JPH0312153B2 - - Google Patents

Description

[Detailed description of the invention]

This invention particularly relates to a surface treatment method for imparting excellent pitting corrosion resistance to the surface of a processed Al alloy containing Zn. Traditionally, Al alloys generally have relatively good corrosion resistance,
It is widely used in various fields because it has excellent thermal and electrical conductivity, weldability, and moldability. In particular, Al alloys exhibit extremely good corrosion resistance in neutral environments such as freshwater and seawater, and this is due to the strong oxide film formed on their surfaces. However, even in such an oxide film on the surface of an Al alloy, if it inevitably contains impurities or is mechanically damaged, these areas will not be able to exhibit sufficient corrosion resistance. , selective dissolution progressed, resulting in significant local corrosion, that is, pitting corrosion. In particular, this tendency for pitting corrosion occurs when the environment to which the Al alloy surface is exposed contains heavy metal ions such as Cu ions;
This was noticeable when the Al alloy contained relatively large amounts of impurities such as Fe, Si, and Cu. Therefore, in order to impart excellent pitting corrosion resistance to the Al alloy, 0.1 to 1.0% by weight of Zn was added, thereby reducing the strength of the oxide film, which had excellent corrosion resistance, that had been formed on the surface of the Al alloy. Zn-containing Al has been designed to have a general corrosion type, thereby suppressing localized corrosion and pitting corrosion.
Al alloys have been proposed, but when processing these Al alloys, for example, during cold rolling the temperature of the cold rolled material rises to over 100°C due to the heat generated, and during extrusion processing it is usually heated to about 500°C. It is exposed to quite high temperatures, and the heating pattern must be uneven, and as a result, the strength of the oxide film formed on the surface will vary, resulting in a considerable improvement in pitting corrosion resistance. However, the current situation is that it is still not fully satisfactory. Therefore, from the above-mentioned viewpoint, the present inventors conducted research on the relationship between the oxide film formed on the surface of Zn-containing Al alloy after processing and pitting corrosion resistance, and found that Zn :The oxide film formed on the surface of the as-processed Al alloy containing 0.1 to 1.0% by weight is strong but uniform as a whole, so localized corrosion, or pitting corrosion, occurs in relatively weak parts of this oxide film. However, this non-uniform oxide film can be etched by dipping and holding in a caustic soda aqueous solution having a temperature of room temperature to 80°C and a concentration of 3 to 20% for 1 second to 10 minutes. The Zn-containing Al alloy can be completely and easily removed, and the oxide film naturally formed on the surface of the Zn-containing Al alloy after this etching treatment is relatively weak and uniform throughout. They obtained the knowledge that the corrosion pattern on the alloy surface is completely general corrosion type, and there is almost no occurrence of pitting corrosion. Therefore, the present invention was made based on the above findings, and the reason why the Zn content and etching treatment conditions were limited as described above will be explained below. (a) Zn content Only when Zn is included in the Al alloy,
By etching the Al alloy surface after processing, a weak and uniform oxide film can be spontaneously formed on the surface.
Therefore, if the content is less than 0.1%, the strength and weakness of the oxide film formed after the etching treatment will become uneven, making it impossible to secure the desired pitting corrosion resistance.On the other hand, if the content exceeds 1.0% If this happens, the oxide film becomes too weak, which not only significantly increases the corrosion rate but also causes local corrosion, so the Zn content in the Al alloy was set at 0.1 to 1.0%. (b) Etching treatment conditions The temperature and concentration of the caustic soda aqueous solution, as well as the immersion retention time, are determined economically.
When the temperature is room temperature to 80℃, the concentration is 3 to 20%, and the immersion holding time is 1 second to 10 minutes, the etching treatment on the Al alloy surface after processing will give the Al alloy surface pitting resistance. It is possible to form an excellent oxide film. That is, if any of the conditions of the temperature and concentration of the caustic soda aqueous solution and the immersion retention time are below the above lower limit values, the desired excellent pitting corrosion resistance cannot be secured; If any one of the temperature, concentration, and immersion holding time exceeds the above upper limit, the loss of the Al alloy surface will be significant, leading to unnecessary loss of the Al alloy. In addition,
The most desirable conditions are that the temperature of the caustic soda aqueous solution is 40 to 60°C, the concentration of the solution is 8 to 12%, and the immersion holding time is 10 to 60 seconds. Next, the surface treatment method of the present invention will be specifically explained using examples. Example Al alloys 1 to 4 having the final compositions shown in Table 1 were melted using a normal melting method, cast into an ingot, homogenized, and then Al alloys 1 to 4 were melted and cast into an ingot. For No. 3, hot rolling was carried out under normal conditions, and further warm rolling was performed while the temperature of the hot rolled material was maintained at 120°C to obtain a rolled sheet material with a thickness of 1 mm. On the other hand, regarding Al alloy No. 4, was made into an extruded square material having a cross-sectional size of 30 mm x 5 mm by extrusion processing, and then subjected to aging treatment at a temperature of 180° C. for 6 hours. First, a test piece with dimensions of 20 mm x 80 mm was cut out from the rolled plate material of Al Alloy 1 obtained as a result, and after washing this test piece with acetone,

