## 1 Introduction
If hydroxide ions migrate into the anode side, in addition of losing cathodic product, it would combine with dissolved chlorine gas to produce hypochlorite ions, which is an unwanted side reaction:  
\[\mathrm{Cl_{2}}\ (\mathrm{g})+\ 4\ \mathrm{OH^{-}}\ (\mathrm{aq})\to 2\ \mathrm{ClO^{-}}\ (\mathrm{aq})+2\ \mathrm{H_{2}O}\ (\mathrm{I})+\ 2\mathrm{e^{-}} \tag{5}\]  
The energy efficiency of the chlor-alkali process depends on the overpotentials associated with each component, namely the membrane, the electrodes and the concentration polarization. The overall cell potential is the sum of cathode, anode, membrane, electrolyte and hardware [6]:  
\[E=E_{\mathrm{therm}}+\ \eta_{\mathrm{a}}+\left(-\eta_{\mathrm{c}}\right)\ +\ (\ R_{\mathrm{mem}}+R_{\mathrm{hw}}+R_{\mathrm{sol}})\cdot j \tag{6}\]  
where \(E_{\mathrm{therm}}\) represents the redox thermodynamic potential for the operating conditions (temperature and composition), the \(\eta_{\mathrm{act}}=\eta_{\mathrm{a}}+(-\eta_{\mathrm{c}})\) represents the anodic and cathodic activation overpotentials. The membrane, hardware and electrolyte-related overpotentials can be calculated using the [PERSON]'s law, \(\eta_{\mathrm{O}}=R_{\mathrm{O}}\cdot j=(R_{\mathrm{mem}}+R_{\mathrm{hw}}+R_{ \mathrm{sol}})\cdot j\)[5]. However, Equation (6) does not consider the concentration overpotential term because the concentration of NaCl and NaOH in the anode and the cathode are high, therefore, the electrolyte resistance can be considered negligible. Most of the maintenance of a membrane electrolyser is related to the membrane, which affects the ohmic resistance proportionally to the extent of the affecting area. A preventive maintenance strategy is usually followed based on the potential of the cell, replacing the membrane before electrolyte exchange between the cathode and anode compartments destroys the electrodes. The DSA(r) activation overpotential grows exponentially at the final stage of its service time due to the deactivation of the electrode [10]. This overpotential increase is a result of the coating erosion, selective electrochemical dissolution of ruthenium, blockage of the active surface site due to insoluble species and the formation of an insulating TiO\({}_{2}\) layer at the coating-substrate interface [10; 16]. The overpotential associated with the cathode surface corrosion grows smoothly making possible, until a certain extent, to keep the current density by increasing the cell's potential [11]. For the reasons given above the deactivation of the electrode, an excessive electrode service time contributes to increase of the energy operating costs of NaCl electrolysis. However, the recoating or replacing of the electrodes is very expensive due to the use of noble metal oxides on the electrode surface. Therefore, scheduling the maintenance instants of an electrolyser should be carefully made, balancing the increase of energy costs over time, based on the evolution of the potential as a function of time for a specific current density, and the obtained productivity with the replacement costs and the substantial catalyst losses increase that happens during the shutdowns and start-ups of the electrolyser [10]. Therefore, it is essential to have a rigorous criterion to decide when to perform the preventive maintenance of the electrodes and membranes. It should be emphasized, for instance, that the under current performance of a cell compromises the whole stack for cells associated in series.