KINETICS OF IRON PASSIVATION AND CORROSION IN MOLTEN ALKALI NITRATES*

-Theelectrochemical behaviour of iron anodes in molten alkali nitrates has been investigated at temperatures ranging from ca 240-320°C. The E/l curves exhibit a potential region where the electrode passivates. Two different oxidized states of the passive iron surface exist. Within definite potential regions, the oxidation of nitrite and iron occurs, the Iatter being characterized by a Tafel slope of ZRT/F_ At high positive potentials the breakdown of passivity is observed. Results are interpreted in terms of the stability of possible iron oxides during the electrode reaction. The kinetics of nitrite and nitrate ion oxidation on the passive surface is discussed on the basis of previous work. A comparison between iron passivation and corrosion in molten nitrites and nitrates is made.

X-ray diffractometry was applied to study the metal surface oxides and the insoluble oxide left in the melt.The latter was also chemically evaluated.

Oxide-film formation
The formation of oxide films on the iron electrode was evidenced by X-ray analysis.When the metal is merely immersed in the molten nitrate eutectic, the X-ray diffractograms exhibit the peaks of a-iron and that of an iron oxide having the structure of magnetite with a little of Fe0. 13 When the sample is anodized at O-025 V higher than the potential obtained on immersion, the X-ray ditfractograms are nearly the same as before, but the intensity ratio of the a-Fe/FqO,(FeO) peaks diminishes.Finally, when the sample is anodized at a potential 1.3 V higher than the initial potential, the peaks corresponding to a-Fe and Fe,O, are observed.

Anodic formation of Fe,O,
Various reactions take place at the iron anode in the molten alkali nitrates.At low anode potentials the electrode acquires a passive state.On increasing the potential further, the anodic oxidation of nitrite, usually present in the meltin traces, takes place, the main reaction yielding NO2.At higher anode potentials the formation of nitrogen oxides and the simultaneous oxidation of iron to Fe20s.

Various other chemical or electrochemical side reactions yield also N,O among the reaction products.
The composition of the solid formed at high anodic overvoltages and left as a suspension in the melt was 942 per cent Fe,O,.
The remaining 5-8 per cent corresponded to water added to the oxide when it was removed from the melt.X-ray analysis confirmed the structure of y- Fe,O,.H,O for the solid separated from the melt.

Current/potential
curves The potentiostatic current/potential curves, as shown in the IY/log i plots defined in terms of the initial geometrical area of the iron sample, exhibit a complex shape (Figs.14).
For a description of the initial II/log i curve obtained with a fresh iron electrode, let us consider a curve recorded by changing the potential upward, at 258°C.When the anode potential is just a few mV above the initial potential obtained when the iron sample is immersed in the melt, a sudden increase of current is noticed, and a maximum current density, i,,,, is reached.This is between 10 and 40 pA/cm2, depending on the temperature.Afterwards, a further increase of the anode potential provokes a sharp decrease of current and the onset of a definite passivity.For an approximate O-1 V range of anodic potential there is a passivity current, z&,~, about ten times smaller than the maximum current.On increasing the electrode potential

Semilogarithmic plot of a potentiostatic anodic E/i curve. 271°C. Partially dehydrated melt.
further, a slow increase of current is observed before the occurrence of the anodic oxidation of nitrite ion.The E/log i curve in this region is complex in shape and not very reproducible.Beyond a AE of about O-5 V, nitrite-ion oxidation takes place and a net limiting current is established.The latter is directly proportional to the amount of nitrite present in the melt.This was confirmed by adding known amounts of nitrite to the nitrate eutectic.In this region NO, is evolved.
Beyond the limiting current region, the E/log i curve satisfies a good Tafel line for about one and a half logarithmic decades.Finally when the anode potential reaches 1.15 V, a sudden breakdown of passivity is observed and, at constant potential, the current attains a large value.The features described for the E/log i characteristics are of the same shape at all temperatures investigated.
If the E/log i curves are recorded at decreasing potential a different situation is met.
Thus, the upper Tafel region is reasonably well reproduced as well as the limiting current region, but the remaining part of the curve is no longer observed as the potential reaches a rather stable value at about 0-450-0~500 V while the current takes a constant value of the order of lo--' A/cm2.Under these circumstances if a second E/log i curve is obtained, it starts from the potential located at 0~450-0*500 V and its shape reproduces that just reported for the returning curve (Fig. 5).The only kinetic parameters that actually do depend on temperature are the current (iJEP and the limiting current, iL, for nitriteion oxidation.The Arrhenius plot for (i,JEp yields an experimental activation energy equal to 27 f 5 kcal/mole (Fig. 6).The temperature dependence of iL coincides with already reported figures for the same process.14

