The mechanism of spongy electrodeposits formation on inert substrate at low over potentials

Popov, Konstantin; Krstajić, N.V.

doi:10.1007/bf00615827

Cited by 36 publications

(23 citation statements)

References 19 publications

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“…From a more practical point of view, it was shown in a flowthrough channel cell that various types of deposits (flat, bulbous, or dendritic) could be obtained as a function of current density and Reynolds number [16]. Further research on the subject concentrated on the deposit morphology [15][16][17][18][19][20][21][22][23], with the aim to improve the control of surfaces with powdery deposits, and also on the quality of dense deposits made, first, by pulsated current or by superimposing alternating current (ac) on direct current (dc) and, second, with organic additives or on the electrode mechanism [24][25][26][27][28][29][30][31][32]. To summarize, because of the high value of the exchange current density (see [30, p. 2657]; values in the literature are between 2 and 31 A dm À2 ), surface diffusion should not be rate determining; diffusion in solution is more likely to play this role [18].…”

Section: Alkaline Noncyanide Zinc Bathsmentioning

confidence: 99%

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Electrodeposition of Zinc and Zinc Alloys

Winand¹

2010

Modern Electroplating

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Zinc has a standard reversible potential [À0.76 V/standard hydrogen electrode (SHE)] that is more negative than that of iron (Fe/Fe 2 þ -0.44V/SHE). For this reason zinc is used for sacrificial cathodic protection of steel against corrosion.According to Fischer [1], in metal electrodeposition, zinc can be considered as an intermediate metal, giving rise to medium values of overpotentials due to secondary inhibition resulting from adsorbed hydrolized species.In acid sulfate solutions without an organic additive, BR metallographic cross-sectional structures are obtained (BR: basis reproduction; coherent deposits with coarse crystals whose diameter increases with the deposit thickness; see Fig. 10.1). In chloride electrolytes, even coarser grains are observed, because of the more activating character of chloride ions against sulfate ions [2] and despite the complex formation in solution [3, p. 464]. In cyanide baths, on the other hand, much finer grains are obtained, and the deposits pertain to the FT (field-oriented texture: coherent deposits, with elongated crystals perpendicular to the substrate, with almost constant diameter throughout the deposit thickness) or UD types (UD: unoriented dispersion type; coherent deposit, with small crystals showing three-dimensional nucleation to occur all the time during electrodeposition). Figure 10.2 shows a deposit obtained in a cyanide solution without an organic additive: the structure is initially FT, but it becomes progressively bad. Going from a cyanide bath to a noncyanide alkaline bath should result in still worse deposits because electrolyte complexation decreases [3] and exchange current density increases [4, pp. 528-543].Accordingly, when rather thick deposits are needed and/or if a bright surface is required, organic additives are always present at high concentration (1 g L À1 or more). This always occurs in barrel-and-rack plating of small pieces of equipment, because the local current densities will vary over a wide range. For continuous plating on steel sheets or wires, constant current density and rather good hydrodynamic control may be achieved over the whole cathodic surface, and organic additives are not so much in use.Whatever the pH, zinc is always more negative than hydrogen [5]. In electrolytes where zinc is complexed, its standard reversible potential is still more negative; for instance, for cyanide baths, the reversible potential for reactionin alkaline solution is À1.26 V/SHE [6]. Consequently hydrogen evolution occurs at the cathode as a competitor to zinc deposition, resulting in a decrease of current efficiency and eventually in some atomic hydrogen diffusion into the substrate.Finally, in industrial processes electrolytic plating is an alternative to hot-dip galvanizing. Both processes have advantages and disadvantages. Continual technological improvements make it difficult to conclude in favor of one process over the other. However, hot dip always includes some surface alloying by diffusion, and the deposit thickness is less easily controlled th...

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Section: Alkaline Noncyanide Zinc Bathsmentioning

confidence: 99%

“…23 Cathode current efficiency as a function of current density for zinc-iron alloy plating. Channel cell.…”

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confidence: 99%

Electrodeposition of Zinc and Zinc Alloys

Winand¹

2010

Modern Electroplating

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“…It is also in agreement with the trends noted in Section 5.5. At low current densities, the morphology is dominated by large protrusions that grow independently; an observation noted by both Popov (23,24,25) and by Naybour (13).…”

Section: The Effect Of the Deposition Conditions On The Formation Of mentioning

confidence: 83%

“…"\ Popov and co-workers (23,24,25) proposed that spongy zinc formation was the result of the deposition process being under mass transport control while the nucleation density is low.…”

Section: Theories Of Mossy Zinc Depositionmentioning

confidence: 99%

Studies on the development of mossy zinc electrodeposits from flowing alkaline electrolytes

McVay¹

1991

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“…On the other hand, formation of spongy deposits at the lower, and dendrites at the higher, overpotentials was explained by long-standing assumption that the electrodeposition processes characterized by large values of the exchange current density are diffusion controlled at all overpotentials [8]. However, formation of individual grains or granules at low overpotentials was not possible to explain by this assumption.…”

Section: Introductionmentioning

confidence: 96%