Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Patent
1993-04-08
1995-02-21
Silverberg, Sam
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
148255, 148247, C23C 2282
Patent
active
053912403
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to a process for the passivating post-treatment of phosphated metal surfaces of iron, steel, galvanized steel, zinc, aluminum and alloys thereof with chromium-free, silicate-containing aqueous solutions.
STATEMENT OF RELATED ART
The protection of surfaces of the above-mentioned metals by phosphate coatings has long been known (Ullmanns Encyklopadie der technischen Chemie [Title in English: Ullmann's Encyclopedia of Technical Chemistry], 4th Edition, Vol. 15 (1978), pages 686-688). The surfaces mentioned are phosphated to increase the adhesive strength of lacquer films and to improve corrosion resistance. More particularly, the phosphate coatings are intended to prevent rusting of the metal surface to which the lacquers are applied.
For some considerable time, after-rinsing with chromium(VI)-containing solutions has been used to improve the effect of the phosphate coatings. These solutions passivate the metal surface still exposed in the pores of the phosphate layer. In addition, the solubility of the phosphate crystals forming the phosphate coating is greatly reduced by ion exchange reactions.
However, the advantages of the improved phosphate coating are offset by major disadvantages of the standard chromium(VI) process. The most serious of these is the high toxicity of chromium(VI).
Accordingly, various proposals for the passivating post-treatment of phosphated metal surfaces with chromium-free solutions can already be found in the literature:
Thus, chromium-free compositions and a process for the treatment of phosphated metal surfaces are known from EP-A-0 085 626. In this case, the phosphate coatings mentioned are after-passivated by application of titanium(III)-solutions at pH values of 2 to 7. On account of the extreme instability of the titanium(III) ion, the compositions are preferably prepared immediately before use or are stabilized by the presence of relatively large quantities of organic compounds.
DE-A-27 01 321 describes a process for the post-treatment of phosphated surfaces of zinc or zinc alloys in which the surfaces are treated with a chromium-free aqueous solution containing titanium ions and, in addition, one or more components from the group consisting of phosphoric acid, phytic acid, tannin and hydrogen peroxide. In this case, too, a pH value of 2 to 6 is largely maintained. However, this process is confined solely to galvanized steel surfaces.
In addition, EP-A-149 720 describes a process for the after-passivation of phosphated metal surfaces using chromium-free aqueous solutions containing titanium(IV), manganese(II), cobalt(II), nickel(II) and/or copper(II) ions. In this process, the phosphated surfaces of iron, steel, galvanized steel or aluminum are first rinsed with water, then treated at 20.degree. to 120.degree. C. with acidic to neutral aqueous solutions containing the cations mentioned above, subsequently re-rinsed with water and optionally dried. However, even this process does not provide entirely satisfactory results from the applicational point of view.
DE-B-12 77 646 describes a process for increasing the corrosion resistance of surfaces of aluminum and aluminum alloys by application of coating using green chromating solutions containing hexavalent chromium, phosphate and fluoride and post-treatment of the coating with aqueous solutions having a pH value of 9 to 13. Sodium silicates (orthosilicates and condensed silicates), for example, may be used for the preparation of these alkaline-reacting post-treatment solutions. The advantages sought by this process cannot be obtained with solutions having a pH value below 9.
A process for the post-treatment of protective oxide coatings or other conversion coatings on metal surfaces with alkali metal silicate solutions is also known from DE-C-16 21 467. In this case, the disadvantages of known processes, namely post-treatment of phosphate coatings with sodium silicate solutions, attributable to the high alkalinity are overcome by aftertreating the coatings applied with solutions o
REFERENCES:
patent: 3615895 (1971-10-01), Freyhold
patent: 4462842 (1984-07-01), Uchiyama et al.
Chemical Abstracts, 101:96199z (JP-A-84/59 885), 1984.
Chemical Abstracts, 87:205007a (JP-A-77/068 830) 1977.
Brands Karl-Dieter
Gottwald Karl-Heinz
Seidel Reinhard
Henkel Kommanditgesellschaft auf Aktien
Jaeschke Wayne C.
Silverberg Sam
Szoke Ernest G.
Wisdom, Jr. Norvell E.
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