The adsorption of the cationic forms of inhibitor would be limited by the concentration of the anions on the metal surface. The hexa methylene diamine tetra methyl-phosphonic acid molecules may also be adsorbed via donor–acceptor interactions between the unshared electrons pairs of the heteroatoms (P, N, O) to form a bond with the va- cant d-orbitals of the iron atom on the metal surface, which act as a Lewis acid, leading to the formation of a protective chem- isorbed film