1.The specimen is a gray iron cast
2. The oxide (rust) formation is due to annealing and maybe includes Fe2O3 and Fe3O4. etc.
Can anyone suggest what acid cleaning can be used to remove the rust?
Phosphoric acid will remove rust and create a rust resistant (relative) layer.
If you want to clean the iron surface, you can indeed use an acid but it is important to use a corrosion inhibitor, so that only the oxide is removed but the metal is never attacked. You can use HCl with hexamethylenetetramine previously dissolved inside. Doing this, the acid will attack strongly the oxide, but when the metal will be reached, the reaction will stop immediately. Note that other acid-inhibitor couples also exist, e.g. sulphuric acid with thiourea.
Hope it helps,
Alain
Due to the solubility of the metal oxide in Sulphuric acid (H2SO4) and Hydrochloric acid (HCl), either of the two acids can be used for the cleaning. However, the material must be protected with an anti-rust solution after the treatment with thorough rinsing.
If you use any acidified oxalate solution, (20% oxalate in 0.1M sulphuric acid) and leve it for over night, then the colour will be removed. Try this.
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