Metal deposition depolarization

Depassivation of metal surfaces Depolarization, due to chloride leakage Deposit... 

Under these conditions the metal ions play the role of hydrogen atoms, as above explained. They discharge themselves in the cathode boundary surface and, depending upon their reaction velocities, affect the reduction of the depolarizer and the metallic deposition. With a great reduction velocity, therefore, no metal whatever is deposited on the cathode so long as sufficient quantities of the depolarizer are present.1... 

Depolarization of metal deposition sometimes occurs when two metals which separate simultaneously form compounds or solid solutions. The reversible potential of a solid solution generally lies in between those of the pure constituents hence an alloy containing both metals may be deposited at a potential that is less cathodic than that necessary for the less noble constituent in the pure state. This probably accounts for the fact that zinc and nickel are deposited simultaneously at a potential of about — 0.6 volt, whereas that required for pure zinc is nearly 0.2 volt more cathodic. The simultaneous deposition of the iron-group metals is partly due to the similarity of the discharge potentials, but the formation of solid solutions also plays an important part. Although the deposition potentials of cobalt and nickel are lower than that of iron, the cathodic deposit almost invariably contains relatively more of the latter metal. ... 

In conventional additive-based electroplating, competitive adsorption for surface sites occurs coincident with substrate immersion and the onset of metal deposition. Additive adsorbate coverage evolves with metal deposition and is often accompanied, to some extent, by incorporation into the growing solid with associated influence on the microstructure and resulting physical properties. The adsorbates can have an accelerating or inhibitory effect on the metal deposition process. Inhibition manifests as an increase in electrode polarization with additive concentration while accelerating or depolarizing additives exhibit the opposite trend. Recent use of... 

The effect of ultrasonic field on the polarization curves of Cu-Pb, and some brasses has been studied in chloride and sulfate solutions in the presence and absence of the respective metal ions [108]. The main effect of the ultrasound at low current densities is the acceleration of diffusion. The passivation current density in solutions free of the respective metal ions is considerably increased when ultrasound is applied. Stable passivity cannot be attained because of the periodic destruction of the salt film. The hydrogen evolution reaction is accelerated because of the destruction of the solvation shell. The oxygen depolarization reaction is also enhanced due to the increased diffusion. The rate of metal deposition is likewise increased by ultrasound. The steady-state potentials of reactions with anodic control are shifted in the negative direction when ultrasound is applied. 

Wherever possible, hydrogen evolution should be avoided this may be accomplished by control of the pH or cathode potential or by addition of a depolarizer, such as nitrate in the constant current deposition of copper. If the metal ion reduction is accompanied by hydrogen evolution, the metal deposit may be brittle, porous, and nonadherent. The main underlying cause is believed to be the physical effect of bubble evolution rather than the formation and decomposition of metal hydrides. 

Since the pH defines the potential of hydrogen evolution, its variation may be used to extend negatively the potential window for metal ion discharge. One may also use hydrogen evolution for depolarizing or for preventing deposition of less noble metals, subject to the deleterious effects on the quality of the metal deposit (see above). 

Crevices, deposits on metal surface or any geometrical configuration which results in differences in the concentration of oxygen or other cathodic depolarizers (e.g., Cu ). Metal in contact with the lower concentration—this follows from considerations of an equivalent reversible cell, although the situation is more complex in practice. 

The deposition of metals or hydrogen at the cathode or the discharge of anions or dissolution of the anode are not the sole processes which may occur at the electrodes in course of electrolysis. If substances are present in solutions which can accept, or release electrons at lower potentials than required by the aforementioned processes these substances are reduced or oxidized with priority. Such substances are generally called depolarizers... 

The possibility of activation of the electrocatalysis for Hj evolution at various materials by introduction of depositable transition metal salts has been recognized for some time. Some practical applications refer to depolarization of amalgam electrodes in the old Hg cell chloralkali process. This procedure can be applied to various other substrates, for example, graphite, Fe, Ni steel, and Ti (164-167). 

Consider now the fate of some metal ion, such as lead(II), that begins to deposit at point A on the cathode potential curve. Lead(II) would codeposit well before copper deposition was complete and would therefore interfere with the determination of copper. In contrast, a metal ion, such as cobalt(II), that reacts at a cathode potential corresponding to point C on the curve would not interfere because depolarization by hydrogen gas formation prevents the cathode from reaching this potential. 

