ANODIC-DISSOLUTION OF ARMCO IRON IN 0.5 M H2SO4 IN THE PRESENCE OF ADSORBED CHLORIDE-IONS

Anodic dissolution of Armco iron was studied in 0.5 M H2SO4 solution w ith and without the addition of NaCl (concentration range 3 x 10(-2)-1 .5 M in Cl-). Comparison with the pure electrolyte showed that Cl- ion s at the concentrations tested act as an anodic inhibitor by a blockin g mechanism, When Cl- ions are present in concentrations higher than c a. 0.1 M they reverse their action with the reaction order with respec t to Cl- ions, n(Cl-) approximate to 1. The observed anodic Tafel slop e of ca. 120 mV dec(-1) in the presence of chloride ions was explained using the iron dissolution mechanism suggested by Bockris, Drazic and Despic (or BDD mechanism). The accelerating action of Cl- ions at hig her concentrations appears to be a consequence of a parallel process i n which Cl- ions from the bulk (i.e., not adsorbed) form a complex bef ore the slow step. Adsorbed FeOHads appears as a common intermediate f or both parallel routes, The kinetics of adsorption of Cl- on Armco ir on electrode was analyzed by different types of adsorption isotherms.