Electrochemical Metal Treatments

Different electrochemical surface treatments have found extensive use for the purposes of providing metal parts with particular properties, appearance, and shape. This includes the apphcation of superficial oxide or salt films (see Section 16.3), metal films (Section 16.5), and a number of methods that exploit the selective anodic dissolution of different segments of the metal surface. We examine briefly a few examples of the latter type. 

Electropolishing is performed in concentrated mixtures of acids (sulfuric, phosphoric, chromic, etc.). Often, organic acids and glycerol are added. It is somewhat inconvenient that almost all metals and alloys require their own solution composition. For electropolishing, intermediate and high current densities are used, between about 0.1 and 500 mA/cm. Depending on current density, the process requires between 30 s and 20 to 30 min. Usually, a metal layer 2 to 5 pm thick is removed under these conditions. 

Different views exist as to the reasons for selective dissolution of the asperities. According to older concepts, convection of the liquid is hindered in the solution layers filling recesses hence, reaction products will accumulate there and raise the concentration and viscosity in these layers. Both factors tend to lower a metal s anodic dissolution rate relative to that at raised points. According to other concepts, a surface condition close to passive arises during electropolishing. In this case, the conditions for passivation of the metal at raised points differ from those in recesses. 

FIGURE 16.5 Electrochemical machining of metals (1) workpiece (anode) (2) tool (cathode). 

Formation of a continuous passivating layer is difficult at raised points the degree of passivation is lower, and dissolution will be faster. The overall phenomenon of electrochemical polishing is highly complex, and a variety of factors contribute to the effect of surface leveling. 

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Finally, electrochemical pre-treatment is performed to obtain a reproducible surface. This is done mainly by cycling the applied potential over the entire potential window limited by the hydrogen and oxygen evolution reaction. Such a treatment has two functions first, removal of adsorbed species and, second, altering the microstructure of the electrode, the latter being caused by the repetitive dissolution and deposition of a metal mono-layer in the scanning procedure. 

Kaba and Hitchens (1989) found that electrolysis of a mixture of urine and feces produced C02, N2, and H2. Some HOC1 is generated this eliminates the pathogens and bleaches the contents. The anodic reactions at 90 °C consume the biomass the cathode evolves hydrogen and can be assumed to deposit the small metal content. The residuum is sodium chloride from the urine. Most of the electrochemical studies that establish the basis of a practical process for electrochemical sewage treatment have been carried out on packed-bed electrodes as shown in Fig. 15.29. 

Electrochemical wastewater treatment — may involve (a) -> electrolysis with the purpose of heavy metal ion removal, (b) electrochemical transformation of anions, e.g., perchlorate, nitrate, (c) oxidation of organic pollutants, (d) - electrodialysis, or (e) - capacitive deionization. 

Electroplating is the most widely used electrochemical surface treatment process, involving the electrochemical deposition of a layer of metal or alloy onto a substrate by means of one or more cathodic metal ion reduction reactions of the form... 

Electrochemical processes for the pre-treatment of metals include both anodic oxidation, for example, of Cu and A1 in alkali, or Ti in NaF/HF, and cathodic oxidation of Cu in sodium bicarbonate. Electrochemical pre-treatments promote adhesion by producing either a porous surface (Anodizing of Al), or a needle-like dendritic oxide structure. The most suitable surface structure may often be determined by the generic nature of the paint that will be applied subsequently. 

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