How can I remove the coating from black iron pipe?
It is "mill varnish" , nothing to do with mill scale. I find no definition in the  ASTM index. Very likely it is a varnish ; A resin plus a solvent/vehicle. Mineral spirits or gasoline should take it off. Different mills may use different products as there is no firm definition. It is applied to new , relatively scale free pipe. A industrial purchaser does not want to see his brand new pipe delivery covered with rust. Oil companies sometime order pipe with no mill varnish as it is a nuisance to remove. I have seen it hard like paint , I have seen is still gooey. Mill scale is thick black iron oxide scale; Fe O and Fe3 O4. The mill removes scale with concentrated sulfuric acid. I have seen some new steel products with scale allowed to weather , rust ( complex hydrated Fe3 O4 - brown) will loosed the mill scale so blast cleaning is easier