Anodic behaviour of aluminium and its alloys in sodium chlorate solutions

The e ect of chlorate ions on the anodic dissolution of aluminium and its alloys with magnesium under conditions of high current densities and intensive electrolyte mixing is investigated using the method of anodic polarisation curve removal on a rotating disk electrode. It is shown that at relatively low anodic potential values the process is limited by the capacity of the electrochemical reaction, but with a further potential shi and a venting of the reaction products from the surface of the anode. The e ective smoothing of the surface microrelief of aluminium alloys in solutions of chlorates is due to the periodic formation and destruction on the treated surface of a speci c oxide lm and inhibition due to the electrochemical heterogeneity of the structural components of the alloys.


Introduction
In solving problems associated with the electrochemical machining of aluminium alloys, an important role is played by an understanding of the laws of their anodic dissolution in an intensive electrolyte fl ow.Th erefore, knowledge of these laws will al-low the optimal composition of the electrolyte to be chosen for the best mode for electrochemical machining of aluminium alloys, providing high accuracy and surface quality.

Research Methodology
In this paper, the study of the behaviour of aluminium alloy anodes used a rotating disc electrode to simulate the actual hydrodynamic conditions of the electrochemical process and provide an intense fl ow of electrolyte at the electrode surface [1].
In order to eliminate the resistive component in the measurement of the electrode potential, a rotating electrode design was created at the Chemical-Technological Institute of the Ural Federal University for the studied vertical movement relative to the very thin tip of the salt bridge of the reference electrode.Th e capillary of the reference electrode, having an external diameter of 0.1 mm, was led into the centre of the disk by a distance not exceeding 0.07 mm.In order to control the value of the resistive component in the values of the electrode potential, polarisation curves were taken at consecutive removing of the capillary from the surface of the electrode and then extrapolated to zero distance.
Moulded PTFE cylindrical rods with a diameter of 2 mm, which had been degreased with ethanol directly prior to the experiment, were used for removing the polarization curves.
In order to reduce dissolution of the electrode and thereby increase the distance between the working electrode and the reference electrode capillary, anodic polarisation curves were recorded in potentiodynamic mode with linear scanning potential of 4B/min in a temperature controlled three-electrode glass cell with the divided anode and cathode spaces in solutions NaClO 3 at 25 °C.A silver chloride electrode in a saturated solution of potassium chloride served as a reference electrode.For the auxiliary electrode, a platinum element was used.
Technological research was carried out on a pilot plant for electrochemical hole cutting in metals.In the experiments, the current density was varied between 50 and 250 A/cm 2 with an electrode gap of 0.05 to 0.1 mm, electrolyte fl ow rate from 10 to 40 m/sec and temperature between 20 and 60 °C.
Highly pure grade A995 aluminium and aluminium alloys AMg1, AMg3, AMg6, having varying magnesium content, formed the objects of study.

Experimental part
In sodium chlorate solutions, the anodic behaviour of aluminium and its alloys, like sodium chloride solutions, is characterised by areas of active dissolution and passivation.
Polarisation curves obtained for aluminium alloys in sodium chlorate solutions show a sharp increase in the anodic current density following activation of the anode potential.
However, when it reaches a certain density, the current reaches a limit in the speed of the anode process of dissolution of aluminium alloys -the current density falls with anode off set potential into the positive area and no further increases take place (Fig. 1).
A slight difference in the chemical composition of alloys is considered here in terms of the main alloying components (magnesium) having no signifi cant impact on their anodic behaviour; this is determined by the base alloy component, i. e. aluminium.
Along with the concentration of the electrolyte, the infl uence of the electrolyte temperature and speed of rotation of the disc electrode on the intensity of anodic dissolution of aluminium alloy (Fig. 2) were also studied.
In the study of the anodic behaviour of the investigated aluminium alloys depending on the temperature of the solution, the rotating speed of the disk electrode showed that the maximum current density on the anodic polarisation curves increases with a decreasing concentration of sodium chlorate solution from 5M to 3M (Fig. 1), as well as with an increase in electrolyte temperature (Fig. 2) and with an increase in the rotational speed of the disk electrode (Fig. 2).
It can be assumed that in sodium chlorate solutions, as in chloride solutions, the passivation of aluminium and aluminium alloys of AMg6 type occurs due to the formation of a salt fi lm on the surface of the electrode.

