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.