It is advisable to protect T1 from surface oxidation by coating it with a layer of paraffin or storing it under glycerol or petroleum. II. Brown and McGlynn report preparation of a good, smooth, cohesive electrolytic deposit of metallic T1 from a thallium perchlorate bath containing peptone as an anodic depolarizer and cresol as a further additive. Current densities of 0.5 to 1.8 amp./ 100 cm. are used. 

The contaminants may be deposited on the surfaces of the materials in the form of anhydrous or hydrated species. Some pollutants, like CO2, SO, NO, and HCl, are typical of urban and industrial areas, give rise to acid rains, and might contribute to the cathodic processes, while others, such as chlorides, are typical but not exclusive of marine and coastal areas and give rise to hygroscopic salts that increase the duration of wetting of surfaces, increase the conductivity of solutions, and make less protective the corrosion products. Some others, such as the sulfides, which can result from microbiological activity, alter the composition of the corrosion products, their protective capability, and the nobility of the metal often they are semiconductors, depolarize the cathodic process of hydrogen evolution, and may be oxidized to sulfuric acid by bacteria. Ammonia alters the composition of corrosion products and the solubility of metal ions it has particularly drastic effects on copper alloys and their corrosion forms. In the transport of these contaminants toward the surfaces, an important role is exerted by the wind and by the orientation of the surfaces, which can promote or hinder the washout by the rains. 

Let us first consider the process without depolarization (a = 1), for example, the deposition of metal Z on an inert electrode surface. 

Depolarizers prevent imdesirable reactions by providing a facile reaction that prevents the cathode potential from becoming too negative or the anode potential from becoming too positive. Cathodic depolarizers are oxidizing agents that are reduced less readily than the depositing metal ion but more... 

The most commonly used working electrode material is platinum, sometimes alloyed with a few per cent of iridium. Two difficulties can arise. First, in chloride media, the Pt anode may dissolve and codeposit with the intended metal on the cathode platinum dissolution can be suppressed using a depolarizer such as hydrazine sulfate. Second, electrode cleaning is difficult when the deposited metal alloys with platinum. This arises with Bi, Cd, Ga, Hg, Pb, Sn, and Zn but can be prevented by precoating Pt with Cu or Ag. 

Copper and lead in brasses and bronzes. Nitric acid treatment of brass leads to chemical separation of tin as SnOi XH2O after filtration and an ignition procedure, the tin can be gravimetrically determined. Thereafter, the analysis is based upon cathodic deposition of Cu and anodic deposition of PbOi. If copper alone is to be determined, deposition from a pH 4 tartrate medium with hydrazine as the anodic depolarizer gives separation from most common metals. If lead is also to be determined, one uses a concentrated nitric acid medium, from which PbOi is deposited... 

Alloys of Cu, Sb, Pb, and Sn. This procedure for analysis of bearing metals relies on two successive depositions of pairs of metals. Each (mixed) deposit is dissolved, and then one of the components is selectively deposited (and determined) under different conditions and the other component determined by the difference. First, copper and antimony are codeposited from HCl, with hydrazine as a depolarizer. The (weighed) deposit is dissolved in nitric/ hydrofluoric acid, which retains antimony in solution as a fluoride complex, allowing deposition of pure copper (at —0.40 V). Tin and lead are codeposited (at —0.70 V) from the initial residual solution. After redissolution of the (weighed) deposit in nitric/hydrofluoric acids, lead is deposited ano-dically as PbOi- The Pb and Sn aspect of this procedure is useful for analysis of solders. An analogous procedure allows Ni and Co separation (via C02O3). 

Phosphate pre-treatments may be either zinc phosphate (from zinc dihydrogen phosphate solutions) or an iron phosphate (fi om alkali phosphate solutions) (see Conversion coating and Pre-treatment of steel). The conversion reactions are promoted by accelerators (depolarizers), for example, bromates or molybdates in alkali phosphate baths or chlorates in zinc phosphate baths (with Ca or Ni grain-refining additions). Iron phosphate pre-treatment coatings are often described as amorphous . In practice, however, they are usually crystalline deposits of iron oxides and iron phosphate. Zinc phosphate pre-treatment coatings are always crystalline. A fine, dense crystal pattern of zinc phosphate on the metal surface is the ideal, as it improves both paint adhesion and corrosion resistance most effectively. 

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