Galvanized or alloy-galvanized steel strips are provided with temporary anticorrosion protection either by simply oiling them with anticorrosion oils or, if exposure to more severe corrosive conditions is expected, by phosphating or chromating them. A multistage process generally takes place prior to final coating with organic binders (primers, paints). When galvanized metal strip or aluminum and the alloys thereof are used in the domestic appliance and architectural industry, optionally after prior removal of the oil layer, the metal surface is firstly provided with an anticorrosion layer. The best anticorrosion measure known in the prior art is chromating, in which the metal surface is coated with a layer containing chromium(III) and/or chromium(VI), generally in amounts of for instance 5 to 15 mg/m2 of chromium. Phosphating as an alternative temporary anticorrosion measure has two disadvantages: on the one hand, the appearance of the metal surfaces may be undesirably changed. On the other hand, phosphating is highly complex in terms of plant engineering, since, depending on the substrate material, it requires an additional activation stage and generally a passivation stage after phosphating. Apart from an anticorrosion effect, the inorganic coating also provides good adhesion to the primer applied thereon. The primer in turn not only has a favorable influence on the anticorrosion action of the inorganic conversion layer, but the primer layer itself in turn also provides good base for the topcoat material.
Strip finishers increasingly supply sheet metal with a preapplied coating, which for example facilitates mechanical processing such as stamping, drilling, folding, profiling and/or deep drawing. This preapplied coating must impart to the sheet metal properties which not only inhibit corrosion but also facilitate mechanical processing. A further function of such preapplied coatings is to provide a base for subsequent decorative topcoat layers. A workpiece produced by mechanical processing from a sheet metal material precoated in this manner may then finally be provided with a topcoat. The preapplied coating according to the invention furthermore preferably acts as a primer, the topcoat layer then being applied immediately thereafter in the coil coating plant. In this case, only once it is fully coated is the sheet metal material sent for further processing. According to the prior art, the coil coating method substantially consists of three sub-steps. In a first step, the metal strip is cleaned and provided with an (inorganic) pretreatment layer, in the second step the primer is applied, and in the final third method step the topcoat is applied. In some applications, clear coats or protective film coverings may additionally be applied. Attempts have been made to provide just with the pretreatment step those functions which facilitate use in the manner of the above-stated preapplied coating. For example, a suitable organic polymer is added to the inorganic conversion treatment for this purpose, which polymer is capable of forming a surface film exhibiting the functional properties required of a preapplied coating.
EP-A-685534 describes a method for protecting a steel substrate by a thin film of an organic/inorganic hybrid polymer based on an alkoxysilane, a further condensable organometallic compound of the formula M(OR)4 and (meth)acrylic acid, water and a polymerization initiator. Coating is effected by thermal or photopolymerization. Zirconium and titanium are mentioned as metals for the organometallic compound. It is stated that such a film protects steel substrates from corrosion and oxidation and this coating is moreover intended to protect the substrate from impact and other mechanical effects.
DE 197 51 153 describes polymerizable chromium-free organic compositions containing titanium, manganese and/or zirconium salts of olefinically unsaturated polymerizable carboxylic acids and further olefinically unsaturated comonomers and an initiator for free-radical polymerization and the use thereof for organic coil coating of metallic materials. These non-aqueous polymerizable compositions allow chromium-free pretreatment of steel materials with anticorrosion properties.
WO 00/69978 describes a polymerizable chromium-free anticorrosion agent containing metal complexes of an α-unsaturated carboxylic acid of the general formula (1):
in which                R1 and/or R2 is/are H, C1- to C12 alkyl, aralkyl or the group —CO—O—Y,        R3 is H or C1 to C12 alkyl,        Me is a titanium, silicon or zirconium ion,        X is H, C1 to C12 alkyl, aryl or aralkyl, alkoxyl, aroxyl, sulfonyl, phosphate, pyrophosphate,        Y is H, C1 to C12 alkyl or Me, and n is 0 to 4,            at least one further olefinically unsaturated comonomer with at least two olefinically unsaturated double bonds per molecule,    optionally further comonomers with one olefinically unsaturated double bond per molecule,    at least one initiator for free-radical and/or cationic polymerization.
In relation to an anticorrosion coating with an antimicrobial action, DE 102005045441.0, which is as yet unpublished, describes polymerizable chromium-free organic compositions containing metal complexes of derivatized α-unsaturated carboxylic acids of the general formula (2):
in which                R1 and/or R2 may be H, C1- to C12 alkyl, aralkyl or the group —CO—O—Y,        R3 may be H or C1 to C12 alkyl,        Me may be a metal atom with an oxidation state of λ selected from silicon, titanium, zirconium, manganese, zinc, vanadium, molybdenum and tungsten,        X may be H, C1 to C12 alkyl, aryl, aralkyl, alkoxyl or aroxyl or 2(—O—X)=acetylacetonate,        Y may be H, C1 to C12 alkyl or a further metal ion Me,        Z is selected from O, NH, a group O—Zb—C(═O)—O, a group —O—Zb—P(═O)—O, a group O—Zb—P(═O)2—O, a group O—Zb—O—P(═O)—O, a group O—Zb—O—P(═O)2—O, a group O—Zb—S(═O)2—O, a group O—Zb—O—S(═O)2—O, in which Zb represents an organic grouping, and        n is 0 to λ, preferably 1 to (λ-1), in which A means the oxidation state of the metal Me.        
A preferred derivatization for the purposes of the above-stated invention for ensuring adequate anticorrosion protection by the cured coating is the condensation of hydroxyethyl methacrylate with succinic anhydride and the corresponding titanium alkoxides to yield metal complexes which are of the general formula (2).
Despite the wide-ranging prior art, a need still remains for improved anticorrosion agents and coating methods for metal surfaces which permit chromium-free pretreatment of the metallic substrates with good corrosion protection while avoiding strong acids and elevated fluoride concentrations. The constituents should here preferably be homogeneously dissolved or dispersed in the composition in order to avoid segregation during production, transport, storage and use.
Application of the coating by the coil coating method, in which the metal strips to be coated are unwound from and rewound onto the coil on conveyor belts at strip speeds of up to 200 m/min, as is conventionally preferred in the prior art, means that the applied coating agent must be completely cured within the shortest possible time.
The coating itself must meet the requirement that stamping and forming of the components from the coated metal strips can proceed without loss of the anticorrosion properties, i.e. without detachment or destruction of the coating in the formed area. The layers on the metal substrates should furthermore withstand the subsequent manufacturing steps until assembly of the products, such as for example cleaning, optionally phosphating, riveting, welding and be overcoatable with a topcoat, either directly or after mechanical processing. For reasons of environmental protection and industrial safety, it should be possible to perform the treatment method without using chromium compounds and, where possible, also with exclusion of organic solvents.