The effect of pH on the electrochemicalbehaviour of iron in hydrochloric acid

Abstract

The anodic and cathodic behaviour of pure Fe electrodes has been investigated in solutionsof constant ionic strength (CHl + NaCl) at pH values from 0 to 3 by means of the potentiokinetic polarization method (3V/h). The following parameters have been obtained: ∂ε corr/∂ pH = −51 mV; ∂ log i corr/∂pH = − 0·06; b − =120 mV ∂ log i −/∂ log a H − ) ε = 0·5; (∂ε/∂pH) i − = − 59 mV; (∂ε/∂ pH) i + = − 58 mV. As the ph value increases, ∂ log i +/∂ log a 0H −) ε changes from 0·83 to 1·32 and b + changes from 70 to 44 mV. The results, which confirm the importance of the surface treatment, are discussed and compared with those obtained in sulphuric solutions. The competitive adsoprtion of Cl − and OH − ions is evident and in solutions at pH > 2 the effect of the latter ion prevails. On the contrary, in solutions of pH < 2, a strong interaction between the Cl − ions and the Fe adatoms must be assumed. Nevertheless, it is not necessary to assume that these ions directly participate in the dissolution via a desorption process. The dissolution reaction may take place in the following way: Fe + OH −FeOH ads − FeOH ads − → FeOH+ + 2 e