JPH0428881A - Aluminum alloy material with surface having superior treatability with zinc phosphate - Google Patents

Abstract

PURPOSE:To improve the treatability of an Al alloy with zinc phosphate by forming a Zn-based metal coating film on the surface of the Al alloy and further forming a composite coating film consisting of Zn, Fe and SiO2. CONSTITUTION:A metal plating layer of Zn, Zn-Fe contg. a small amt. of Fe, etc., is formed as an underlayer by 0.3-2g/m<2> on the surface of an Al alloy material as a lightweight metallic sheet for the panel of an automobile, etc. The plating layer is then plated with 0.3-1g/m2 composite coating film contg. fine SiO2 particles in a Zn-Fe alloy with>40wt.% Zn so that the total Zn content of both the plating layers is regulated to 50-90wt.%. The Al alloy material with both the plating layers has improved treatability with zinc phosphate, considerably superior finishability and filiform corrosion resistance.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to an aluminum alloy material having a surface that is highly susceptible to zinc phosphate treatment. , automobile panel materials)
The present invention relates to a surface-treated aluminum alloy material that provides excellent coating film adhesion and thread rust resistance, and also has excellent image clarity.

(Prior art and problems to be solved) In recent years, aluminum alloys have been used in automobile parts for the purpose of weight reduction, but they are usually used in applications that require thread rust resistance, such as panel materials. Zinc phosphate treatment is used as a base treatment on automotive paint lines, and in this case,
At present, sufficient effects in terms of thread rust resistance have not been obtained as a surface treatment for aluminum alloys.

As an aluminum alloy material for automobile panels, AQ
-Mg-Cu alloys are mainly used;
This is because water-based alloys tend to cause precipitation of zinc phosphate and have excellent thread rust resistance compared to AQ-S i -Mg-based alloys.

On the other hand, AQ-5i-Mg alloy has high strength after paint baking and has excellent properties as an automobile panel material, but conversely, precipitation of zinc phosphate is difficult to occur and it has sufficient thread resistance. Since rust resistance cannot be obtained, the actual situation is that it is hardly used in automotive panel applications where zinc phosphate is used as a base treatment as described above.

Therefore, as a surface treatment method to promote the precipitation of zinc phosphate, a method of applying Zn plating to the surface has already been proposed in JP-A-61
No. 157693, etc., specifically, electroplating and melt plating are mentioned.

Further, as a method for forming a Zn plating layer on the surface of an aluminum alloy, for example, chemical substitution plating (zincate treatment), which is commonly performed as a base treatment for plating, is well known. However, the zincate treatment is carried out as a base treatment for plating, and the following steps are essential to ensure the adhesion of the zincate film.

Alkaline etching → nitric acid neutralization → 1st stage zincate → nitric acid stripping → 2nd stage zincate (Water washing is required between each step) Furthermore, since it is performed for the purpose of plating, the surface is highly active and exposed to the atmosphere. Since it is easy to cause corrosion such as discoloration and powdering, for example, when used as a surface treatment for automobile panel materials, the treatment cost increases and the corrosion resistance of the treated film is not sufficient.

As mentioned above, zinc phosphate treatment properties can be improved by forming a Zn-based metal film on the surface of an aluminum alloy, but with conventional surface treatment techniques, the adhesion of the film is insufficient and corrosion resistance is poor. is also insufficient. Although the adhesion of the film can be improved by performing two-stage zincate treatment, the growth rate of the film is slow and the treatment process is complicated as mentioned above, making it difficult to put it to practical use in industrial mass production. Met.

The present invention was made in order to solve the problems of the above-mentioned conventional technology, and it is easy to deposit zinc phosphate, has excellent zinc phosphate treatment properties, and has excellent paint finish properties, thread rust resistance, etc. The purpose of the present invention is to provide a surface-treated aluminum alloy material that can improve the properties of aluminum at a low cost.

(Means for Solving the Problems) In order to solve the above problems, the present inventors have conducted intensive research on the development of a coating base treatment method that is low cost and has excellent thread rust resistance. It has been found that by uniformly and finely precipitating zinc phosphate, a remarkable improvement in thread rust resistance after painting can be obtained.

To do this, first, Z is applied to the surface of the aluminum alloy material.
A layer of n-based metal is formed, and then a layer of a metal and oxide composite film composed of metal Zn containing Zn, Fe, and Si oxide is formed to a predetermined film thickness, and phosphorus is formed. It has been discovered that zinc phosphate precipitation can be promoted at the cathode portion on the surface of an aluminum alloy material by acting as an anode in a zinc oxide solution, and the present invention has been made based on this discovery.

