Will cast iron drainage grates rust?

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17/04/2024
There are two conditions that cause cast iron to rust are:
  • Moisture (cast iron will begin to patina when the relative humanity exceeds about 64%)
  • Oxygen
Factors that accelerate rusting:
  • Carbon Dioxide
  • Acids
  • Impurities
Like most metals, cast iron undergoes a natural oxidation process, resulting in an outer protective coating known as rust. This process is called 'patina'.
Patina is a naturally occurring process and should not be confused with corrosive rust. Patina is good for iron as it provides a protective layer, as it is a thin layer of oxides on the surface that acts to slow down further corrosion. Therefore patina on cast iron grates does not harm the structural integrity. This 'rust' layer shields the cast iron from further oxidation. Think of rust as a corroded armour that protects against additional corrosion. This property allows iron to remain strong & intact for several decades. Unlike steel, cast iron is durable and will not flake. The patination of cast iron grates is predictable & the duration of each stage depends on local moisture conditions, foot traffic, foreign substances etc.
Once the oxidation process begins, cast iron will turn a bright orange then fade to a chocolate brown, similar to the colour of manhole covers.
Austinlane 13
SABdrain 707 Cast Iron grates installed at Austin Lane, Darwin in 2020; photographed in 2022
Gledswoodhills 29
SABdrain 704 Cast Iron grates installed at Gledswood Country Club in 2016,

Preventing Rust

Patina cannot be prevented but the process may be slowed by applying various coatings or treatments to create a barrier between the metal and environmental factors.
  • Paint
    Applying a high quality rust resistant paint specificaly formulated for surfaces. Make sure to clean and prime the surface before painting for better adhesion & durability.
  • Clear Coatings
    Clear coatings such as polyurethane or acrylic sealants can be applied to the surface to protect it from moisture and oxygen
  • Oil or Wax
    Applying a thin layer of oil or wax can create a barrier against moisture & oxygen, inhibiting oxidation. This requires regular reapplication to maintain effectiveness
  • Galvanization
    Galvanizing involves coating the iron with a layer of zinc, which provides excellent protection against corrosion. This process is commonly used in industrial applications but may not be practical for smaller scale projects due to cost & complexity.
  • Powder Coating
    Powder coating applies a dry powder to the surface and cures it with heat. This creates a durable & protective finish that can prevent patina formation.
  • Linseed Oil
    The application of linseed oil can be effective to some extent. It acts as a barrier against moisture & oxygen, thus slowing down the oxidation process.
All advice in this article is of general nature, seek professional advice for tailored guidance