Anodes coating life

Because mercury chlorine cells operate at higher current densities and because the mercury cell antxle can be adjusted during operation to minimize the unode-to-ealhode gap. anode coating life in these cells is much shorter, Because of limited commercial experience with anode coinings in membrane cells, commercial lifetimes have yet to be defined. Expected lifetime is 7- 12 years. 

Some impurities can also affect electrode performance. Those that affect anode coating life or performance include F, Fe, Mn, Pb, Ba, SO4, and organics. Impurities such as Hg and Pb may migrate through the membrane to affect the cathode coating. There is also evidence that iron brought in as ferrocyanide in the feed salt can affect the cathode. 

A similar improvement in expectation of life for thin anodic coatings has... 

Another option to lower the Ru losses is to dope the anode coatings with IrO2, which has been shown [53, 54] to markedly decrease the Ru corrosion rate during electrolysis in NaCl solutions. This would minimise the surface depletion of Ru, as shown by the SIMS analysis of the TiC>2 + RuC>2 + IrCU coatings (Fig. 5.19), and thus extend the operating life of the anodes. 

Recent studies performed with deactivated anodes show [55] that electroless or electrolytic platinum deposition on failed anodes, not only lowered the polarisation behaviour of these anodes (see Fig. 5.20), but also demonstrated an equivalent lifetime as that of a new anode in accelerated life tests in the sulphuric acid solution (see Fig. 5.21). These results unequivocally demonstrate that the deactivation of anodes, for which the Ru loading is still high, is a direct consequence of the depletion of Ru from the outer region of the anode coating. Note that this process of surface enrichment by conducting electroactive species will not lead to reactivating a failed anode, if there is a TiC>2 build-up at the Ti substrate/coating interface. 

In addition to yields, current density and anode life are also important in evaluating an electrochemical synthesis. Although the current density should drop as water (a strong electrolyte in HF) is consumed, it does not always do so. Instead, for the first 15-30 minutes of electrolysis it increases in both continuous and interrupted electrolysis. This may be caused by a breakdown in a resistive anode coating. Once a maximum current is reached, the current density remains constant however, it drops as the last few tenths percent of water are consumed. Also, high water levels (>3%) cause low current densities. The current density maximum was at 0.5-1.0 mole % water. 

FIGURE 4.5.10. Variation of the anode potential and the operating anode life with mol% Ru02 in the anode coating (plotted from the data in ref. [68]). Operating life was determined by tests in 300 gpl NaCI at 3 kA m (pH = 2.0-2.5 temperature = 80°C) with coatings containing 5 gms of Ru per m. ... 

Table 8.4 Formulae for coating and anode design life.

Before the brine enters the electrolysis cells, it should be acidified with hydrochloric acid to pH < 6, which increases the life of the titanium anode coating, gives a purer chlorine product with higher yield, and reduces the formation of hypochlorite and chlorate in the brine. 

Corrosion creep of diecast Mg-5A1 and Mg-9Al-lZn alloys anodized by a 8p.m thick coating showed that anodic coating delays metal dissolution facilitating plastic deformation of the stressed metal, and for this reason alone increases the corrosion creep life of metals (Fig. 9.15). For instance, time-to-fracture of anodized Mg-5A1 alloy increases from 340 to 637 hours... 

Anodes are connected to the object to be protected or to the transformer-rectifier by insulated conductors that are resistant to mineral oil (e.g., Teflon-coated cable) with a cross-section of 2.5 mm of Cu. The transformer-rectifier must meet the demands according to Ref. 6 and have the capability for monitoring and controlling its operation. The life of the anodes is in every case designed to be at least 15 years. 

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