JP2003082484A - Surface treatment method for aluminum or aluminum alloy - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface treatment method for aluminum or an aluminum alloy by which stable degreasing treatment can be performed in a rising period after initial make-up of an electrolytic bath. SOLUTION: An aluminum can is treated by the following stages including degreasing treatment: prerinsing-prewashing-washing (degreasing treatment)- primary rinsing-treatment (chemical conversion film treatment)-secondary rensing-deionized water rinsing-drying. In the degreasing treatment stage, on initial make-up of an electrolytic bath, aluminum salt is added to an acid bath so that the concentration of aluminum ions is controlled to the prescribed one, and, even in the rising period, the level of the aluminum ion concentration of the stable bath is held.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a surface treatment method for aluminum or an aluminum alloy, and more particularly to a method for degreasing aluminum or an aluminum alloy in an acid bath mainly containing sulfate ions.

[0002]

2. Description of the Related Art Aluminum or aluminum alloys are used in a wide range of fields such as household articles, furniture, electric appliances, and building materials. Further, for example, in beverage can applications, heavy steel cans are used. A large amount of aluminum or aluminum alloy cans are used, which are lighter than those in.

[0003] These aluminum or aluminum alloys have a poor natural corrosion resistance due to the non-uniform natural oxide film formed, and when the surface is coated, this oxide film hinders the adhesion of the paint. Surface treatment is essential.

As for the surface treatment of aluminum or aluminum alloy, similar to the case of steel, chemical treatment is carried out in addition to mechanical treatment such as sandblasting and buffing.

Among these, the standard process for the latter chemical treatment is degreasing → washing → film formation → washing → drying. Furthermore, prior to degreasing, chemical cleaning or water washing may be performed in advance, or etching, water washing, or the like may be performed before film formation.

[0006] For example, when the surface treatment of a beverage can using aluminum or an aluminum alloy is performed, in the treatment process in which a series of baths for performing each treatment described above is arranged, the can with the opening facing downward is endless. Is placed on the mesh conveyor, continuously conveyed, and sequentially processed.

In the degreasing process, for example, an acid solution is sprayed onto the can from above and below the can conveyed on an acid bath to degrease the can. The acid solution after spraying is collected in an acid bath and is recycled.

A series of surface treatments including the above degreasing treatment are started in a state where a liquid having a predetermined composition is prepared in each bath in advance, that is, after the bath is constructed. As the surface treatment is repeatedly applied to innumerable cans, the concentration of the acid liquid component decreases in the degreasing process, so that new acid liquid is sequentially replenished.

In this case, the liquid composition is likely to be unstable for a predetermined period after the work is started, for example, for 2 to 3 days, and the liquid composition is in a so-called unstable bath state.
A so-called stabilizing bath is reached.

In the stable bath, it is possible in principle to carry out stable treatment infinitely thereafter. Depending on various conditions such as supply and demand conditions of the cans to be treated and work conditions, for example, in the case of a pattern in which processing is done only during the day and processing is suspended at night, when restarting processing after suspension, use the bath before suspension to stabilize immediately. The processing that has been performed can be performed.

Therefore, in the degreasing treatment, it is necessary to focus more on operation management for several days after the bath is built up until the stabilizing bath in which the composition of the acid solution is stabilized.

[0012]

However, in the case of the conventional degreasing treatment, as a method of operation control for several days after the building bath until the stabilizing bath in which the composition of the acid solution is stable, the electric conductivity of the acid solution used is changed. Automatic replenishment management of the new acid solution is performed based on the online measurement, and in addition to this, the free acid concentration of the acid solution used is measured off line at a predetermined frequency to grasp the accurate concentration of the acid solution and measure the new acid. Efforts are being made to carry out stable degreasing treatment by modifying the replenishment conditions of the solution, etc., but such management is complicated, and despite such complicated management, The composition is out of the control range,
The fact is that it has not been possible to completely prevent defective degreasing. Then, a large amount of defective cans will be produced within a short time after such degreasing failure.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a surface treatment method for aluminum or aluminum alloy, which enables stable degreasing treatment during the rising period after the bath is built. .

[0014]

The method for surface treatment of aluminum or aluminum alloy according to the present invention is a method for surface treatment of aluminum or aluminum alloy, which comprises degreasing aluminum or aluminum alloy in an acid bath, wherein the acid is applied during construction. It is characterized in that an aluminum salt is added to the bath so as to have a predetermined aluminum ion concentration.

