Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Reexamination Certificate
2000-07-06
2003-03-25
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
C148S262000, C428S472300
Reexamination Certificate
active
06537387
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a new process for the corrosion protection of steel strips coated with zinc or zinc alloy. The present process provides, on the one hand, a temporary corrosion protection for transport and storage purposes. In this connection, the term “temporary corrosion protection” means that the metal surfaces are effectively protected from corrosion during transport and storage periods until they are coated with a permanent anticorrosive layer, such as a lacquer. On the other hand, the process according to the present invention serves as a preliminary treatment of the metal surfaces prior to a coating, which may be carried out on the metal surfaces immediately after the application of the present process.
BACKGROUND OF THE INVENTION
As a measure for providing temporary corrosion protection, steel strips coated with zinc or zinc alloy are either simply oiled or, where greater corrosive stresses are anticipated, they are phosphated or chromed. These measures are, however, inadequate in the case of particularly high corrosive stresses, such as ship transport in a salty sea atmosphere or storage in a tropical environment. The best-known temporary corrosion protection measure is chroming, during which the metal surfaces are coated with a chromium(III)- and/or chromium(VI)-containing layer generally in a layer weight generally of about 5 to about 15 mg/m
2
chromium. Owing to the well-known toxicological problems of chromium compounds, this process is disadvantageous and expensive from the aspects of industrial safety, the environment and the necessary disposal.
Moreover, chromed metal sheets are not very suitable for a subsequent phosphating, as, on the one hand, they result in a contamination of the cleaning solutions by chromium and, on the other hand, the whole of the metal surface cannot as a rule be phosphated. A phosphating as an alternative measure for providing temporary corrosion protection may alter the appearance of the metal surfaces in an undesirable way. A phosphating is, moreover, expensive as regards plant, as, depending on the substrate material, it requires an additional activating step and, after the phosphating, it generally requires a passivating step. The passivating is frequently carried out using treatment solutions containing chromium, as a result of which the above-mentioned disadvantages of the use of chromium-containing treatment solutions also arise here.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a process for the corrosion protection of steel strips coated with zinc or zinc alloy, which is less problematic environmentally and is simpler to carry out technically than are the above-mentioned corrosion protection measures. This new process is to be at least equal to the conventional processes with regard to coatability and adhesion of the coating, but in addition, is to improve the corrosion protection for storage purposes.
This object is fulfilled by a process for the corrosion protection of steel strips coated with zinc or zinc alloy, characterised in that the steel strips coated with zinc or zinc alloy are brought into contact with an aqueous treatment solution having a pH of from 1.5 to 3.5, which contains:
1 to 20
g/l manganese(II) ions and
1 to 150
g/l phosphate ions,
and the solution is dried without intermediate rinsing.
DETAILED DESCRIPTION OF THE INVENTION
Electrolytically zinc-coated or hot-dip zinc-coated steel strips accordingly are suitable as substrate materials for the process according to the present invention. The steel strips may also be coated with zinc alloy, that is, they may have a layer of zinc alloy applied electrolytically or in the hot-dipping process. Here, the most important alloying components for zinc are iron, nickel and/or aluminum. The thickness of the zinc layer or of the layer of zinc alloy is generally between about 2 and about 20 micrometers, in particular between about 5 and about 10 micrometers.
The applied treatment solution is dried without intermediate rinsing. Processes of this type are known in the industry as no-rinse processes or dry-in-place processes. In the process, the treatment solutions may be sprayed onto the metal surfaces or applied by passing the steel strips through the treatment bath. The required quantity of the treatment solution remaining on the metal surface which results in the intended layer weight of 1 to 5 g/m
2
may be adjusted here by squeezing rollers. However, it is useful to apply the treatment solution directly in the required layer weight by a system of rollers, for example, of the type known as “Chemcoater”.
The treatment solution for the process according to the present invention contains from 1 to 150 g/l, preferably 10 to 70 g/l, phosphate ions. Here, the phosphate content is calculated as phosphate ions. The person skilled in the art is, however, aware that, at the pH within the range of 1.5 to 3.5 to be established according to the present invention, only a very small proportion of the phosphate is present as triply negatively charged phosphate ions. Rather, there is an equilibrium of free phosphoric acid, primary and secondary phosphate ions, which is dependent on the acid constants of the phosphoric acid for the various protolysis steps and on the pH actually selected. Within the selected pH range, the greater part of the phosphate is present as free phosphoric acid and as primary and secondary phosphate ions.
The corrosion protection to be achieved by the process according to the present invention may be further improved if the treatment solution contains in addition one or more of the following components:
up to 10 g/l, preferably between 2 and 4 g/l, zinc ions,
up to 10 g/l, preferably from 3 to 6 g/l, nickel ions,
up to 20 g/l, preferably between 3 and 7 g/l, titanium ions, which are used preferably as hexafluorotitaniate ions,
up to 50 g/l, preferably between 15 and 25 g/l, silicon ions in the form of silicon compounds, such as hexafluorosilicate ions and/or finely-disperse silica having an average particle size of less than 10 &mgr;m,
up to 30 g/l fluoride ions, which may be introduced as free fluoride in the form of hydrofluoric acid or of soluble alkali metal fluorides or ammonium fluoride or in the form of hexafluoro anions of titanium or of silicon. Free fluoride, irrespective of whether it is introduced into the solution as free acid or as soluble salt, at the pH to be established in the treatment solution, will be present as a mixture of hydrofluoric acid and of free fluoride ions.
The treatment solution may also contain up to 150 g/l, preferably between 60 and 125 g/l, of one or more polymers or copolymers of polymerisable carboxylic acids selected from acrylic acid, methacrylic acid and maleic acid, and esters thereof with alcohols having 1 to 6 carbon atoms. When in this connection reference is made generally to the “treatment solution”, this means that the organic polymers, depending on type, may also be present as a suspension in the solution of active substances. It is also the case here that, depending on acid constants of the carboxylic acids used, at the established pH of the treatment solution, these are present as a mixture of free acids and acidic anions. It is particularly preferred that one or more polymeric carboxylic acids be used, together with at least one of the above-mentioned optional components zinc, nickel, titanium, silicon and fluoride.
Manganese and, if desired, zinc and nickel may be introduced into the treatment solution in the form of water-soluble salts, for example, as nitrates. It is preferred, however, not to introduce any other foreign ions into the treatment solution apart from the components given above. Manganese, zinc and nickel are therefore introduced preferably in the form of the oxides or carbonates, so that they are ultimately present as phosphates in the treatment solution. It is also preferable that titanium, silicon and fluoride be used in the form of hexafluoric acid. If need be, the silicon may be introduced in the form of finely disperse si
Harper Stephen D.
Henkel Kommanditgesellschaft auf Aktien
Oltmans Andrew L.
Seifert Arthur G.
Sheehan John
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