Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Reexamination Certificate
1999-10-12
2001-09-11
Wyszomierski, George (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
C148S251000, C148S261000, C148S262000
Reexamination Certificate
active
06287394
ABSTRACT:
This invention relates to anti corrosion treatment of galvanised steel having galvanising coatings of aluminium, zinc or alloys thereof, particularly aluminium/zinc alloys.
Alloys of aluminium, for example 55% by weight of aluminium, the balance substantially zinc (55 Al/Zn) are widely used as protective surface coatings for steel in order to prevent rust and corrosion of the steel substrate. However when surfaces of such coated steels are held in close proximity (as occurs, for example, in coils or packs of sheets) and are exposed to moisture during storage and the moisture is retained for a period, then the protective coatings can themselves readily become susceptible to corrosion. Based on their appearance, the residues of such corrosion are referred to as white rust for principally zinc only coated steels and black rust for coated steels whose coatings have a significant amount of aluminium contained within them. These residues generally make such coated steel unsaleable despite the fact that the overall service life of the coated steel may remain the same. The ability to resist such corrosion is referred to as wet stack performance.
In order to prevent the formation of such rust the coated steel surfaces are usually treated, prior to dispatch to customers, with a chromium containing solution, that treatment being hereinafter referred to as chromium passivation. While this provides a reasonable level of protection against rust, chromium solutions are highly toxic and therefore disposal of chromium residues from such treatment is difficult and expensive.
It is known to use a non-chromium passivation system such as that which involves the use of phosphates and/or molybdates as is described in U.S. Pat. Nos. 4,385,940 and 4,264,378. However, such systems do not provide adequate wet stack performance.
It is also known to apply a resin coating on top of the chromium passivation and/or incorporate the chromium within the resin as described in U.S. Pat. Nos. 3,053,692, 3,053,693, 3,630,791 and 4,637,840. While the use of such resins can increase the wet stack performance of steel coated with aluminium, zinc or alloys thereof in comparison with the performance of chromium passivation on its own, the resins provide another advantage in that they can remove the necessity for applying lubricating oil to aluminium or aluminium alloy coated steel during shaping, for example roll forming. This is because aluminium and aluminium alloy coatings do not possess the self lubricating properties of zinc coated steels, as recognised in the latter U.S. Pat. No. 4,637,840. Such lubricants add an additional cost to forming operations and may cause their own problems in regard to health and safety.
It is an object of the present invention to ameliorate one or more of the above disadvantages of the prior art. It is an object of preferred embodiments of the invention to provide a coated steel which does not necessitate use of a lubricating oil during ordinary shaping, and which is not chrome passivated but which nevertheless has acceptable wet stack performance.
According to one aspect, the present invention relates to a method for treating a surface of zinc or of an alloy of zinc and aluminium, said method comprising the step of treating said surface with an aqueous solution having a pH of below 3 and comprising a complex formed from at least one species of metal oxo ion in conjunction with at least one species of hetero ion, the metal oxo ion being selected from the group consisting of molybdate, tungstate and vanadate; the hetero ion being selected from the group consisting of phosphorus (V), aluminium (III), silicon (IV), manganese (II), manganese (IV), zirconium (IV), titanium (IV), tin (IV), cerium (III) and nickel (II); and with a film forming acid tolerant resin compatible with the selected ions.
According to a second aspect, the present invention consists in a dispersion comprising:
at least one metal oxo ion species in conjunction with at least one hetero ion species, the metal oxo ion being selected from the group consisting of molybdate, tungstate and vanadate; the hetero ion being selected from the group consisting of phosphorus (V), aluminium (III), silicon (IV), manganese (II), manganese (IV), zirconium (IV), titanium (IV), tin (IV), cerium (III) and nickel (II) in an aqueous medium at below pH 3, said medium having dispersed therein a compatible film forming acid tolerant resin.
In preferred embodiments of the invention the resin is a non ionic surfactant stabilized suspension. In highly preferred embodiments the ratio of weight of metal of the complex to resin in the bath is less than 0.5 moles per kilogram of dry resin solids. Desirably the film forming resin comprises one or more acid tolerant crosslinking agents.
Molybdenum is preferred as the metal of the oxo ion while phosphorus is the preferred hetero ion.
The invention will now be more particularly described by way of example only.
Resins suitable for the invention should be selected so as to remain stable during storage and application at the pH involved which is preferably below pH 2 and so as to avoid destabilisation by the presence of metals in the solution. The acid stable resins may be selected from the groups of water soluble and water dispersible resins and for preference are combined with the aqueous solution.
Such resins typically consist of addition polymers prepared by the incorporation of unsaturated monomers such as methyl methacrylate, butyl acrylate, ethyl acrylate, styrene, and the like, and unsaturated functional monomers such as hydroxyethyl acrylate, acrylic acid, methacrylic acid, and the like into acrylate ester or copolymer emulsions.
Additionally, preferred resins comprise sterically stabilised latex particles formed using non-ionic surfactants and which tolerate the low pH involved, typically below pH 2, and remain stable. Suitable resins may be thermosetting or thermoplastic by nature, or may contain functional groups as known to the art for the purpose of crosslinking with other resin additives. However, should they be reactive in this manner, they are most suitable if not rapidly crosslinked by the presence of the metals in the solution.
Highly preferred resins are those dispersions of polymer particles described in International Patent Application No. PCT/AU90/00565 or U.S. Pat. No. 4,504,618. These dispersions utilize non ionic surfactant stabiliziers and have been found to be surprisingly tolerant to the addition of metal ions and to remain stable at an acidic pH below pH 3 and typically below pH 2.
Molybdates such as ammonium molybdate, sodium molybdate or molybdic acid are usually employed as the source of the metal oxo ion species, but other compounds containing a metal oxo ion may also be used. Typically the molybdate is present in a concentration of from 0.02 to 0.12 moles of molybdenum per kilogram of resin solids and more preferably from 0.03 to 0.06 moles of molybdenum per kilogram of resin solids.
If the concentration of metal is excessive, the integrity of the resin film deteriorates with consequent reduction in corrosion protection. If the concentration is too low the treatment is ineffective.
The hetero ion species used in the present invention may be added to the metal oxo ion solution as an acid or salt. Preferably, phosphoric acid is used as the source of phosphorus, aluminium chloride is used as the source of aluminium and sodium silicate is the preferred source of silicon whereas zirconium tetrachloride is the preferred source of zirconium.
Desirably, aluminium etching agents are added to the solution for example, sodium fluoride or sodium tetrafluoroborate, but other aluminium etching agents may also be employed. Typically from 0 to 0.012 moles of fluoride per kilogram of resin solids is used in the present invention and more preferably from 0.004 to 0.008 moles of fluoride per kilogram of resin solid. Desirably, the resin is selected to be compatible with the fluoride so that the fluoride can be added to the latex and applied from a single bath.
It has been found that the anti
Buxton David Peter
Riley Peter John
BHP Steel (JLA) Pty. Ltd.
Kerins John C.
Miles & Stockbridge P.C.
Wyszomierski George
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