That is, in the present invention, 0.3
g/m2 or more and 2 g/m2 or less of a metal Zn-based metal layer, and further thereon, 0.3 g/m2 or more and 1 g/m2
Below, an aluminum alloy having a surface that is excellent in zinc phosphate treatment property is characterized in that it is provided with a metal and oxide composite film made of Zn and Fe-based metals with a Zn content of 40% by weight or more and 81 oxide. The gist is the material.

The present invention will be explained in further detail below.

(Function) In the present invention, Zn precipitated on the aluminum alloy surface
Forms a cell on an aluminum surface in a zinc phosphate treatment and acts as an anode. Precipitation of zinc phosphate occurs due to an increase in pH on the surface of the material to be treated, and the increase in pH occurs in the cathode region.

Further, the pH of the anode portion also increases due to the pH-adhesive action of the eluted metal. In addition, the composite film consisting of metallic Zn, Fe, and Si oxides dissolves during zinc phosphate treatment, and the pH
promotes the buffering effect of Therefore, according to the invention:
By proactively configuring the local battery, it is possible to increase the amount of precipitation.

In the zinc phosphate treatment of aluminum alloys, at the initial stage of immersion in the zinc phosphate bath, only the dissolution of aluminum progresses;
Although there is an induction period during which zinc phosphate precipitation does not occur, it has been found that by shortening the induction period, the number of precipitation nuclei can be increased and uniform and fine zinc phosphate crystals can be obtained. The composite film on the aluminum surface of the present invention is effective in shortening the induction time, so that it is possible to ensure uniform and fine precipitation of zinc phosphate in an amount necessary and sufficient to improve paint finish and thread rust resistance. Therefore, it is possible to significantly improve the surface quality and thread rust resistance of, for example, an automobile panel material which is subjected to zinc phosphate treatment, cationic electrodeposition, intermediate coating, and top coating.

Next, the reasons for limiting the film structure in the present invention will be explained.

First, Zn is first formed on the surface of the aluminum alloy.
The metal layer (lower layer) effectively incorporates Fe in the composite film formed on the Zn metal and Fe-based and Si oxides into the zinc phosphate bath, thereby improving the alkali resistance of the zinc phosphate. This is intended to contribute to improving the adhesion of the film.

If the amount of precipitated Zr-based metal film in the lower layer is less than 0.3 g/m, the necessary amount of zinc phosphate may not remain due to dissolution during the degreasing and cleaning process before zinc phosphate treatment. , the lower limit is 0.3 g/m2.

Note that an even smaller amount may be used depending on the precipitation situation and the zinc phosphate treatment process.

On the other hand, the upper limit of the precipitation amount of the lower layer Zn-based metal film may be within a range in which Zn does not exist as a layer after the zinc phosphate treatment, and the upper limit is usually 2 g/m2. Zn
Although there is no problem even if the amount of precipitation is further increased depending on the form of precipitation, the upper limit is set at 2 g/m2 in consideration of manufacturing costs and the like.

The composite film formed on the above-mentioned lower layer of the aluminum alloy surface must be composed of metal Zn and Fe and Si oxide, and Si oxide is essential for corrosion protection of the metal layer consisting of Zn and Fe. It is also necessary to improve the adhesion of the film to the aluminum interface. In addition, the amount of this composite film only needs to be the amount necessary to maintain the anodic reaction in the zinc phosphate bath; further precipitation will result in a layer containing active Zn remaining on the coating base. Undesirable.

Specifically, the Zn content in this composite film must be at least 40% by weight in order to act as an anode point during zinc phosphate treatment, and if it is less than 40% by weight, the precipitation rate of zinc phosphate will be reduced. This is not preferable because it will be insufficient.

The composite film is necessary to supply Fe to the zinc phosphate crystals for the purpose of improving the alkali resistance of the phosphoric acid sub-button girders. If the amount is less than 0.3 g/m2, the effect is insufficient, and if it exceeds 1 g/m2, the effect is saturated and the manufacturing cost increases, so the amount of the composite film should be 0.3 g/m2 or more,
g/m2 or less.

Note that the Zn content in the entire film is preferably 90 to 50% by weight. If it exceeds 90% by weight, the corrosion resistance of the film will be insufficient and discoloration will occur easily during storage.
If it is less than % by weight, the elution of Zn will be delayed and it will tend to remain after zinc phosphate treatment, which may lead to a decrease in corrosion resistance, which is not desirable.

Lower layer (metallic Zn-based metal layer) and upper layer (metallic Zn and Fe
and S1 oxide composite film), the method for forming these films is electroplating.

There are various methods such as substitution plating and thermal spraying, but in consideration of the Zn precipitation form, precipitation treatment properties, cost, surface characteristics after coating, finishing properties, etc., substitution treatment by chemical treatment is preferable.

