JPH0153353B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a post-treatment method for imparting corrosion resistance to an aluminum-zinc composite plated steel sheet hot-dipped in an aluminum-zinc bath and preventing black discoloration. Recently, due to the demand for high corrosion resistance for plated steel sheets, some aluminum-zinc composite plated steel sheets (hereinafter abbreviated as Al-Zn composite plated steel sheets), which are steel sheets melt-plated in an aluminum-zinc bath, have been put into practical use. I'm getting used to it. There are two types of Al-Zn composite plated steel sheets: one with a Zn-based Al-Zn alloy and one with an Al-based Al-Zn alloy.
Conventionally, when exposed to the atmosphere or in a humid environment while still plated, the Al immediately after plating
-The metallic luster characteristic of Zn composite plated steel sheets is lost, resulting in a gray-black appearance (hereinafter abbreviated as black discoloration).
The drawback was that it significantly reduced the product value. This black discoloration usually does not occur uniformly over the entire surface, but preferentially occurs in areas that have been machined, and in some cases, it takes a short period of 2 to 3 months after construction on roofs and walls to reach customers. It occurred within the interval.
For this reason, it has been difficult to store and ship Al-Zn composite plated steel sheets without post-treatment. As a post-treatment for Al-Zn composite coated steel sheets, chromate treatment, which has been commonly used for post-treatment of hot-dip galvanized steel sheets and hot-dip aluminum coated steel sheets, can be considered. After conducting various investigations, it was found that although chromate treatment improves corrosion resistance, it actually promotes black discoloration. In particular, the effect of promoting black discoloration due to this chromate treatment is more pronounced as the Al-Zn composite plated steel sheet has a lower Al%.
-Zn composite plated steel sheet was not so great when it was 55% Al-Zn based, but it was significant when it was 5% Al-Zn based. For this reason, when conventionally applying chromate treatment to Al-Zn composite plated steel sheets for the purpose of corrosion resistance, the amount of chromate film was kept to a minimum in terms of corrosion resistance, taking into account the effect of promoting black discoloration. However, if the amount of chromate film is minimized, corrosion resistance will be poor, and white rust may occur during product inventory, especially during the rainy season, making it difficult to harmonize corrosion resistance and black discoloration through chromate treatment. It was hot. In general, for Al-Zn composite plated steel sheets, a method of forming a film containing a chromium compound as a post-treatment for corrosion resistance is inexpensive and has excellent corrosion resistance. However, chromate treatment promotes black discoloration as described above and cannot be applied. Therefore, the present inventors formed a film containing a chromium compound using a method other than chromate treatment, and furthermore, the chromium compound in the film does not come into contact with the surface of the Al-Zn composite plated steel sheet even when exposed to the air or in a humid environment. As a result of considering a post-treatment method to achieve this, we found that by using an aqueous solution containing hexavalent chromium and resin as a post-treatment liquid, we determined that the dry film thickness and amount of chromium compounds in the film after treatment with this post-treatment liquid would be appropriate. It has been found that if the composition is adjusted so as to provide excellent corrosion resistance, and the resistance to black discoloration is also significantly improved. In other words, the post-treatment method of the present invention uses a chromium compound to impart corrosion resistance and a resin to prevent the chromium compound from leaching onto the surface of the steel sheet. An aqueous solution containing emulsion resin or both of these resins is
It is characterized in that it is directly applied onto a composite plated steel plate and then dried to form a resin film with a dry film thickness of 0.5 to 4 μm and a chromium content of 5 to 50 mg/cm ³ . The present invention will be explained in detail below. First, the post-treatment liquid is an aqueous solution containing (1) chromic acid or chromate, or both, and (2) a water-soluble resin, an emulsion resin, or both. The reason for containing chromic acid or chromate or both is to mainly supply hexavalent chromium compounds into the film. For this reason, these substances need to be aqueous hexavalent chromium compounds, and the preferred ones are chromic acid anhydride, and potassium chromate or dichromate in the case of chromate. , ammonium, magnesium, cobalt, nickel,
Examples include salts of manganese, zinc, calcium, and strontium. However, among chromates, those whose cations serving as counterions of chromic acid or dichromic acid form hygroscopic chromium compounds in the film (for example, sodium ions) are not preferred. This means that Al-Zn composite plated steel sheets have good corrosion resistance and good corrosion resistance even if they are used unpainted or painted.
Moisture resistance is extremely important, and if a hygroscopic chromium compound were dispersed in the film, it would absorb moisture in the air or in a humid environment, which would be disadvantageous in terms of moisture resistance. The reason for containing a water-soluble resin, an emulsion resin, or both is to prevent the chromium compound contained in the coating from coming into contact with the surface of the Al--Zn composite plated steel sheet. By the way, when incorporating these resins into an aqueous solution,
Coexisting chromic acid and chromate are strong oxidizing agents, so if they are easily oxidized, they will decompose and form a film, making it impossible for the resin to achieve its original performance, and chromium compounds are also trivalent chromium. As the amount increases, corrosion resistance also decreases. Further, due to the oxidation reaction, a precipitate is formed in the aqueous solution, or the aqueous solution is turned into a gel. Therefore, it is necessary to use a resin that has excellent oxidation resistance and does not cause precipitation or gelation even after being stored for two weeks or more. As such resins, polyacrylic resins, polyacrylonitrile resins, or modified resins containing these as main components are preferable. In addition, these resins have extremely excellent weather resistance, and when painted, have good film adhesion with various paints, and are not very selective with paints. The present invention uses an aqueous solution having the above composition as Al-Zn.
It is applied to a composite plated steel plate and dried.
The coating may be performed by a known method such as a roll coating method or a roll drawing method. Further, drying may be performed using a known method such as a hot air drying oven or an infrared lamp at a substrate temperature of 60 to 250°C. When applying, make sure that the resin film has a thickness of 0.5 to 4 μm after drying. This is because the surface of the Al-Zn composite plated steel sheet has a microscopic surface roughness of 1 to 3 μm, and if it is less than 0.5 μm, it is difficult to form a uniform film on such a surface, which is industrially difficult. It is from. In particular, the aqueous solution to be applied has a low viscosity, often ranging from a few CPS to several hundred CPS at most.
After application, it flows along the irregularities of the surface roughness, and the concave parts become liquid reservoirs, and the convex parts become extremely thin or partially exposed, providing sufficient corrosion resistance and black discoloration prevention function. I can't. On the other hand, if the thickness exceeds 4 μm, spot welding, which is widely used in processing this type of galvanized steel sheet, becomes impossible as shown in FIG. In general, it is better to thicken the resin film in order to improve corrosion resistance and black discoloration resistance.
This increases the cost and is not desirable. Further, during coating, the chromium content in the resin film after drying is adjusted to 5 to 50 mg/cm ³ as chromium element. This is because if the chromium content is less than 5 mg/ ^cm3 , the corrosion resistance required for the application of Al-Zn composite plated steel sheets cannot be obtained, as shown in Figure 2.
This is because if it exceeds mg/cm ³ , the L value (lightness) measured by a color difference meter becomes small as shown in FIG. 2, and black discoloration becomes significant. The reason why black discoloration becomes significant as the chromium content increases is as follows. In other words, the resin film is 0.5~
As it is thin at 4μm, it allows moisture to pass through easily. For this reason, the resin film undergoes a cycle in which it swells with water when the humidity is high and shrinks due to drying when the humidity is low. During this cycle, water-soluble chromium compounds in the film migrate downward, ie, toward the steel plate, and concentrate at the interface between the resin film and the plating layer, promoting black discoloration. Therefore, as mentioned above, if the chromium content in the resin film is limited to a certain level, black discoloration can be prevented and corrosion resistance can also be maintained high. In the specification of the chromium content in the present invention, mg/cm ³ and the amount per volume are used, and the chromium content is not expressed as the amount per unit surface area, which is generally used for conventional chemical conversion coatings. This is due to the dispersion density theory. The famous conventional Al-Zn composite plated steel sheets are 5% Al-Zn based and 8% Al-Zn based.
Al-based systems such as 15% Al-Zn systems, and 55% Al-Zn systems and 75% Al-Zn systems are known as Al-based systems, but the post-treatment method of the present invention can be applied not only to these systems. Generally, it is applicable to Al-Zn composite plated steel sheets whose plated layers mainly contain Al and Zn.
For example, Mg, Mn, Si, Ti,
It is also possible to use materials to which Ni, Co, Mo, Pb, Sn, Cr, rare metals (La, Ce, Y, Nb, etc.) are added. Example Post-treatment was performed by directly applying a post-treatment solution with the composition shown in Table 1 on various Al-Zn composite plated steel sheets with different plating layer compositions using a roll coater, and then drying. Pieces were taken and subjected to a salt spray test, a wet test, and a spot welding test to investigate corrosion resistance, black discoloration resistance, and weldability, respectively. The results are shown in Table 2. Next, the remaining post-treated pieces were painted, and test pieces were taken in the same manner to conduct a paint film adhesion test and a post-painting corrosion resistance test.

