Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Patent
1995-05-23
1996-05-28
Silverberg, Sam
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
148273, 148275, C23C 2242
Patent
active
055207502
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to anti corrosion treatment of aluminium or aluminium alloy surfaces. In particular, the present invention relates to an anti corrosion treatment of metal coated steel having an aluminium or aluminium alloy surface.
BACKGROUND ART
Zinc, aluminium and/or combinations of aluminium and zinc (Al/Zn), are widely used as surface coatings, particularly but not exclusively for steel for protection against rust and corrosion. In practice, however, the zinc or Al/Zn coatings are susceptible to white rust or black rust respectively when exposed to the atmosphere due to reactions with moisture. Such rust is detrimental to the surface and generally makes coated steel substrates unsaleable despite the fact that the overall service life of the coated steel may remain the same and the formation of rust generally interferes with finishing operations. The ability to resist such corrosion is referred to herein as wet stack performance.
In order to inhibit the formation of rust on coated surfaces it is generally accepted that the treatment of a surface with a chromate imparts anti corrosive properties and this type of treatment is generally referred to as chromate passivation. However chromate is highly toxic to exposed workers and due to its high toxicity, disposal of chromium residues is difficult. Further in various markets the yellow discolouration of the treated coated surfaces is considered to be an unacceptable product attribute.
In order to overcome the problems associated with chromate passivation, phosphate coatings have been used. However the anticorrosion properties of phosphate have been found to be far inferior to the above mentioned chromate treatment.
U.K. Patent Application No. 2,070,073 discloses an anti corrosive treatment for preventing white rust on galvanized steel comprising applying to the surface of a galvanized steel sheet an acidic solution containing molybdic acid or a molybdate in a concentration of 10-200 g/l calculated as molybdenum and adjusted to a pH of 1 to 6 by addition of an organic or inorganic acid. However, it has been found that the corrosion resistance of aluminium and aluminium alloy surfaces treated with the above solution is inferior to the chromate treated substrates under certain conditions and the treated surfaces suffer from an undesirable degree of discolouration. Moreover modybdate treated surfaces have been shown to change from a pale yellow/blue to a strong green colour when stored for periods of time in excess of 24 hours.
Accordingly it is an object of the present invention to provide means for avoiding and/or ameliorating at least some of the above discussed disadvantages of the prior art.
DISCLOSURE OF THE INVENTION
According to one aspect, the present invention consists in an anti corrosion treatment of an aluminium or aluminium alloy surface comprising treating said surface with a solution having a pH of from 0 to 2 and comprising at least one metal oxo ion complexed with phosphorous (V) wherein the metal oxo ion is vanadium or tungsten or a mixture thereof.
Surprisingly, it has been found that the treated aluminium or aluminium alloy surfaces of the present invention provide substantial advantages over the prior art. In particular the vanadate and tungstate treating solution of the present invention provides improved high temperature corrosion resistance and reduce discolouration of the treated surface.
Ammonium and alkali metal salts such as ammonium vanadate and tungstate or sodium vanadate and tungstate are generally the preferred source of the metal oxo ion. Preferably the metal oxo ion is present in a concentration from 10 to 100 g/l and more preferably from 10 to 60 g/l.
In a highly preferred embodiment of the present invention, typically aluminium etching agents are employed during the treating step. Preferred etching agents are those containing at least one fluoride containing compound such as sodium fluoride and sodium tetrafluoroborate. Desirably between 0.4 g/l and 1.5 g/l of fluoride and more preferably betwee
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patent: 4385940 (1995-05-01), Kirihara
Derwent Abstract Accession No. 92-312635/38.
Derwent Abstract Accession No. 74460.
Derwent Abstract Accession No. 249388/13.
BHP Steel (JLA) Pty. Ltd.
Silverberg Sam
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