Corrosion diaphragm cell cathode

The fused diaphragms are ready to be installed in the cells, and it is preferable to use them within 2 days in order to minimize corrosion of the cathode, unless precautions are taken to ensure that the diaphragm is dry. It is essential that all the procedures and the specifications be adhered to in order to realize reproducible and optimal diaphragms. Run charts are suggested to monitor the various parameters discussed in this section. 

Cathode Corrosion. One of the problems encountered with MDC-29 diaphragm cells is the severe cathode corrosion at the cefi liquor/hydrogen interface near the cathode screen, which prevents deposition of good diaphragms. The primary current distribution profiles were calculated [15] using the finite difference method and the results... 

The cathode material is carbon steel in diaphragm cells, and nickel, often with a catalytic coating in membrane cells. As discussed in Section 4.6.6, exposure to anolyte containing active chlorine (CI2. HOCl, and OCl ) without cathodic protection is the primary reason for the corrosion of these components, unless the cathode coating is pore-free and noble metal based. Another species contributing to the corrosion of iron and nickel is the hydroxyl ion in the catholyte. 

It should be noted, however, that the operating environments for the electrodes are not identical in the three technologies, e.g. in a diaphragm cell the anode operates in a more alkaline medium (due to diffusion of hydroxide from cathode to anode) than in membrane or mercury cells. Also, it is only in a diaphragm cell that the cathode sees significant chloride ion in solution and this can increase corrosion problems, particularly when the cell is open-circuited. These particular diflicultLes will be discussed in section 3.3.2. 

Conventional potassium hydroxide electrolytic cells are made from carbon steel. Areas with high corrosion potential are frequently clad with nickel, plastic, or ceramic material. The cathode is constructed of steel coated with a catalyst. The anodes and cathodes of bipolar cells are usually made from nickel or nickel-coated steel. Diaphragms were originally made from asbestos reinforced with nickel nets. Because of the health hazards associated with the use of asbestos, ceramics and polymers are being considered as substitute materials. 

In a special series of experiments anodic dissolution of niobium in NaCl-KCl equimolar mixture was carried out at 750 °C under potentiostatic conditions. The initial potential of niobium anode (corrosion potential) was equal to -1.5/-1.6 V (vs a chlorine reference electrode). During potentiostatic anodic dissolution we varied the applied potential from -1.35 to -1.15V. The EAS recorded (Figure 4.4.4) were similar to the spectra obtained when niobium was dissolved at a constant current rather than a fixed potential (Figure 4.4.3). Therefore we conclude that, at potentials below -1.15 V, the anodic dissolution of niobium proceeds in one three-electron step. Unfortunately we were not able to perform the anodic dissolution experiments in spectroscopy cells at higher electrode potentials due to geometrical limitations, the diaphragm we employed to separate the cathode from the bulk of the melt did not conduct high currents required. 

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