Results and Discussion
Processing of the experimental data using the temperature-kinetic method showed that the value of the eff ective activation en-ergy of the dissolution process of the alloy in AMg6 NaClO 3 solutions decreases from 7.05 to 7 > 2.4 kcal / mol with increasing  Th erefore, at relatively low anode potential values, the alloy dissolution process is limited by the electrochemical reaction, and, when a certain potential is reached -withdrawal of the reaction products from the surface of the anode [2].Th e presence of diff usion limitations at the time of active dissolution of the alloy also shows the maximum dependence of the current density on the polarisation curve on the speed of the rotation disc electrode (Fig. 2).
Th is may also indicate the fact that the maximum current value before the beginning of passivation increases almost linearly with an increase in temperature of the electrolyte owing to increased solubility of the salt fi lm.
With a further shift of the potential of the aluminium electrode to a positive region in sodium chlorate solutions, no new sharp increase in current density was observed after reaching the maximum current density, as was the case in the solutions of sodium chloride.Th is behaviour of the aluminium anode in solutions of NaClO 3 can be explained by the fact that fi lm passivation of a more complex nature may occur aft er the formation of a salt fi lm on the electrode.
To some extent this shows the dependence of current on time (Fig. 3.).
Although they were in evidence in the sodium chloride solution, no clearly expressed peaks were present with potentiostatic closure in the region of signifi cant polarisations.More complex secondary passivation layers, probably on the part of aluminium oxide compounds, are formed in sodium chlorate solutions.Th e formation of a number of diff erent oxide fi lms on the metals in the sodium chlorate solutions was noted in [3].Since in the passive region, the current of the anodic dissolution is only somewhat less than the maximum it is possible to assert that the resultant oxide fi lm on the anode does not prevent the anodic dissolution of aluminium alloys in the solutions of sodium chlorate.When directly viewed through a microscope, a fi lm can be clearly seen forming on the surface of the aluminium electrode during dissolution; this almost immediately disintegrates into fragments, exposing the surface of the electrode.
Moreover, transportation of metal ions from the anode surface may occur by migration through the fi lm.According to [4] structural defects are present in an ionic oxide lattice: either cationic and anionic components are unoccupied, or, alternatively, ions introduced in the interstices of the lattice, whereby the diff usion of ions and electrons can occur in the oxide layer.Th e intensity of the dissolution of the metal will depend on the speed of movement of ions through the atomic lattice of the oxide.
In electrochemical treatment of aluminium alloys in sodium chlorate solutions, a high purity of the treated surface is achieved.Effective smoothing of the microrelief and high purity of the treated surface following electrochemical treatment of aluminium alloys in sodium chlorate solutions can be linked to the formation of specifi c oxide fi lms on the surface of the electrode and suppression thereby of the electrochemical heterogeneity of structural components of aluminium alloys by an additional potential barrier at the metal boundary, consisting of a fi lm that also reduces the extent of etching.In this case, there is a redistribution of the potentials of various structural components of the alloy, which leads to their levelling and a consequent smoothing of the microrelief.
Th is is to some extent confi rmed by metallographic studies of the aluminium alloy surface following electrochemical treatment in sodium chlorate solutions, which showed the absence of any intergranular violations of the grain boundary.
In nitrate solutions, the activation potential of aluminium and its alloys with magnesium is almost two volts higher than in sodium chlorate solutions.Th erefore, from the standpoint of energy consumption, treatment of aluminium alloys using a sodium chlorate based electrolyte is preferable.
Th us, along with high quality electrochemical machining in the complete absence macroscopic defects on the treated surface, a high level of productivity in processing aluminium alloys is achievable using chlorate solutions.

Fig. 2 .
Fig. 2. Eff ect of temperature (1) and speed of rotation of disc electrode (2) on the maximum current density dissolution of aluminium alloy AMg6 in 4M sodium chlorate

Fig. 3 .
Fig. 3. Anodic polarisation curve of current density -time at diff erent potentials, taken on a rotating disk electrode for AMg6 alloy in 4M sodium chlorate solution 1 -forward stroke of polarisation curve, 2 -return stroke of polarisation curve