The two conditions that cause cast iron to rust are:
Factors that accelerate rusting:
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 occuring process and shouldnt be confused with corrosive rust. Pantina is good for iron as it provides a protective layer. A metals 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 armor 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, 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.
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 powder coat the grates. This coating must be reapplied whenever there is chipping to maintain rust protection.
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