To answer that question, one must understand the factors that cause rust.
The two conditions that cause cast iron to rust are:
- Moisture (Cast iron will begin to patina when the relative humidity exceeds about 64%)
- Oxygen
Factors that accelerate rusting:
- Carbon Dioxide
- Acids
- Impurities
Like most metals, cast iron goes through 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 shouldn’t be confused with corrosive rust. Patina is good for iron as it provides a protective layer. A metal patina is a thin layer of oxides on the metal 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 and intact for several decades. Unlike steel, cast iron is durable and will not flake. The patination of cast iron grates is predictable, and 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 and then fade to a chocolate brown, similar to the colour of manhole covers.
Preventing rust
Rust cannot be prevented but the process may be slowed by introducing a buffer material between the iron and atmosphere. Linseed oil can be applied to the grates prior to installation. Another common practice is to spray or powdercoat the grates. This coating must be reapplied whenever there is chipping to maintain rust protection.