[Table] Etching treatment was performed using a 10% caustic soda aqueous solution at a temperature of 50°C. piece (hereinafter referred to as test piece)
(b)), (3) Test piece etched for 30 seconds (hereinafter referred to as test piece).
(c)), (4) Test piece etched for 5 minutes (hereinafter referred to as test piece).
(d)), respectively, and then the test pieces of (a) to (d) above were subjected to a neutralization treatment by immersion in a 10% nitric acid aqueous solution,
A test piece for anode polarization measurement was prepared by washing with water and drying. Then, using these four types of test pieces, the temperature: 40
The relationship between electrode potential (saturated calomel reference) and current density was measured in 3% saline at 0.degree. C., and an anode polarization curve was created. The results are shown in FIG. From the results shown in Figure 1, it is generally said that pitting corrosion begins to occur when the electrode potential becomes higher than -800mV. ), pitting corrosion occurs at an extremely low current density of 6 μA/cm ² , whereas in both test pieces (b) ~
(d), that is, a test piece with an etching time of 3 seconds
In (b), the current density for pitting corrosion was increased to approximately 11 μA/cm ² , and in particular, the current density for specimens (c) and (d) that were etched for 30 seconds and 5 minutes was significantly higher. The electrode potential is saturated over the range, indicating that pitting corrosion is unlikely to occur. In addition, for the above test pieces (a) to (d), in a state where an anode current of 10 μA/cm ² was applied in 3% saline solution,
Tests were conducted for immersion for 1 hour, 5 hours, and 20 hours, and the occurrence of pitting corrosion on the surface of the test piece after the test was observed. The observation results are shown in Table 2. As is clear from the results shown in Table 2, test pieces (b) to (d) etched according to the surface treatment method of the present invention all exhibit excellent pitting corrosion resistance. Therefore, pitting corrosion occurred significantly in the test piece (a), which had a surface that had not been washed with acetone.

[Table] Furthermore, the rolled plate materials of the above Al alloys 1 to 3 and
A test piece with dimensions of 50 mm x 30 mm was cut out from an extruded square material of Al alloy 4, and the above test piece was subjected to etching treatment except that the temperature and concentration of the caustic soda aqueous solution and the immersion time were as shown in Table 3. Prepare a test piece under the same conditions as in (a) to (d), and use the resulting test piece to
A tap water immersion test was conducted under the condition of immersion in tap water containing 1 p.pm of ions for 30 days, and after the test, the number of pitting corrosion and the maximum pitting depth on the test piece surface were measured. The measurement results are also shown in Table 3.

【table】

[Table] As shown in Table 3, etching according to the surface treatment method of the present invention provides much better pitting corrosion resistance than when no etching is performed. It is clear that it can be done. As described above, according to the surface treatment method of the present invention, extremely excellent pitting corrosion resistance can be imparted to the processed Zn-containing Al alloy surface with an extremely simple operation.

[Brief explanation of the drawing]

FIG. 1 is an anode polarization curve diagram.

Claims (1)

[Claims] 1. On the surface of processed Al alloy containing at least 0.1 to 1.0% by weight of Zn, 1.
A surface treatment method for imparting excellent pitting corrosion resistance to an Al alloy surface, which is characterized by performing etching treatment under conditions of immersion and holding for seconds to 10 minutes.