Steel reinforcing cables in concrete are protected against corrosion by injecting a carrier fluid and corrosion inhibitors into interstitial spaces between the wires of the cable at a first location along the cable and causing the fluid to pass through the interstitial spaces between the wires of the cable to a second location along the cable. The cable comprises an array of wires confined together and intimately surrounded by a covering material which is engaged with a periphery of the cable so that there are insufficient interconnected spaces between the cable and the covering material to allow passage of fluid longitudinally along the cable outside the cable itself. The method can be used with pre-stressed concrete, with post-tensioned bonded cables and with extruded un-bonded mono-strand cables.