An Experimental Method to Calculate Average Metal Ions Charge by Electrolysis at Different Temperatures

An experimental method to calculate average charge of metal ions by electrolysis at different temperatures is proposed. Aluminium undergoes dissolution to the Al 3+ ions at all temperatures. Iron undergoes dissolution to the Fe 2+ or the Fe 3+ ions and copper undergoes dissolution to the Cu + or the Cu 2+ . It depends on temperature and electric current density. Direct electric current value and anode mass decreasing were measured during electrolysis into concentrated NaCl solution in water (5 mol/kg or 23.1%, freezing point equals -22 o C, pH 6.5–7.5) at room temperature and 100 o C. The average charges of copper, iron, and aluminium ions were calculated using Faraday’s law of electrolysis at electric current density 3,000 A/m 2 (or 30 A/dm 2 ): +3 for aluminium; +2 for iron; and +1 for copper at room temperature, and +3 for aluminium; +2 for iron; and +1.5 for copper at temperature 100 o C. The main condition was z Al =3. We concluded that calculations of the average metal ions charges, z Fe and z Cu , were correct since z Al =3. The result is as follows: the Al 3+ , the Fe 2+ , and the Cu + ions dissolve into concentrated NaCl solution in water at room temperature; the Al 3+ , the Fe 2+ , the Cu + and the Cu 2+ ions (50%/50%) dissolve into the solution at temperature 100 o C. We have obtained experimentally and by mathematical modelling that aluminium anodes (cylindrical or spherical) dissolve into the solution more rapidly with temperature increasing during electrolysis accordingly to the Arrhenius law, while copper anodes (cylindrical or spherical) dissolve more slowly with temperature increasing from room temperature to temperature 180°C like “inverse Arrhenius law”. Iron electrochemical corrosion rate practically does not depend on temperature below 100°C (and, obviously, up to 180°C) like “zeroth Arrhenius law”. The spherical anode effect is greater than the cylindrical anode effect in 1.5 times.


Introduction
The container which consists of an outer copper canister and an inner carbon steel (Fe) tank is used to dispose of spent nuclear fuel [1].Copper (Cu) coatings on an aluminium (Al) wire, that are widely used in the automobile and aerospace industries to reduce of total weight of the electric wires, can be quickly destroyed during working since they are heated up to 200 o C [2].
Aluminium undergoes dissolution to the Al 3+ ions in all types of electrodes at different temperatures [3].Two aluminium electrodes were used for the removal of cadmium (Cd) from wastewater through the EC process and various EC tests were conducted for the different initial temperatures of 18, 30, 50 and 70 °C [4].Cadmium (Cd) removal rate increased with temperature increasing [4].Rate of aluminium anodes dissolving (Al→{electrolysis}Al 3+ ) increases with temperature increasing [5 -8], so quantity of coagulants Al(OH)3 and pure Al also increases with temperature increasing, and, as result, the cadmium removal rate increases with temperature increasing too.
Iron produces the Fe 2+ ions during dissolution.Fe 2+ oxidation to Fe 3+ by dioxygen becomes significant only at higher pH values (pH > 7) although Fe 3+ is a stronger coagulant than Fe 2+ [9,10].We can conclude that Fe(OH)3 is better coagulant than Fe(OH)2.Rate of iron anodes dissolving practically does not depend on temperature [5 -8].We have planned to investigate what coagulants, Fe(OH)2, Fe(OH)3, or/and pure Fe are formed near iron anode during electrolysis in concentrated NaCl solution at temperatures 20 o C -100 o C. It depends on electric current density [9,10].

The amount of aluminium loss due to electrochemical corrosion per unit time calculation
The dependence of corrosion rate (CR), or the amount of aluminium (only Al, not Fe or Cu) electrochemical corrosion per unit time, on temperature could be expressed in the form of Arrhenius equation in the following way [6]: where A is the pre-exponential factor, EA is the activation energy, R is the gas constant and T is temperature.We can calculate the aluminium activation energy and the pre-exponential factor using experimental results during electrolysis at room temperature and 100 o C [3,4]: ( ) ln 1.637 / 0.017 ( ) (11) where kAl is the aluminium anode radius-decreasing rate constant, L is anode length immersed into the electrolyte, ρAl is the aluminium density.

Conclusions
Copper electrochemical corrosion is higher than aluminium or iron electrochemical corrosion at room temperature T1≈20 o C (average charge of copper ions equals +1) and at temperature T2=100 o C (average charge of copper ions equals +1.5).The ratio of electrochemical corrosion rates, kCu/kAl (or kCu/kFe), decreases with temperature increasing.Iron electrochemical corrosion rate practically does not depend on temperature below 100 o C (average charge of iron ions equals +2).It is obvious because the melting point of iron is higher than the melting point of copper or aluminium (Fig. 7).
Increasing temperature leads to dissolution rate value of copper anodes decreasing while the electric current value increases since average charge of the Cu ions increases too.
Increasing temperature leads to dissolution rate value of aluminium anodes increasing and electric current value increases too.
The dissolving rate value of the copper anodes decreases approximately in 4 times due to the temperature increasing effect while the dissolving rate value of the aluminium or iron anodes increases all the time due to the cylindrical shape effect.
Increasing temperature practically does not influence on rate value of iron anodes dissolving as on the direct electric current value too.The spherical shape effect is greater than the cylindrical shape effect in 1.5 times.