PSI - Issue 28

Nina Ogoreltceva et al. / Procedia Structural Integrity 28 (2020) 1340–1346 Nina Ogoreltceva et al. / Structural Integrity Procedia 00 (2020) 000–000

1345

6

The cross-section analysis (Fig. 3) showed that the microstructure and the interface between the coating and the carbon cathode substrate are homogeneous. No cracks were observed along the interface between the coating and the carbon cathode substrate. The finding correlates also well with the results of pull-off test performed to determine the adhesion of the coating based on the TD1 powder to the carbon cathode surface. The adhesion strength value is 3 MPa (Table 2). The EDS results indicated the predominant presence of titanium (Ti), boron (B), and carbon(C) that are the basic elements of the composite. In addition, the presence of small amount of oxygen (O) was observed that could be justified by XRD analysis as TiBO 3 (1.5-3 wt. % ). 3.3. Industrial application The developed method of the cathode blocks protection was tested in operating conditions on an industrial high amperage electrolysis cell. The surface of the cathode blocks was coated with proposed coating composition based on the TD1 powder according to cathode wear profile, in areas with the most intensive wear. Two bands were applied as shown in Fig.4. along the periphery sides of the electrolysis cell of a width of 600 mm with the layer thickness of 3±1 mm.

Fig. 4. Schematic representation of coating application to protect the most defenseless areas of cathode blocks.

No blisters or large cracks were observed visually on the coating surface. The coating was protected against oxidation using a graphite powder bed with the thickness of 15-30 mm. The TiB 2 -C composite has been formed the following conventional procedure for cathode start-up heating and operating at 950°C during 48-72 h. The industrial cell has being operated experimentally for more than 4 mounts using coated cathode blocks. The current distribution uniformity over blooms and the voltage drop as well as wear rate of the coating were monitored according to the standardised procedures of OAO RUSAL Sayanogorsk. The wear monitoring data showed the stability of the coating in the corrosive cryolite-alumina environment and its reducing from 60 cm/year to less than 3 cm/year with application of the TiB-C coating in indicated areas. The expected lifetime of the cathode blocks has been increased at least for 4 mounts. It has been also shown that the cost of manufacturing and applying of the TiB 2 -based protective coating pays off by increasing the lifetime of the electrolysis cell even within 0.5 month. 4. Conclusions The cathode wear is regarded as the main limiting factor for the lifetime of the aluminium reduction cells. The composition of the TiB 2 -based protective coating and the cost-effective method of the coating deposition to protect the most defenseless areas of the cell bottom have been developed. The results are promising and show the efficiency of the proposed method which is applicable to existing cells without the cost and time of a complete cell redesign.