This part of ISO 4404 specifies a procedure for the determination of the corrosion-inhibiting properties of non-aqueous hydraulic fluids within the category HFD, as classified in ISO 6743-4. It provides a qualitative assessment of corrosion of five of the most common metals used in the construction of hydraulic systems, but other metals and/or alloys could be added or substituted for these metals for particular installations. NOTE The determination of corrosion resistance of aqueous hydraulic fluids within categories HFA, HFB and HFC is described in ISO 4404-1.