A system for fixing anodized aluminium

Abstract: A system for fixing anodized aluminium surfaces comprises a bath (1) of nickel fluoride solution in which the items are immersed following anodization, and a tank (2), connected with the bath by way of flow and return pipelines (4, 5) and containing a negative ion exchange resin bed charged with F-­ fluorine ions; the solution is circulated between bath and tank by a recycle pump (3), and the resin bed will be recharged by a regeneration device (7) whenever the fluorine ions have been totally or almost totally replaced by other negative ions.

Description: The present invention relates to a system for the fixing of anodized aluminium. Following the anodization of aluminium, an oxide layer forms on the surface of the metal; this layer is porous and therefore readily open to attack from extraneous substances that cause its deterioration. Accordingly, subsequent fixing is required. The traditional method of fixing formerly employed was to immerse the anodized items in boiling water containing salts of nickel, for a period of some 20...40 minutes duration. Being slow and costly One of these hot methods but including triethanolamine additive rather than Ni salts is described in FR 2438098; in this document, the application of anion resins is also described, to eliminate polluting anions from the bath. Being slow and costly, this method has been replaced latterly by a process that is quicker and consumes less energy, whereby the anodized items are immersed in a cold bath containing predominantly nickel fluoride dissolved at a rate of 5 grams/litre. The cold method in its turn has certain drawbacks: not only is the cost of nickel fluoride relatively high, but the nickel fluoride solution is easily contaminated by external agents; this leads to a marked reduction in the quantity of fluoride ions in solution, and ultimately to substandard results in fixing. Contamination occurs, for example, through the presence of calcium ions; these form fluorides which precipitate and steal fluoride ions from the solution. Also, the solution contains aluminium ions that form fluoraluminate compounds, the effect of which is to render the fluoride ions unusable. Such, by way of example, are the principal sources of contamination to which the fixing solution is subject. One expedient adopted in order to eliminate these difficulties consists in adding ammonia to induce precipitation of the aluminium and successively eliminate the precipitate, aluminium hydroxide, from the solution; the effectiveness of this method is offset by the drawback of its being long and laborious. The object of the invention is to overcome the aforedescribed drawbacks through the provision of a system by means of which anodized aluminium fixing solutions can be purified simply, swiftly and economically, while maintaining the concentration of fluoride ions at prescribed levels. The stated object is realized, together with other objects, with a system as characterized in the claims appended. The system according to the invention will now be described in detail, by way of example, with the aid of the accompanying drawing, which provides a schematic illustration of the essential components. The system disclosed comprises a bath 1 filled with solution containing nickel fluoride added at a rate of 5 grams/litre approx.; in solution, the nickel fluoride dissociates into Ni++ and F- ions. It is into this bath 1 that the anodized items to be fixed are introduced. Inevitably, the bath 1 is a repository of certain contaminants such as calcium, aluminium, etc. which are carried in the solution in the event that this is not completely demineralized (often the case), or brought in either as traces of the preceding processes effected on the aluminium, or indeed by the aluminium itself. Such agents tend to steal fluoride ions from the solution, replacing them with other negative ions such as Cl- chlorine and SO4-- sulphate; in addition, there is a tendency toward the formation of AlF6--- type fluoraluminate ions. 2 denotes a tank accommodating ion exchange resins in a bed of conventional type, by which negative ions are exchanged; the resin is charged initially by the usual methods (shortly to be mentioned) with F- fluoride ions. The tank 2 is connected to the bath 1 by way of a flow pipeline 4 and a return pipeline 5. A pump 3 installed on the flow line 4 permits recycling of the solution from the bath 1 through the tank 2 as and when required; more exactly, the solution is transferred by the pump 3 from the bath 1 to the tank 2 by way of the flow line 4, and restored to the bath from the tank by way of the return line 5. A conventional monitoring and control device 6 is used to verify the concentration of a selected ion at a given point along the return line 5; in the preferred embodiment described and illustrated, it is the concentration of Cl- chloride ions that is monitored by the device 6. Finally, 7 denotes a conventional regenerating device which, when operated, once the initial charge of fluoride ions has been exchanged wholly or in part, recharges the resin bed in the tank 2 with new fluoride ions (to be described in due course). Operation of the system according to the invention will now be described. First, it should be stated that the pump 3 can be operated either continuously or intermittently, at the user's discretion. The continuous mode is to be preferred, generally speaking, as this ensures a continuous purification of the solution contained in the bath 1. The solution is pumped from the bath 1 through the tank 2, and thus brought into intimate contact with the resin bed; the resins have a particularly low affinity with the fluoride ion, so that this ion is retained far less readily than all other negative ions. Following passage of the solution through the tank 2, negative ions in the solution other than fluoride are retained by the resin bed, which for each negative ion captured releases one negative F-fluoride ion. In short, while in the bath 1 one has an exchange in which fluoride ions are taken from the solution and replaced by other negative ions, the exact opposite occurs in the tank 2, so that the concentration of fluoride ions in the bath 1 is maintained as required. Throughout operation, a constant check is kept by way of the monitoring device 6 on the concentration of chloride ions in the return pipeline 5; the fact of selecting the chlorine ion for control purposes is due to the, ease and precision with which this particular ion can be monitored. Once the depletion of the charge of fluoride ions in the resin bed has reached a fairly advanced stage, the chloride ions (always preponderant in number) will no longer be replaced as the solution passes through the tank 2; accordingly, their presence begins to register in the return pipeline 5, and once the concentration at the point sensed by the monitoring and control device 6 exceeds a given threshold, the device will shut off the recycle pump 3 and switch in a further pump 8, connecting the tank 2 to the regeneration device 7 which then proceeds to recharge the resin bed with F- negative fluoride ions. Regeneration of the resin bed 2 can be effected using a variety of conventional media, from which the selection will in any case be made according to whether weak or strong resins are utilized. In a preferred system, sodium hydroxide (NaOH) could first be introduced, followed by hydrofluoric acid (HF), and the bed then flushed. Needless to say, the system will incorporate valves (not illustrated) to prevent any circulation of the solution through bath 1 and tank 2 while the resin bed is recharging. In addition to the advantages already mentioned, a system as described and illustrated affords the facility of preparing a bath 1 with nickel salts far less costly than nickel fluoride; for instance, nickel sulphates might be used; in the example illustrated, indeed, it becomes possible to recycle the solution in such a way that SO4-- ions brought into solution using nickel sulphate are exchanged entirely for F- ions, and whilst it is true that a solution of this kind imposes the requirement for more frequent, hence costlier regeneration of the resin bed, the overall cost of operating the system can nonetheless be reduced from that which would be incurred using a nickel fluoride solution for the fixing bath 1.

