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Termokinetics processes at electrodeposition of chromium and alloys Co-W


Author: Natalia Ţînţaru
Degree:doctor of chemistry
Speciality: 02.00.05 - Electrochemistry
Year:2007
Scientific adviser: Alexandru Dicusar
doctor habilitat, professor, Institute of Applied Physics of the ASM
Institution:
Scientific council:

Status

The thesis was presented on the 16 February, 2007
Approved by NCAA on the 19 April, 2007

Abstract

Adobe PDF document0.64 Mb / in romanian
Adobe PDF document0.67 Mb / in russian

Thesis

CZU 541.13 + 621.35

Adobe PDF document 4.45 Mb / in russian
137 pages


Keywords

termokinetics processes, surface heat production, electrodeposition of chromium, codeposition of alloys Co-W, throwing power, and pulse electrodeposition

Summary

Influence of surface heat production on electrodeposition rate, on current den-sity-deposition rate dependences; on technological parameters (first of all, for the case at chromium plating), including electrolyte throwing power and structure of the deposited layers were investigated for electrodeposition of chromium in standard chromic electro-lyte and Co-W alloys in citrate electrolyte, at various bulk electrolyte temperatures and current densities of constant and pulse currents. It is shown, that observable surface heat production (during chromium electrodeposition from standard electrolyte for chromium plating with direct current in the current densities interval 0,5 - 6,25 А/сm2) is the func-tion of current density and leads to reduction of current efficiency with the current den-sity in the region of high current densities.

It is shown, that in the case of unipolar pulse current application the current ef-ficiency essentially depends on pulse duration, increasing (at the determined duty cycle) with its duration. In the millisecond pulse current range (and below) the current effi-ciency of chromium plating is close to zero, when the pulse duration it is more than 0,1s - the current efficiency is more then observable at direct current (for average current densities usually used).

Surface temperature (and, as consequence, deposition rate and current effi-ciency) dependence on pulse current parameters is established. At average constant cur-rent densities and bulk electrolyte temperatures the surface temperature reduction with increase of pulse duration is observed. It is shown, that use of a pulse current in an inter-val of pulse duration 0,1 s  р  20 s leads to decrease of p /d ratio (pulse current ef-ficiency to direct current efficiency ratio) depending on average current density. This ef-fect is the reason of increasing of electrolyte throwing power (TP) from negative values for direct current up to ТР ~ 90 % for a pulse current (iavg = 0,5А/сm2, duty cycle – 25 %, р =2 s). Influence of the surface heat production on structure of the electrodeposited chromium is studied and it is shown, that there exist the threshold values of current den-sity above which ambiguity of properties is observed under identical conditions of elec-trodeposition.

It is established, that an increment of surface temperature is negligible at elec-trodeposition of Co-W alloys; and the increment value practically does not depend on current density. It is shown, that the absence of current density influence on surface temperature increment is connected with change of film structure during the current density increase. This film structure change leads to "replacement of an organic phase" out of film and therefore with increase of current density the plated film becomes more "metal", that leads to reduction of its resistance, and, hence, to decrease of surface heat production in it.