally, not only is the matrix structure consistently polished following molten iron’s nurturing treatment, but the internal stress is markedly decreased, which can prevent stress corrosion.

Compostion of nurture cast iron

It must be emphasized that depending on the amount of nickel and chromium added, nickel’s graphitization coefficient at the eutectic temperature is + 0.4. Chromium’s graphitization coefficient is – 1.2, so to avoid carbide precipitation at the grain boundary, Ni: Cr> 3:1, There should also be a margin to consider Mn’s potential to form carbides. The practice has shown that gray cast iron with 0.5% Cr will generate many carbides, making cutting challenging. Therefore, the more Cr content, the worse.

There are other several ways to increase the corrosion resistance of cast iron valves, including:

  1. Material selection: Choosing a cast iron material specifically designed for improved corrosion resistance can significantly enhance the performance of the valves. For example, using materials such as ductile iron or alloyed cast iron with higher levels of chromium, nickel, or other corrosion-resistant elements can help prevent corrosion.
  2. Protective coatings: Applying protective coatings on the surface of the cast iron valves can provide an additional layer of protection against corrosion. Coatings such as epoxy or enamel can be applied to the valves to create a barrier that prevents direct contact between the metal surface and corrosive substances.
  3. Cathodic protection: A cathodic protection method uses a metal surface as the cathode of an electrochemical cell to shield it from corrosion. This can be achieved by attaching sacrificial anodes or employing impressed current systems to the valves. These helps prevent corrosion by transferring the corrosion potential to the sacrificial anodes or supplying a protective current to the valves.
  4. Proper installation: Ensuring cast iron valves are installed correctly can also help improve their corrosion resistance. Proper installation techniques, such as avoiding galvanic coupling between dissimilar metals, ensuring adequate sealing, and protecting valves from exposure to corrosive environments, can help prevent corrosion.
  5. Regular maintenance: Regular inspection, cleaning, and maintenance of cast iron valves can help identify and address potential corrosion issues before they become severe. This can include cleaning valves regularly, repairing any damaged coatings or protective layers, and monitoring the valves for signs of corrosion or other damage.
  6. Environmental controls:Managing the environment where cast iron valves are used can also improve their corrosion resistance. For example, controlling the pH, temperature, and chemical composition of the fluid or gas that flows through the valves can help reduce the risk of corrosion.

It’s essential to consult with a qualified engineer or corrosion specialist to determine the most effective approach for improving the corrosion resistance of cast iron valves based on the specific operating conditions and requirements of the valves.

Conclusion

 As a result, there is no carbide precipitation at the grain boundary, the casting can only have minor internal stress, and the surface and interior of the casting can be reduced. Consistent organization can effectively prevent local, intergranular, and stress corrosion. Seawater and reclaimed water production used in the valve thickness of the average corrosion rate of ≤ 0.5mm / a, thereby extending the service life of the valve.

Leave a Reply

Your email address will not be published. Required fields are marked *

Featured Products

Professional Industrial Valve Manufacturer

Industrial Valve

CONTACT US Professional Industrial Valve Manufacturer Get in Touch with Us Facebook Youtube Whatsapp Linkedin BCST -Your Expert

Read More »