If you discovered that your iron chimenea is rusting on you, you might be able to rescue it.
How Bad is the Rust?
A cast iron chimenea can look very rusty, but on close inspection the rust may be superficial. It may only be a surface infection. This sort of rust can be cleaned away with a wire brush. Wear a face mask and scrub the whole surface. Finish off with wire wool pads and water with a little detergent in it. If it is a garden chimenea and you are cleaning it in the garden, the dust will do no harm to plants.
More Serious Rust
If the rust has eaten into the metal and the wire brush will not remove it, you will need a rust remover. Naval jelly is often recommended. Apply the naval jelly to the affected part of the chimenea using an old brush. Allow it to set for five to ten minutes before washing it off with water.
You should be left with a clean metal surface. If not, repeat the treatment as needed. When using naval jelly wear rubber gloves.
Once the chimenea is rust free, check that it is structurally sound. Protect the refurbished chimenea with heat proof paint or lamp black.