Claims: A system for fixing anodized aluminium, of the type comprising a bath (1) of nickel salt solution at room temperature in which items of anodized aluminium are immersed, characterized in that it comprises: a tank (2), connected with the bath (1) by way of a flow pipeline (4) and a return pipeline (5) and accommodating a conventional negative ion exchange resin bed charged initially with F- fluoride ions; pump means (3) serving to recycle the solution contained in the bath by way of the flow line, the tank and the return line; a conventional device (7) by means of which to recharge the resin bed with F- fluoride ions as and when required. A system as in claim 1, comprising a monitoring and control device (6) by means of which to verify the concentration of a given ion through the return pipeline. A system as in claim 2, wherein the monitoring and control device is of a type designed to verify the concentration of Cl- chloride ions. A system as in claim 2, wherein operation of the regeneration device (7) is triggered by a signal supplied from the monitoring and control device (6) whenever the concentration of the ion in the return pipeline exceeds a selected threshold. A system as in claim 1, wherein the bath(1) contains a nickel fluoride solution.

Inventors: GARUTI GIUSEPPE, GARUTI, GIUSEPPE
Applicants: GARUTI GIUSEPPE, GARUTI, GIUSEPPE
Application year: 1990
Main classification: C25D11
IPC codes: C25D11, C25D21