Anticorrosion treatment of metal coated steel having coatings of

Details Anticorrosion treatment of metal coated steel having coatings of Anticorrosion treatment of metal coated steel having coatings of

1996-09-30

1999-11-16

Sells, James

Metal treatment

Process of modifying or maintaining internal physical...

Processes of coating utilizing a reactive composition which...

148251, 148261, 148262, 148273, 148275, C23C 2200

Patent

active

059850477

DESCRIPTION:

BRIEF SUMMARY
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: ion species, the metal oxo ion being selected from the group consisting of molybdate, tungstate and vanadate; the hetero ion being selected

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