Anode Failure Mechanisms

FIGURE 4.5.13. Schematic of the cross-section of Ru02 + Ti02 coatings on Ti. 

Inadequate surface preparation of titanium before coating can result in surface oxides of Ti with the O content approaching two. Also, if the anode potential is high, the oxide films on the Ti can break down, leading to the anodic dissolution of Ti. It is essential to ensure that the intermediate layer containing mixed oxides of Ti and Ru is conductive. This can be done by proper thermal treatment of the coating. Otherwise, the anode potential will be high from the start. 

The increase in anode potential as the ruthenium content decreases (at constant loading) is a consequence of two factors  

As the mol% Ru02 decreases, the conductivity of the mixed oxide decreases (Fig. 4.5.3). The reason for this transition in conduction from that of metal to that of an insulator is associated with changes in physical properties and in the nature of the chemical bonding [33]. 

As the mol% Ru02 decreases, the mechanism of the chlorine evolution (Fig. 4.5.7) changes from one involving adsorbed intermediates to a slow electron transfer step, presumably because of the limited number of electrocatalytic sites available for the reaction. Note that if adsorbed intermediates participate, the Tafel slope is 30-40 mV, and when the charge transfer step is slow, the Tafel slope is 120 mV [70]. 

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Labossiere, P.E.W., Dunn, M.L., and Cunningham, S.J., Application of bimaterial interface comer failure mechanics to silicon/glass anodic bonds, Jourruil of the Mechanics and Physics of Solids 2002, 50,405-433. 

Aurbach, D. Zinigrad, E. Cohen, Y. Teller, H. A short review of failure mechanisms of hthium metal and hthiated graphite anodes in hqirid electrolyte solirtions. Solid State Ionics, 2002,128,405-416. 

Several gold based alloys used for electrical contacts have been evaluated by exposure to oxidation, HjS or SOj whilst the corrosion failure mechanism associated with gold metallisation in electronic circuits have been reported. Growth of gold shorts from a cathodic conductor occurs if chloride ions are present whilst a voluminous reaction product of Au(OH)3 is produced by the anodisation of an anodically biased conductor. No electronic circuit using gold conducting paths, is totally immune to corrosion failure. 

This failure mechanism can have significant impact on the ability of the anode to tolerate adsorbed contaminants. Similar to the impact of carbon corrosion on the cathode, the reduced electrochemically active catalyst surface area becomes very sensitive to the presence of contaminants. This is very important, for example, for operation on reformate where even small amounts of carbon monoxide can result in significant performance loss. 

It is significantly accelerated by the presence of hydrolyzable ionic contaminants (for example, hahdes and acids from flux residues or extracted from polymers). Delaminations or voids that promote the accumulation of moisture or contaminants can promote dendritic growth. Conductive anodic filament growth (discussed later) is a special case of dendritic growth. Time to failure is inversely proportional to spacing squared and voltage. The failure mechanisms in accelerated tests have been reviewed. ... 

The electrochemical equivalent of about 480 Ah kg is one of the lowest for all metallic anodes, and the OCV of 1.35 V for the Nicad is not favorable for many applications. Studies of failure mechanisms [26] revealed that the cadmium electrode is responsible for capacity loss and memory effect of the nickel/cadmium battery. Additionally, it is desirable to restrict the use of cadmium for environmental reasons. The consequence is a continuous retreat of this system from many applications, and battery packs for electric tools may eventually be the only remaining use. 

Surface analysis has made enormous contributions to the field of adhesion science. It enabled investigators to probe fundamental aspects of adhesion such as the composition of anodic oxides on metals, the surface composition of polymers that have been pretreated by etching, the nature of reactions occurring at the interface between a primer and a substrate or between a primer and an adhesive, and the orientation of molecules adsorbed onto substrates. Surface analysis has also enabled adhesion scientists to determine the mechanisms responsible for failure of adhesive bonds, especially after exposure to aggressive environments. The objective of this chapter is to review the principals of surface analysis techniques including attenuated total reflection (ATR) and reflection-absorption (RAIR) infrared spectroscopy. X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and secondary ion mass spectrometry (SIMS) and to present examples of the application of each technique to important problems in adhesion science. 

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