Chemistry: electrical and wave energy – Processes and products
Patent
1985-07-19
1986-09-09
Kaplan, G. L.
Chemistry: electrical and wave energy
Processes and products
204 41, 204 431, 420428, 428632, 428667, C25D 356, C25D 512, C25D 548
Patent
active
046107630
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to the deposition of corrosion resistant alloys on electrically conductive substrates.
Various reports have appeared in literature relating to electrodeposition of chromium bearing alloys, e.g.: 1964 pp. 61-65 Shenoi., Metal Finishing December 1967 pp. 67-70. Elsie, and B. A. Shenoi, Metal Finishing August 1969 pp. 65-71 Metal Finishing February 1970 pp. 57-63 Shenoi Metal Finishing June 1972 pp. 30-34 L. Rama Char Metal Finishing September 1972 pp. 36-42.
We have also tested electroplating according to the baths used by Gowri and others and by Chisholm and Carnegie with resultant coatings having severe defects such as poor adhesion and non-uniformity of deposit and discolouration of deposit.
Various patents also relate to electrodeposition of chromium bearing alloy viz: U.S. Pat. Nos. 2766196, 2990343, 2927066, 3093556, 3795591, 3888744, 3374156, 3092556, 4141803 and 4142948 and UK 830205, 914866, 912950, 1149011.
We have carried out carefully documented tests on many such processes but have been unable to obtain satisfactory results especially from appearance, corrosion resistance and adhesion to the substrate.
In spite of all these efforts extending over many years, and in spite of the obvious advantages if a successful process could be developed, none of these efforts have been introduced into commercial production due to all the defects such as surface finish, adhesion and macrocracking.
Electroplating chromium (as distinct from chromium bearing alloys) has, of course, been commercially successful. However, all (except a few as mentioned hereafter) commercial chromium electroplating has been effected with baths based on hexavalent chromium compounds. This has considerable disadvantages which do not arise when using trivalent chromium compounds. Thus with hexavalent compounds the bath must be used at a much higher temperature e.g. 40.degree.-60.degree. C., than with trivalent chromium compounds and this gives rise to fumes and spray which can be exceedingly harmful to operators. However, the use of trivalent compounds has heretofore involved disadvantages especially the strong tendency to produce discoloured or striped coatings and undue lack of tolerance to contaminating ions e.g., Fe, Ni, Cu, Zn, in the bath which may arise from articles being coated and/or from carry-over from pre-plating or pre-treatment baths. Furthermore, internal stress of deposits when using trivalent chromium compounds for alloy deposition are greater than when using hexavalent compounds so that there is a greater tendency towards macrocracking. Microdiscontinuities have advantages compared with macrocracking e.g. improves corrosion resistance and accordingly it is very desirable to achieve coatings with microdiscontinuities e.g. at least 250 cracks per linear cm as defined in British Standard 1224 or pores of 10000 per 100 mm sq. The use of trivalent chromium also has the advantage that the bath can be effective with much lower concentrations of chromium than is required with hexavalent chromium compounds which is much better for various reasons e.g. disposal of effluent. Also with hexavalent chromium compounds a temporary break in current supply produces grey deposits which does not occur when using trivalent chromium compounds. With hexavalent chromium compounds also the degree of current density is much more critical than with trivalent.
A process for electrodepositioning a fine grained nickel coating is described in British Specification No. 936172 (Canada Pat. No. 689276) in which the bath contains finely divided inert particles which produces microporosity when subsequently covered with a thin coating of chromium which has "a favourable porosity pattern".
With the deposition of iron-chromium and iron-chromium nickel alloys there have been numerous patents and papers relating to this subject but there is not a commercially desirable process for the deposition of chromium alloys marketed based on baths containing trivalent chromium. In the case of trivalent chromium a process is proposed base
REFERENCES:
patent: 2927066 (1960-03-01), Schaer
patent: 2990343 (1961-06-01), Safranek
patent: 3093556 (1963-06-01), Machu et al.
patent: 3917517 (1975-11-01), Jordan et al.
patent: 3954574 (1976-05-01), Gyllenspetz et al.
patent: 4142948 (1979-03-01), Tajima et al.
WO 82/03095, Published 16 Sept. 1982, 204/43.1.
Chemical Abstracts, vol. 91, p. 580, 184047d, (1979).
Inter Metals and Minerals, S.A.
Kaplan G. L.
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