Nickel electrodes are used in different interesting technological applications, as well as example of not-inert electrodes. Their electrochemical behaviour depends on the production methods and particularly on their surface morphology. The polycrystalline nickel electrode has been the subject of extensive studies in acid aqueous media, where dissolution and passivation processes are observed. Those processes were postulated through reactive schemes which begin with two common electrochemical steps, where an intermediate specie of Ni(II) is produced and then it leads to two competitive reactions, to yield Ni²⁺ in solution or/and the formation of a non-conductive passive layer containing Ni(OH)₂ in different hydration degrees [1,2].

Depending on the pH-solution, the phenformin cation can be found in the neutral, monoprotonated or diprotonated form. The values of the pK_a determined by UV-spectroscopy are pK_a1= 2.7 and pK_a2 =10.7 [3]. The monoprotonated cation is a planar Π-electronic system with an electronic dislocation around the overall biguanide group, stabilized by the hydrogen bonding between the nitrogen atoms:

In this work, the effect of phenformin hydrochloride on the polycrystalline nickel electrode in aqueous sulphuric acid media was studied. The results indicate that phenformin plays an active role in the anodic dissolution of nickel electrode. The experimental results are explained from a kinetic model where the electrical elements measured by means electrochemical impedance are related with the kinetic constants of the electrochemical mechanism. This biguanide adsorbs on the nickel surface and also reacts with the corrosion products, being able to inhibit the nickel electro-dissolution, while at low concentrations of phenformin the Ni dissolution process is enhanced.

From the obtained results, it could be concluded that biguanides such as phenformin show interesting properties for different surface treatments as are electro-polishing and corrosion inhibition of nickel materials. Because its chelating action on Ni(II) ions, its overall positive charge, the located pair of electrons located on nitrogen atoms, its adsorption on the metallic surfaces, and pH effects, phenformin could be used with different promising objectives in technological processes.

The kinetic model proposed allows to be correlated the kinetic parameters with the electrical magnitudes of the equivalent circuit. Therefore the electrochemical impedance is useful in order of characterise the double layer and also how depends the rate of the steps of the electrochemical dissolution of nickel on the media composition. The impedance Z was fitted to the equation

Z = R_u + Z_T, where:

Table 1. Impedance parameters obtained by fitting the impedance experimental data at h = 25 mV

In 0.245 M K₂SO₄ + x M H₂SO₄ (pH=3.0) + (0.1-z) KCl + C_F M phenformin hydrochloride aqueous solution. T = 298 K. h = + 0.25 V. R_u is the uncompensated resistance. C_dl is the double layer capacity. R_ct is the charge transfer resistance. a₂ is the CPE exponent.

In spite of the observed inhibition of the nickel corrosion caused for the phenethyl biguanide cation across the formation of chelate compounds, the chloride anion plays an important role in the overall process due their aggressive character [4]. Another important factor in the overall process is the increase of the pH when increases the amount of phenformin, which enhances the formation of the passive layer.

REFERENCES

[1] S.G. Real, J.R. Vilche and A.J. Arvia, Corros. Sci., 20, 563 (1980).

[2] M.R. Barbosa, S.G. Real, J.R. Vilche and A.J. Arvia. J. Electrochem. Soc. 135, 1077 (1988).

[3] F. Vicente, J. Trijueque, F. Tomás, Química e Industría, 28, 307 (1982).

[4] J. Gregori, J. J. García-Jareño, D. Giménez-Romero, F. Vicente. J. Electrochem. Soc.: 153 B206-B212 (2006)..

ACKNOWLEDGEMENTS

Part of this work was supported by FEDER-CICyT CTQ2015-71794-R