When forming the lower and upper layers, wash with water after forming the lower layer,
Since the upper layer can be formed continuously, continuous processing becomes possible and cost reduction can be expected.

As mentioned in JP-A No. 61-157693, in the case of electroplating, a combination of degreasing process and surface treatment for chemical plating is essential, and in fused Zn plating, due to the material properties of the material, both Undesirable.

In the present invention, the precipitation site of zinc phosphate is not on the surface of the Zn plating layer but on the aluminum alloy surface near the precipitated Zn, and this point is different from the above-mentioned JP-A No. 61-157693.
This is a major difference from the issue.

Therefore, in the present invention, pretreatment for precipitation includes:
It is only necessary that the surface of the treated material is degreased to the extent that it does not repel water, and no special pretreatment is required when performing precipitation treatment after annealing. Since the amount of precipitation is small and no special pretreatment is required, productivity is improved and production costs can be significantly reduced.

When the aluminum alloy material according to the present invention is used for automobile panels, it is necessary to perform simultaneous treatment with iron, so it is subjected to zinc phosphate treatment, followed by cationic electrodeposition, intermediate coating, and top coating. Since the amount of zinc phosphate precipitated is large and is precipitated uniformly and finely, the finish quality of the coating is improved, and the thread rust resistance of the coating material, which was the biggest problem when using aluminum panels, is also significantly improved.

The component system and composition of the aluminum alloy material to which the present invention can be applied is not particularly limited, but conventionally, although it has excellent workability, strength, and corrosion resistance for automobile panels, the amount of zinc phosphate precipitation is small and the thread rust resistance is insufficient. AQ-Si-
The effect is remarkable when applied to Mg systems.

Examples of the present invention are shown below.

(Example) First, the material is A6009 (Ai2-5i-Mg type)
and A5182 (AQ-Mg-based) rolled plates (1 mm x 75 mm x 150 mm) of each alloy were prepared.

The A6009 alloy rolled plate described above was subjected to various surface treatments shown in Table 1, and then the coating was dissolved with concentrated nitric acid, and the weight of the coating was measured. The reason why concentrated nitric acid was used here is that concentrated nitric acid does not dissolve metal aluminum, and the film can be dissolved and chemically analyzed. The results are shown in Table 1.

From Table 1, it is clear that the examples of the present invention have a coating amount and a coating structure within the range of the present invention.

A3182 and A600 with the same surface treatment as Katsuomata Example 1
9 alloy rolled sheet was degreased and surface treated in the same way as automotive panel materials, and then coated with cationic electrodeposition (film thickness 20 mm).
μm) to evaluate thread rust resistance. The groundwork is
Employed silicate-based degreasing cleaning → surface activation treatment and phosphoric acid resistant tincture treatment. Thread rust resistance was determined by salt spray (5%, 35°C,
1 day)→humidity test (50° C. x 85% RH 16 days) was defined as one cycle, and thread rust resistance was evaluated after 5 cycles. In addition, zinc phosphate treatment properties and film adhesion were also evaluated. The results are shown in Table 2.

As is clear from Table 2, all of the examples of the present invention have excellent zinc phosphate treatment properties, and are excellent in thread rust resistance and film adhesion.

[Blank 1 (Effects of the Invention) As detailed above, according to the present invention, the amount and morphology of zinc phosphate precipitated are significantly improved, and zinc phosphate treatment properties are excellent, resulting in improved paint finish properties, Thread rust resistance can be greatly improved. In particular, when the present invention is applied to the Afl-5i-Mg alloy material, which has conventionally been considered to be insufficient in terms of thread rust resistance due to the difficulty in precipitation of zinc phosphate, and use as an automobile panel material, Currently used surface treatment line (for iron)
Since direct treatment is possible, the cost is low, and the problem of thread rust resistance, which has conventionally been the biggest problem with zinc phosphate treatment, is solved, so the effect is remarkable.

Patent applicant: Kobe Steel, Ltd. Representative Patent Attorney Takashi Nakamura

Claims (3)

[Claims] (1) Form a metallic Zn-based metal layer of 0.3 g/m^2 or more and 2 g/m^2 or less on the surface of the aluminum alloy, and further layer 0.3 g/m^2 or more and 1 g/m ^2 or less and has a surface that is excellent in zinc phosphate processability, characterized by being provided with a metal and oxide composite film made of Zn and Fe-based metals and Si oxides with a Zn content of 40% by weight or more. Aluminum alloy material. (2) The aluminum alloy material according to claim 1, wherein the Zn content in the entire film is in the range of 90 to 50% by weight. (3) The aluminum alloy material according to claim 1 or 2, which is used as a panel material for an automobile.