There were small variations in the initial delay before the electric
current was switched on, but the fact that the electrodes were
immersed in the solution or on the metal surface state could explain
the difference measured between the aluminum concentrations
obtained with Cells A + C and Cell C in pH 3. According to
Sasson et al. iron electrodes are significantly corroded without
electricity at pH < 8 [22]. In this study, aluminum electrodes were
used and dissolution rate of electrodes in the solution without
electricity was not measured. However, it is well known that aluminum
corrodes when pH is either acidic or alkaline [23]. Aluminum
is naturally passive because continuous protecting alumina
(Al2O3) layer is formed on the surface. This layer protects aluminum
efficiently at the pH range 4–8. In higher and lower pH environment,
alumina layer dissolves at considerable rate. The
corrosion behavior of aluminum depends also on the other parameters
of solution, such as the concentration of anions. Sulphate anions
are passivating agents and therefore reduce the production of
metal cations [24]. Chlorides, on the other hand, induce the breakdown
of the passive layer and therefore cause pitting corrosion
[23,25]. It is known that passivation and formation of deposits on
electrodes increases the voltage of electrocoagulation system when
current density is kept constant and hence reduces the power efficiency
[1]. However, this was not observed in these short batch
studies.
The dissolving of Cell A follows theoretical values calculated
according to Faraday’s law of electrolysis (Eq. (7)).