Abstract: Process for conditioning metal surfaces to inhibit their corrosion and/or to inhibit scale deposition thereon by treating said surfaces:(A) prior to contact with a corrosive or scaling environment, with 2-hydroxy-phosphonoacetic acid or a metal salt thereof and optionally with a metal ion component (b) as hereinafter defined(B) during contact with an aqueous system capable of corroding said metal surfaces or depositing scale thereon, with a combination of:(a) 2-hydroxy-phosphonacetic acid or a water-soluble salt thereof; and(b) a metal ion which enhances, synergistically, the metal conditioning effected, individually, by 2-hydroxy-phosphonoacetic acid and the metal ion.