The effect of a new class of corrosion inhibitors, namely piperidin-1-yl-phosphonic acid (PPA) and (4-phosphono-piperazin-1-yl) phosphonic acid (PPPA) on the corrosion of iron in NaCl medium has been investigated by electrochemical measurements. Potentiodynamic polarization studies clearly reveal the type of the inhibitor. The addition of increasing concentrations of phosphonic acids causes a shift of the pitting potential (E-pit) in the positive direction, indicating the inhibitive effect of the added phosphonic acid on the pitting attack. The potential of corrosion is moved towards negative values and the corrosion current is reduced. The values of the current are lower in the presence of PPA and PPPA. This is explained by the fact that most of the surface of the electrode is covered by the molecules adsorbed. PPPA has a strongly inhibitive effect on chloride pitting corrosion, it seems that the addition of the NCH2PO3H group (center adsorption) in the PPA para-position, giving PPPA, reinforces the active sites of this molecule and consequently increases its inhibition efficiency. (C) 2003 Elsevier B.V. All rights reserved.