Anodic dissolution uniformity

Electroplating of one metal onto another is widely used for protection against corrosion and wear or for cosmetic purposes.16 Again, the source of metal for deposition could be anodic dissolution or a prepared solution with an inert anode. In contrast to electrolytic refining, only a very thin layer (typically on the order of 1 to 10 pm) of the plating metal is wanted, but usually this layer must be uniform, cohesive, and nonporous, and often a shiny appearance is desired. To understand the roles of some of the variables in electroplating, it is useful to consider the electrodeposition... 

The rate of oxide formation relative to dissolution of the oxide determines the surface coverage, thickness, and properties of oxide, occurrence of passivation and current oscillation as well as uniformity of anodic dissolution. 

The effect of ultrasound on the rate of anodic dissolution of metals was studied by Karavainikov [118] in 1973. He found that the rate of dissolution of Fe in 10% HC1 was slightly increased with ultrasonic vibration in the current density range of 0.08-0.4 A/m2. The surface after dissolution under ultrasound had a uniform, fine-grained structure giving diffuse dispersion of light. 

Similar to the formation of porous aluminum oxide a passivation - dissolution mechanism can be used to form nanopo-rous structures on InP. If (OOl)n-InP is polarized anodically under illumination in HCl solutions, nanoscaled pores are formed [117]. For potentials up to 1.2 V vs. SGE the main reaction is uniform anodic dissolution. Above this potential porous InP with a surface oxide is formed. The overpotential and anodizing time influence pore diameter (110-250 nm), wall thickness (16-50 nm) and pore length... 

The increase of the apparent density of powder particles by the application of RC regimes can be explained by the effect of the anodic time in square-waves RC on selective dissolution of the electrode surface [62], The selective dissolution of the electrode surface during the anodic time only occurs at points with a very small radii of curvature, which dissolve faster than flat parts of the surface or of points with larger tip radii. In the minute range, the duration of selective dissolution must be shorter compared to the overall anodic dissolution time, because the tip radii of dendrites or dendrite branches very quickly become sufficiently large to make the effect of selective dissolution negligible and the particles dissolve uniformly. A decrease of the overall dissolution time leads to a decrease of the... 

From these two examples, which as will be seen subsequently, present a very oversimplified picture of the actual situation, it is evident that macroheterogeneities can lead to localised attack by forming a large cathode/small anode corrosion cell. For localised attack to proceed, an ample and continuous supply of the electron acceptor (dissolved oxygen in the example, but other species such as the ion and Cu can act in a similar manner) must be present at the cathode surface, and the anodic reaction must not be stifled by the formation of protective films of corrosion products. In general, localised attack is more prevalent in near-neutral solutions in which dissolved oxygen is the cathode reactant thus in a strongly acid solution the millscale would be removed by reductive dissolution see Section 11.2) and attack would become uniform. 

Table 10.9 lists some common zinc anode alloys. In three cases aluminium is added to improve the uniformity of dissolution and thereby reduce the risk of mechanical detachment of undissolved anode material . Cadmium is added to encourage the formation of a soft corrosion product that readily crumbles and falls away so that it cannot accumulate to hinder dissolution. The Military Specification material was developed to avoid the alloy passivating as a result of the presence of iron . It later became apparent that this material suffered intergranular decohesion at elevated temperatures (>50°C) with the result that the material failed by fragmentation". The material specified by Det Norske Veritas was developed to overcome the problem the aluminium level was reduced under the mistaken impression that it produced the problem. It has since been shown that decohesion is due to a hydrogen embrittlement mechanism and that it can be overcome by the addition of small concentrations of titanium". It is not clear whether... 

As mentioned, corrosion is complexly affected by the material itself and the environment, producing various kinds of surface films, e.g., oxide or hydroxide film. In the above reactions, both active sites for anodic and cathodic reactions are uniformly distributed over the metal surface, so that corrosion proceeds homogeneously on the surface. On the other hand, if those reaction sites are localized at particular places, metal dissolution does not take place uniformly, but develops only at specialized places. This is called local corrosion, pitting corrosion through passive-film breakdown on a metal surface is a typical example. 

The electrochemical characterisation studies, discussed in the previous section, showed that a 40 at.% Ru electrode, when subjected to extended electrolysis or potential or current cycling in NaCl solutions and when the chlorine overpotential reaches 300-400mV, behaves like a fresh, low at.% Ru (about 5 at.%) electrode. This strongly suggests that Ru losses from the Ru/Ti oxide coating occur during electrolysis. To determine whether or not the Ru losses in failed anodes take place by uniform dissolution across the entire coating or whether only localised surface... 

If the Ru loss in the deactivated anode is a result of uniform dissolution across the entire coating layer, resulting in a Ru loading of less than 2 g m-2, the anode has to be recoated to regain its electrocatalytic activity for the chlorine evolution reaction. Under these conditions, the existing anode coating must be stripped prior to recoating. However, if surface depletion of Ru is the cause for increased anode potential, then replenishment of these surface sites should result in the rejuvenation of the deactivated anodes. 

Corrosion of metals by aqueous acids with hydrogen evolution is usually rapid and fairly uniform across the surface (general corrosion), since the reductive dissolution of the oxide film that helps maintain the distinction between anodic and cathodic sites is favored by low pH (reaction 16.9). Thus, although local anodic and cathodic areas persist, pitting becomes less important than overall loss of metal. If the oxide film is sufficiently insoluble in acids and is also resistant to reductive dissolution, as with titanium or stainless (>11% Cr) steels, the metal may remain unaffected by aqueous acids, except at quite negative Eh values. In cases where the cathodic discharge of hydrogen ions... 

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