Picture: The same exhaust manifold before and after citric acid treatment (24h at 50 degrees). The right exhaust manifold is not polished but wiped with WD40.
Household citric acid is a good rust remover and it is environmentally friendly. The left picture shows a heavily corroded exhaust manifold . On the right is nice to see how the remaining chrome parts are rustproof.
Give two tablespoons of citric acid powder to a cup of water and keep the solution on a hot plate at about 50-60 degrees. The mixture removes rust from iron parts within 12-24 h. At room temperature the time needed increases to several days.
Attention: Brass and tin will be strongly attacked. Don't use it to clean carburettors.