Effect of Perchlorate Ions Concentration on Passive State of Iron

The iron in a borate buffer solution is passive. [1-3]. Whereas the passivation film is of a phase nature and includes metal oxides and hydroxides. The chemisorbed borates on the surface of the oxidized iron increase its protective properties and the presence of the Cl¯-ions in the liquid phase disturb the stability of the passive state of the metal [2,4]. An analogous, but less pronounced effect is observed under the influence of expressed ClO₄¯-ions [2]. However, the nature of the latter is not fully understood. In this relation, in this paper we will attempt to establish general and specific mechanisms of the aforementioned process in the perchlorate media with borates.

For an integrated solution of this task a combination of physical and chemical methods was used (electrochemical, microscopic (MIM-7 microscope, JEOL JSM-6510LV scanning electron microscope (SEM)), energy dispersion analysis (EDA)). The experiments were performed with a stationary iron electrode (0.013% С) in a classic three electrode cell in a borate buffer solution (рН = 8.4) at 20±2 °С with free air access. The concentration of the activating additive in the form of NaClO₄ varied in the range of C=(1.0 – 5.0)×10^-2 M, potential scan rate  - v_p= 10 mV/sec.

The voltammograms obtained (Fig.1) and combined with the SEM/ EDA research results showed that the composition of the passivating layer on the iron did not go through changes dependant on the activating additive concentration. 

Nevertheless, in the 0.300 to 1.000V potential range, the system becomes meta-stable (the element of chloride and increased content of the element of iron detected on the oxidized surface of the metal confirm the surface activation, which is also registered by the optical microscopy).

At the same time, persistent pitting formation, probably, takes place at the potentials higher than 1.000 V. As for the spotted instability of the iron/ electrolyte system in the potential range given above it can be related to two other causes conditioned by both: the transformation (cracks) of the passivation layer as well as the increasing absorption of ClO^-₄ – ions which increases their degree of participation in the anodic process insignificantly. In the potential region of peak A3 (Table 3), ClO₄¯-ions go through electrochemical oxidation releasing Cl¯- ions (Е(ClO₄¯/Cl¯) = 0,56 V), which are stronger activators. Meanwhile, the concentration at which an intensive pitting is initiated is reached.

Table 1. Potentials of anodic peaks of the iron voltammogram in the borate perchlorate media with varying concentrations of the activators.

 

С(ClO4¯), М	Е(А1), В	Е(А2), В	Е(А3), В
0,00	-0,412	0,111	0,644
0,01	-0,401	0,116	0,664
0,02	-0,399	0,134	0,657
0,03	0,395	0,128	0,612
0,04	-0,404	0,130	0,614
0,05	-0,388	0,125	0,669

Thus, systematization and analysis of the results of the described researches show that the activating effect of the ClO₄¯- ions on the passive iron in a borate buffer solution is of a complicated mechanism within which an additional electrochemical generation of Cl¯-ions is not a dominant stage.  

1. References
2. 1. S.A. Kaluzhina, N.G. Nafikova Passive State of Iron in Weak-Basic Borate Solution / EUROCORR 2013 for a blue sky: European corrosion congress: book of  abstract, Estoril (1-5 September 2013).
3. 2. А.M. Sukhotin, Physical Chemistry of Passivation Layer on Iron, 320 p., Chemistry, L. (1989)
4. 3. Yu.I. Kuznetzov, M.E. Garmanov, Electrochemistry, 23, 381 (1987).
5. 4. R.W. Revi, H.H. Ulig, Corrosion and Corrosion Control. Introduction to Corrosion Science and Engineering, 479 p., Published by John Wiley & Sons, Inc., Hoboken New Jersey (2008).

Fig. 1. Anodic areas of the iron voltammograms in 0,2M H₃BO₃ + 0,05 М Na₂B₄O₇ + х М NaClО₄ (рН = 8.4) (х: 1 – 0×10^-2; 2 – 1×10^-2; 3 – 2×10^-2; 4 – 3×10^-2; 5 – 4×10^-2; 6 –5×10^-2).