Iron, copper, and aluminium electrochemical corrosion in motionless and moving electrolytes investigation during electrolysis

Abstract

An experimental method to calculate average charge of metal ions by electrolysis at different temperatures is proposed. Aluminium undergoes dissolution to the Al3+ ions at all temperatures. Iron undergoes dissolution to the Fe2+ or the Fe3+ ions and copper undergoes dissolution to the Cu+ or the Cu2+. 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 °C, pH 6.5–7.5) at room temperature and 100 °C. The average charges of copper, iron, and aluminium ions were calculated using Faraday's law of electrolysis at electric current density 3000 A/m2 (or 30 A/dm2 = 3 mA/mm2): +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 °C. The main condition was zAl = 3. We concluded that calculations of the average metal ions charges, zFe and zCu, were correct since zAl = 3. The result is as follows: the Al3+, the Fe2+, and the Cu+ ions dissolve into concentrated NaCl solution in water at room temperature; the Al3+, the Fe2+, the Cu+ and the Cu2+ ions (50 %/50 %) dissolve into the solution at temperature 100 °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 practically the same as the cylindrical anode effect. The galvanic current dependence on ratio of cathodic surface area to anodic surface area in the system Fe-chromium (Cr) stainless steel and the time of the iron anodes dissolution in motionless electrolyte and in moving electrolyte are also analysed using literature experimental data.