INVESTIGATION OF INHIBITION EFFECTS OF 1- HYDROXY-ETHANE-1,1-DIPHOSPHONIC ACID BY RADIOTRACER TECHNIQUE, I - A STUDY OF SORPTION PROCESSES ON POLYCRYSTALLINE GOLD

The aim of the present work is to investigate the time, potential, con centration and pH dependences of the adsorption of 1-hydroxy-ethane-1, 1-diphosphonic acid (HEDP) on polycrystalline gold electrode by in-sit u radiotracer 'foil' method and cyclic voltammetry. The experimental r esults reveal several important findings on the sorption and redox beh aviours of the HEDP molecule, which may contribute to the better under standing of its inhibition mechanism on alloys of industrial importanc e. The following conclusions can be drawn from the experimental data: (a) The adsorption of HEDP on polycrystalline gold is dependent on ele ctrode potential, solution concentration and pH. (b) The surface exces s value depends on the starting point and actual value of the electrod e potential, and the direction of polarization. Reversible accumulatio n processes have been observed on the oxide-free surface of gold elect rode in the potential range of 0.05<E<1.00 V. Formation and irreversib le adsorption of oxidation products of HEDP can be found at E>1.00 V, which cause a hysteresis between the Gamma-E curves for polarization i n anodic and cathodic direction. The HEDP accumulation reaches the qua si-stationary condition in less than 1 min. (c) The adsorption of HEDP on polycrystalline gold surface increases by increasing the solution concentration. The coverage of adsorbed species is less than one monol ayer at c less than or equal to 10(-3) mol . dm(-3). (d) The pH depend ent adsorption of HEDP reveals that the relative adsorbability of the HEDP molecule is predominantly influenced by its dissociation degree. The surface excess value is higher, at pH=9, than in neutral or acidic solution. It is most probable that HA(3-) and A(4-) species, formed f rom HEDP in solution under equilibrium circumstances, have the highest relative sorption strength. The effect of pH on the adsorption of spe cies can be observed in the anodic and catodic polarization directions as well.