Here, the predetermined aluminum ion concentration is not uniform and varies depending on the specific degreasing treatment process such as the treatment conditions such as the required degreasing amount of aluminum or aluminum alloy, the conditions of the acid bath, etc. The optimum aluminum ion concentration is experimentally determined and set for each process. The concentration of this aluminum ion is
Even if the degreasing process is different, it is generally within the range of several hundreds to several thousands ppm.

In the conventional degreasing treatment, 2 to
During the rising period of 3 days, as described above, the automatic replenishment management of the new acid solution is performed based on the online measurement of the electric conductivity of the acid solution during the cyclic use. However, as the treatment progresses, aluminum ions eluted from the can accumulate in the acid bath, and the aluminum ions in the acid solution during cyclic use gradually increase, resulting in a change in the measured electrical conductivity of the acid solution. However, it does not correspond to the composition of the true acid solution. That is, even if the composition of the true acid solution is constant, as the concentration of aluminum ions dissolved in the acid solution increases, the hydrogen ions in the acid solution are reduced to hydrogen gas, and the hydrogen ions in the acid solution are reduced. The concentration decreases. Therefore, the measured value (indicated value) of the electric conductivity of the acid solution is lowered. As a result, the fresh acid solution is injected in excess of the required amount, and the actual concentration of the acid solution in the bath becomes higher than the specified value.

If this state is overlooked, aluminum or aluminum alloy will be over-etched, causing a problem that the surface becomes rough and the appearance becomes too white. On the other hand, when the concentration of the acid solution in the bath becomes lower than the specified value, the aluminum or aluminum alloy will be insufficiently etched, and a chemical conversion film having excellent corrosion resistance can be formed in the film chemical conversion treatment in the subsequent step. In addition, the adhesion is insufficient. Therefore, for example, the acid solution in the acid bath is manually sampled at intervals of 2 hours, the free acid concentration is manually analyzed, and the correct composition of the acid solution is grasped. Based on this result,
The replenishment condition of the new acid solution is corrected by gradually lowering the set value of electric conductivity.

However, such an action based on the measurement result of the free acid concentration may sometimes cause a delay in the measurement timing of the free acid concentration or a situation in which the measurement timing is missed depending on the work situation. However, in this case, the composition of the acid solution deviates from the specified value. Further, even if the action based on the measurement result of the free acid concentration at 2 hour intervals is surely performed, the composition change of the acid solution which occurs during the 2 hours cannot be avoided.

In order to obtain the aluminum ion concentration condition of the stabilizing bath in a short period of time, in the initial time after the bath construction,
It is conceivable that the amount of make-up water to be supplied separately from the new acid solution will be extremely narrowed or the make-up water will not be supplied until a certain period of time elapses. When the residence time in the bath is increased, the surfactant deteriorates and causes degreasing failure.

On the other hand, according to the present invention, the aluminum ion concentration in the acid solution of the stabilizing bath in the degreasing system of interest is ascertained by a preliminary experiment, etc. Since ions are added and the aluminum ion concentration in the acid solution of the stabilizing bath is maintained from the beginning of the bath, the electrical conductivity and free acid concentration can be measured and controlled in the same manner as before even during the rising period after the bath. By doing so, a stable degreasing treatment can be performed.

In this case, the acid bath contains sulfate ions as a main component and ferric ions, nitrate ions and a surfactant as auxiliary components, and the aluminum or aluminum alloy is a beverage can material. Is preferable.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION A preferred embodiment of the surface treatment method for aluminum or aluminum alloy according to the present invention (hereinafter referred to as the present embodiment) will be described below by taking an aluminum can for beverage as an example. To do.

An aluminum DI (drawing and ironing) can (hereinafter, simply referred to as an aluminum can) is, for example, a 350 ml beer can and is subjected to surface treatment at a unit of several thousand cans per minute.

The aluminum can is treated in the following steps including degreasing treatment.

Pre-rinse → pre-wash → wash: degreasing treatment → first rinse → treatment (chemical conversion coating treatment) → second rinse → deionized water rinse → dry Incidentally, after drying the aluminum can, it is further sent to the painting process, and then the can. Design is applied to the surface of.

As described in the conventional example, the cans are conveyed between the respective steps by arranging the cans with the openings facing downward on the endless mesh conveyor.

The degreasing treatment is carried out by spraying an acid solution at a temperature of, for example, 70 ° C. from spray nozzles provided both above and below the mesh conveyor with respect to a can conveyed by the mesh conveyor on a bath storing an acid solution as a degreasing solution. It is performed by spraying both the inner and outer surfaces of the can for about 30 to 50 seconds. The acid solution discharged from the spray nozzle is the acid solution extracted from the bath by the pump, and the excess acid solution after spraying falls into the bath, is collected, and is circulated and used.

