Cathodic hydride formation

Cathodic currents causing titanium hydride formation and embrittlement. 

A cathodic current is in principle possible, but in case of hydrogen evolution, titanium will be destroyed by hydride formation. 

Analogous to the DSA manufacture, a pure htanium dioxide coating can be prepared, which shows a high activity and stabihty (also against titanium hydride formation), for electroorganic cathodic reductions (e.g. [40], see Chapter 7). 

The hydride is also prepared electrochemically by cathodic charging of Ti in 1 N HjSO. Hydride formation, however, is limited to a thin surface layer owing to slow hydrogen diffusion. 

Cathodic charging also is used to prepare ZrHj. Hydride formation again is limited to a thin surface layer as in the formation of TiHj by the same method. 

In order to obtain reproducible results, the working electrode must be pretreated as explained in the following text the electrode used herein was polished with 600 emery paper and alumina suspension (0.3-0.2 pm granularity), washed in distilled water, immersed in a previously deaerated solution, and cathodically polarized at —0.9 V for 5 min. The latter step is important to reduce the oxide layer formed during polishing however, polarization times longer than this period have to be avoided, otherwise hydride formation and metal lattice reconstruction turn up as a consequence of molecular hydrogen inclusion. 

The positive electrode, the cathode, is similar to that in nicad cells and consists of a mixture of NiO(OH)/Ni(OH)3 and Ni(OH)2. An alloy that supports hydride formation replaces the cadmium as the negative anode. The alloy most commonly used is derived from LaNis, in which a mixture of other lanthanides replaces the lanthanum, and a nickel-rich alloy replaces the nickel, to give a general formula LnA/5. The anode is composed of an agglomeration of alloy powder. A small amount of potassium hydroxide is added as an electrolyte. The cell voltage is 1.3 V, making these cells suitable for the direct replacement of nicad batteries. The cell construction is identical to that of the nicad cell (Figure 9.10), with the cadmium replaced by metal hydride. The approximate cell reactions are as follows. 

Attempts to develop an activated cathode for chlorate cells have not yet been successful, and a material for the application faces many constraints. Some important properties for a chlorate cathode are (a) low overpotential for hydrogen evolution, (b) high stability during hydrogen evolution (resistant to the mechanical stress from gas bubbles and no detrimental hydride formation), (c) resistant during shut downs (low corrosion rate at open circuit in chlorate electrolyte), (d) low activity for hypochlorite decomposition, (e) low activity for reduction of hypochlorite and chlorate in the presence and in the absence of the chromium hydroxide film (the latter a step in the search for a chromate-free process), (f) relatively resistant to impurities in the electrolyte, (g) easy to manufacture, (h) easy to install in existing cell concepts, and (i) cost-effective. 

Titaniuni is also prone to hydrogen adsorption leading to the possibility of hydride formation this limits the use of titanium as a cathode material in electrochemical reactors involving addic electrolytes. Such conditions may not allow a stable passive filni to be retained on the electrode surface. 

Current on cathode dissolution, particularly due to combination of electroreduction of protective surface films and active generation of hydrogen. This may be especially marked in the case of titanium, due to facile hydride formation. The result may be pitting, embrittlement or exfoliation of the electrode surface. Electrode corrosion may significantly alter electrocatalytic properties (resulting in a reduced selectivity) contaminate electrolytes (and, hence, products) block cells, dividers or manifolds cause electrical shorting, or provide parasitic redox couples (e.g. Fe /Fe ) which decrease current efficiency or promote deposit or bimetallic corrosion efficiency elsewhere. 

Cathodic disintegration can occur with lead, observable as a grey cloud of fine metal particles. Hydrogen evolved on the surface of the lead can be absorbed if the current density is sufficiently high . Above this level, avalanche penetration can occur, feadipg to the formation of lead hydride, which leads to disintegration in the manner described . Electrochemical implantation pf alkali metals Can also lead to disintegration, ... 

A simple example of the redox behaviour of surface-bound species can be seen in Figure 2.17, which shows the behaviour of a bare platinum electrode in N2-saturated aqueous sulphuric acid when a saw tooth potential is applied. There are two clearly resolved redox processes between 0.0 V and 0.4 V, and these are known to correspond to the formation and removal of weakly and strongly bound hydride, respectively (see section on the platinum CV in chapter 3). The peak currents of the cathodic and anodic reactions for these processes occur at the same potential indicating that the processes are not kinetically limited and are behaving in essentially an ideal Nernstian fashion. The weakly bound hydride is thought to be simply H atoms adsorbed on top of the surface Pt atoms, such that they are still exposed to the... 

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