Anode-potential decay
The anode-potential decays are depicted as overvoltage/log(time) plots in Fig. 7, the overvoltage being referred to the passivation potential.
The plots are rather complex as they should correspond to the E/log i curve previously described.However, in the range of potential between 085 and 1.20 V, they exhibit the shape predicted for a rather simple electrode process.This potential range coincides precisely with the Tafel line region already represented in the E/log i curves.Therefore, only from that portion of the overvoltage/log(time) plot can reliable kinetic parameters be in principle derived.The decay slope, bd, and the apparent electrode differential capacitance, C,, evaluated from the t' times, defined by the decay slope at the potential of current interruption, are assembled in Table 2.The decay slope is close to 2*3(2RT/F) V, a figure which agrees with that derived from the E/log i plot.The apparent electrode differential capacitances seem to attain a constant value at high anodic overvoltages.Unfortunately, due to the complexity of the electrode reaction at lower overvoltages, no further conclusions can be safely drawn from the decay slopes.

Triangular wave voltammetry
Voltammetric E/I curves were recorded after repetitive cycles at potential sweeps from 0.157-7-80 V[s.At lower sweep rates, covering the range of anode potential from O-l.0 V, the E/I display shows one anodic current peak and one cathodic current peak separated by O-2 V, as shown in Fig. 8a.The former corresponds to the potential region preceding net Fe,O, formation, and the latter is related to reduction of products, probably NOz, yielded during the anodic cycle.Figure 8a  seems to be related to the destruction of the oxide film, while the anodic current peak corresponds to the reformation of the passive film.If the first is related to the passivity potential, the anodic current peak appearing at higher anodic potentials should be assigned to the initiation of Fez03 formation or nitrate-ion discharge.At low sweep rates, the EjIcurves for a passive electrode exhibit the already mentioned features, and on expanding the scale adequately, the Ej1curve presents a shoulder at the beginning of the anodic peak assigned to nitrate ion (Fig. SC).That shoulder belongs to nitrite-ion discharge, which exists as an impurity in the molten nitrate.Its potential, at the lower sweep rates, is close to the potential of the cathodic current maximum mentioned above.The identification of the nitrite peak was established by adding nitrite to the nitrate eutectic and observing the corresponding current-peak increase.The separation of the peaks related to nitrite-ion discharge, and NO, reduction is achieved at the highest sweep rate, as shown in Fig. 86.If the nitrite electrode under these circumstances behaves irreversibly, the predicted shift of the anodic and cathodic current maxima at the sweep rates indicated in Fig. 8c and Fig. 8d, should be close to O-3 V, as indeed found.
At higher anode potentials the oxide film approaches the Fe,O, structure, as also shown by X-ray diffractograms.
No definite potential region can be assigned to the composition change of the film which apparently occurs at potentials in between the passivity potential and the breakdown potential.The rest potential, after electrolysing the melt at high anodic potentials, eventually approaches (after a long time) the value corresponding to equilibrium VII.Any further continuous use of the electrode as anode involves as starting surface a predominantly Fe,O,/iron electrode.A further increase of anode potential produces FezO, formation.The kinetics of this reaction is not possible at present to be interpreted in terms of a reaction pathway.It comprises a Tafel slope equal to 2RT/F which suggests that an initial one electron transfer step is rate determining.
NOz and NO participate in the oxidation of the base metal to ferric oxide, the former being partially reduced to NsO, as indicated by the composition of the gaseous products.The existence of NO as a stable product is prevented if oxygen atoms are formed as reaction intermediates.

