Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Patent
1994-05-13
1997-03-04
Silverberg, Sam
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
148273, C23C 2242
Patent
active
056075219
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method for posttreatment of an article with a metallic surface, where the metallic surface is made of one or more metals having standard oxidation potentials within the range -2.5 to +0.5 V, and where the metallic surface is subjected to a treatment by means of an aqueous treatment solution in order to form a thin coating, said treatment solution containing a) a molybdenum compound selected among molybdic acid and salts thereof, and b) a compound capable of forming a heteropolymolybdate together with a molydate, as well as to a treatment solution to be used in the method.
BACKGROUND ART
It is a known procedure to aftertreat metal articles and metal surfaces with chromate in order to obtain a surface coating having corrosion-protecting and/or decorative properties. The treatment is called a chromate treatment and is known for instance in connection with zinc-coated, cadmium-coated or silver-coated copper or iron, including steel. Conventionally, aluminium and aluminium alloys are also treated by a chromate treatment.
The protective effect of a chromate treatment is due to a chemical conversion of a thin metallic surface layer of zinc, cadmium, silver or aluminium by reaction with chromic acid or chromates to form chromium hydroxide/chromate, The resulting layers have also been found useful in treating metal surfaces which are corrosion-resistant per se as such layers are distinguished by being very thin and can be used for achieving a particularly decorative effect.
Although the chromate treatment has the advantages of excellent anticorrosive and decorative properties, and although the method is simple and inexpensive, the use of chromate is restricted by the regulations applying to environmental pollution, and chromate causes problems, such as toxicity to the workers exposed to chromate during the treating process, and difficult disposal of the chromate sludge after the precipitation from the spent solution. In addition, a possibility exists of chromate being released from the chromate-treated products.
Buttner, Jostan and Ostwald, Galvanotechnik 80 (1989) No. 5, pages 1589-1596, have tested various methods for their applicability as replacements of the chromate treatment. Among these possible methods, Buttner et al mention formation of layers containing molybdenum and tungsten by treatment with molybdenum and tungsteniso- or heteropolyacids or salts thereof in connection with zinc-coated surfaces. The isopolyacids form polymeric anions with the same metal atom, such as HW.sub.6 O.sub.21.sup.5-. The heteropolyacids are formed from the isopolyacids with mineral acids and provide mixed anions, such as P(W.sub.12 O.sub.40).sup.3-. Molybdic acid H.sub.2 MoO.sub.4, ammonium heptamolybdate (NH.sub.4).sub.7 Mo.sub.6 O.sub.24.4H.sub.2 O, molybdatophosphoric acid H.sub.3 [P(Mo.sub.3 O.sub.10).sub.4 ].XH.sub.2 O, ammonium paratungstate (NH.sub.4).sub.10 H.sub.2 W.sub.12 O.sub.42.XH.sub.2 O, phosphotungstic acid H.sub.3 [P(W.sub.3 O.sub.10).sub.4 ].XH.sub.2 O, and silicotungstic acid H.sub.4 [Si(W.sub.3 O.sub.10).sub.4 ].XH.sub.2 O are examples of such compounds which according to Buttner et al. can form molybdenum-containing or tungsten-containing layers on zinc coatings. The layers are precipitated from 2% solutions at room temperature and in some cases with addition of small amounts of acid, base or oxidation agents, such as hydrogen peroxide or sodium perborate. In connection with phosphomolybdic acid, a 2% solution contains approximately 12 g/l of molybdenum and approximately 0.3 g/l of phorphorus corresponding to a molar ratio Mo/P of 12.9. The tests performed by Buttner et al. demonstrate that the materials in question provide a certain passivating effect, but the protecting effect thereof cannot compete with the effect of conventional chromate treatment.
GB-PS No. 1,041,347 discloses a process and a treatment solution for corrosion-protection of metal surfaces, where for instance Example 2 of This publication describes the treatment of steel or zinc- or c
REFERENCES:
patent: 2839439 (1958-06-01), Stapleton
patent: 4264378 (1981-04-01), Oppen
Patent Abstracts of Japan, vol. 12, No. 344, C-528, abstract of JP, A, 63-100194 (Kawasaki Steel Corp.), 2 May 1988.
Dialog Information Services, File 351, World Patent Index 81-92, Dialog accession no. 009011675, WPI accession no. 92-139008/17.
Von U. Buttner et al; "Suche nach Chromatierungsalternativen", Galvanotechnik, pp. 1589-1596 (1989).
Bech-Nielsen Gregers
Tang Peter T.
IPU Instituttet for Produktudvikling
Silverberg Sam
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