【table】

【table】

[Table] As is clear from Table 2, the products treated with the post-treatment method of the present invention have excellent black discoloration resistance, retain metallic luster immediately after plating, and have superior corrosion resistance compared to the conventional reinforced chromate treatment. It is equivalent to or better than Comparative Example No. 6). As is clear from Comparative Example No. 7, black discoloration can be prevented by treating the emulsion resin alone with a post-treatment liquid that does not contain chromic acid or chromate, but the corrosion resistance is significantly inferior. In addition, products that are chromate-treated and then subjected to post-treatment to form only a resin film have excellent corrosion resistance, but the chromium compound contacts the plating layer, resulting in significant black discoloration. The spot weldability is the same as that of the chromate treated one (Comparative Example No. 6) and the untreated one (Comparative Example No. 9).
Does not impair weldability. In addition, it has excellent paint film adhesion and corrosion resistance even when coated with materials having considerably different properties such as epoxy-polyester paints and alkyd paints. Although no abnormalities were observed in the post-treatment liquid used in the present invention even after two weeks had passed since its preparation, decomposition and gelation were observed in Comparative Example No. 5. As described above, since the post-treatment method of the present invention forms a film in which the chromium compound is dispersed in the resin, the chromium compound does not come into contact with the surface of the Al-Zn composite plated steel sheet, and the metal immediately after plating is removed. Maintains shine. In addition, its corrosion resistance is comparable to that of conventional chromate treatment.
It also has excellent paintability. Note that the post-treatment method of the present invention can also be applied to post-treatment of aluminum alloys and aluminized steel sheets.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between the film thickness of the resin film formed when the Al--Zn composite plated steel plate is subjected to the post-treatment of the present invention and the weld failure rate. Figure 2 shows the chromium compound content (i.e. dispersion density) and brightness (50
400 hours after a humidity test at 98% RH) and corrosion resistance (occupancy rate of white rust occurrence after a salt spray test).

Claims (1)

[Claims] 1. An aqueous solution containing chromic acid or chromate, or both, and a water-soluble resin or emulsion resin, or both, is directly applied onto an aluminum-zinc composite plated steel sheet and then dried to form a film after drying. A post-treatment method for an aluminum-zinc composite plated steel sheet, characterized by forming a resin film having a thickness of 0.5 to 4 μm and a chromium content of 5 to 50 mg/cm ³ .