The breakdown of passivity
When the anode potential exceeds 1.3 V with respect to the passivation potential, passivation abruptly disappears.
The potential for the occurrence of this process is close to the thermodynamic potential of the oxygen/oxide-ion electrode.The latter in molten systems containing molten oxyanions and in oxide melts at high temperature14 constitutes a reversible electrode represented by reaction (XI), which has been the subject of previous research.l*raAt this high anode potential, the source for oxide ions may well be located either in the ferric oxide layer or in the melt, where they are in equilibrium with nitrate and nitronium ions, as indicated by reaction (I).If the oxide ion discharge takes place in the film, the ferric ion, which is thermodynamically unstable in the molten nitrate, decomposes into FezO, and nitrogen oxides.Consequently the oxide layer breaks down and massive ferric oxide formation results.Iron dissolution is enormously accelerated as the passive oxide film no longer protects the metal surface.Therefore, if reaction (XI) is a fast process, the rate of the over-all reaction appears as potentialindependent, the kinetics probably being governed by a chemical reaction.

Comparison of iron electrode behaviour in molten nitrates and nitrites
Iron electrodes in both melts definitely acquire a passive state and any electrochemical oxidation occurs on the passive iron electrodes.However, some differences are established for the behaviour of iron in the two melts.Thus, although initially the passivation process predominantly involves an iron oxide of the magnetite structure for both nitrite and nitrate melts, only for the latter does the oxidation to ferric oxide proceed.
This difference is also reflected in the composition of gaseous products yielded by both anodic processes.In the nitrite melt NO and NO2 are the main gaseous products, whereas for the nitrate melt NO, and NsO appear as the predominant products.
Another clear difference exists at high anode potentials where iron dissolution in the nitrate eutectic becomes practically potential-independent.This region has not been observed for the nitrite melt, perhaps because the region corresponding to the oxide/oxygen electrode is not reached.
INTRODUCTION KNOWLEDGE of the kinetic behaviour of metals in melts, particularly for iron is rather limited.1 Recent work carried out in this laboratory referred to alkali bisulphates*n3 and alkali nitrites,4 and as part of a systematic investigation, iron-electrode reactions in fused nitrate eutectic are reported.Reactions of fifteen metals, including iron, in the fused eutectic LiNO,-KNO, were studied to establish the main reaction products yielded by decomposition of the melt.sThe behaviour of other metal electrodes, such as copper and silver in nitrate melts, has also been studiedaeg and some results regarding the behaviour of iron in fused alkali nitrates have been obtained by potentiodynamic polarization technique.lOHowever, the kinetic and mechanisms of the different processes invoIved were not established, and the present research attempts to establish the possible course of the different processes.The calculation of the potential/p02-diagram for iron in molten nitrate recently made lL facilitates interpretation of the reactions involved.Furthermore the possibility of iron passivation in molten alkali nitrates is of interest, because of the low solubility of metal oxides therein.* Manuscript received 15 September 1970.3 33 EXPERIMENTAL TECHNIQUE The cell arrangement, the quality of the iron electrodes and their preparation have been described in previous papers.L* Sodium nitrate.(45.6 %)-potassium nitrate (54-4 %) melts (Carlo Erba) were used as electrolytes under two different conditions : (i) dried at room temperature and (ii) thoroughly dehydrated in the molten state under vacuum for 24-48 h.No particular saturation of the melt was made except that produced by gases generated during the anodic reactions.The electrochemical processes were studied at temperatures between 240 and 320°C by applying galvanostatic and potentiostatic techniques both under steady and non-steady conditions, covering a potential range of 1.5 V. Triangular wave voltammetry was employed following procedures already described in the literature.12Gaseous products were analysed by ir spectrometry.

FIG. 6 .
FIG. 6. Arrhenius plot for the current density extrapolated at Ep.

The
anodic discharge of NO, and Fe,O, formation Once the iron electrode is passivated, nitrite-ion oxidation first takes place, according to the over-all reaction NO, -+ NO, + e. (XII) The formation of NOz may also contribute to the oxidation of the passivated film to Fe,O,.The discharge of nitrite ion on the passivated iron surface is under these circumstances mainly a convective-diffusion controlled process, the steady E/i curves being characterized by a net limiting current and the voltammetric E/I curves by two current peaks that occur at nearly the same potential if low sweep rates of potential are employed.Gaseous NO, in the presence of nitrite ions undergoes the reaction NO,