This document describes the magnetic flux leakage (MFL) testing method and evaluation of results for in-use equipment made of ferromagnetic materials. It is mainly used for the detection of volume defects such as corrosion and mechanical damage on the inner and outer surfaces of plate and tubular structures. A certain thickness of coating is allowed on the detection surface. This document applies to external MFL testing of in-use ferromagnetic seamless steel pipes and base metal of pressure vessel shells with an outer diameter of not less than 38mm and a wall thickness of not more than 20mm. It is also applicable to MFL testing of base metal of pressure vessels and storage tank bottom plates from the inside.