When the degreasing treatment is continuously carried out, the acid solution is consumed in the bath and the water content is reduced by evaporation or the like.
Therefore, the consumption and decrease of the acid solution are grasped by online measurement of the electric conductivity, and the acid solution is automatically replenished so that the electric conductivity maintains a predetermined value, and the liquid level in the bath is maintained at the predetermined value. Add fresh water as you would. In addition, as described above, in parallel with the measurement of the electric conductivity, the free acid of the acid solution is manually analyzed to grasp the true composition of the acid solution, and the electric conductivity of the free acid is measured according to the measured value of the free acid. Correct the supply amount of acid solution by changing the set value. The series of operations themselves have the same contents as the above-mentioned conventional example.

In the present embodiment, a new acid solution filled in the bath at the start of work, that is, at the time of building a bath is prepared to have the following composition.

That is, as the base composition, the chemical shown in FIG. 1 is added in a predetermined amount of water so that the composition of the acid solution shown in FIG. . As the drug, for example, sulfuric acid, ferric sulfate, and aluminum sulfate are used as a sulfate ion source, ferric sulfate is used as a ferric ion source, reagent nitric acid is used as a nitrate ion source, and polyoxyethylene is used as a surfactant. Alkyl ether is used. The concentration of the acid solution is controlled during operation so as to maintain this base composition.

When, for example, a 14-18 hydrate of aluminum sulfate is added as a part of the sulfate ion source at the time of building the bath, the predetermined concentration of this aluminum ion varies depending on the degreasing treatment process conditions. As shown in 2, the specific value is within the range of 500 to 1500 ppm.

At this time, as shown in FIG. 2, the electric conductivity of the acid solution varies depending on the concentration level of aluminum ions in the acid solution even if the base composition shown in FIG. 1 is constant.
As the concentration of aluminum ions increases, the electrical conductivity increases, for example, from 53 (mS / cm) to 49 (mS / c).
m). By the way, as shown in FIG. 2, the electric conductivity of the conventional acid bath in which aluminum ions do not exist at the time of building bath is 53 (mS / cm). The measurement temperature of electric conductivity is 70 ° C.

In the surface treatment method of the above-mentioned embodiment,
Since the aluminum ion concentration in the acid solution of the stable bath is maintained from the beginning of the bath, the electrical conductivity and free acid concentration can be measured and managed in the same manner as before even during the rising period after the bath is built up. As a result, the automatic replenishment control of the acid solution based on electrical conductivity can always maintain the state of the acid solution with less composition fluctuation, and the measurement and control of the free acid concentration corrects and maintains the composition of the acid solution more accurately To be done. This makes it possible to perform a stable degreasing treatment even during the standing-up period after the bath is built up, and to prevent the occurrence of defective degreasing cans.

[0034]

According to the surface treatment method for aluminum or aluminum alloy according to the present invention, in the surface treatment method for aluminum or aluminum alloy in which degreasing treatment of aluminum or aluminum alloy is carried out in an acid bath, during the construction bath, in the acid bath Since an aluminum salt is added to the container so that it has a predetermined aluminum ion concentration, it is possible to stabilize by measuring and controlling the electric conductivity and the free acid concentration in the same manner as before even during the standing up period after the construction bath. Degreasing treatment can be performed, aluminum or aluminum alloy is overetched and the surface becomes rough and the appearance becomes too white, or the etching becomes insufficient and corrosion resistance is insufficient in the film chemical conversion treatment which is a post process. Can be prevented.

According to the surface treatment method for aluminum or aluminum alloy according to the present invention, the acid bath contains sulfate ions as the main component and the second component as the sub-component.
Aluminum or aluminum alloy containing iron ions, nitrate ions and a surfactant is preferably used as a beverage can material.

[Brief description of drawings]

FIG. 1 is a table showing a base composition of a degreasing treatment liquid.

FIG. 2 is a table showing the concentration of aluminum ions added and retained in the bath during the construction bath and the electrical conductivity at that time.

Claims (2)

[Claims] 1. A method for surface-treating aluminum or an aluminum alloy for degreasing aluminum or an aluminum alloy in an acid bath, wherein an aluminum salt is added to the acid bath at a predetermined aluminum ion concentration during the construction bath. A method for treating the surface of aluminum or an aluminum alloy, which comprises: 2. The acid bath contains sulfate ions as a main component and ferric ions, nitrate ions and a surfactant as auxiliary components, and the aluminum or aluminum alloy is a beverage can material. The surface treatment method for aluminum or aluminum alloy according to claim 1, wherein