JPS5854199B2 - Anti-corrosion coating method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for successfully painting dissimilar metal joints without causing paint film peeling.

Various methods have been considered for painting parts where two or more metal members are joined together, but peeling of the paint caused by battery action between the joints has become a major problem, especially after painting the joints of dissimilar metals. There is.

That is, a steel plate whose surface is plated with a different metal, such as a metal with a higher ionization tendency than iron, such as zinc or aluminum, or a steel plate in which a coating film is further formed on the plating layer of these plated steel plates, If the part where a steel plate is joined to an unplated steel plate is corroded with, for example, a 5% sodium chloride aqueous solution, the metal in the plating layer, which has a higher ionization tendency than iron, becomes an anode due to the battery action between the metals. , the following reaction formula: (In the above formula, when M is Zn, n=2, and when M is A7, n=3.

), metal ions are eluted and rust occurs.

On the other hand, the iron of the steel sheet becomes a cathode, and a reaction expressed by the following reaction formula occurs, so that the elution of iron ions is suppressed in the iron part, and no rust is observed.

The purpose of so-called zinc plating on galvanized iron and the like is to take advantage of the battery sacrificial corrosion property of zinc as expressed by the above reaction formula.

Therefore, a product made by joining dissimilar metals, for example, a product made by joining a cold-rolled steel sheet and a galvanized steel sheet, which is subjected to pre-treatment for the paint base and then electrodeposited, is immersed in a 50I) sodium monochloride aqueous solution. Or, when the aqueous solution is sprayed to cause rust, white rust of zinc oxide occurs on the zinc surface, but no rust occurs on the iron surface.

However, in this case, the electrodeposited coating on the iron surface swells and peels off from the iron surface, resulting in a serious coating defect.

This is because the electrodeposited coating has acidic groups such as carboxyl groups as functional groups in the resin, so the coating swells and peels off due to the alkaline-rich aqueous solution generated by the cathode reaction that occurs in the iron part. It's for a reason.

When the iron surface where the paint film has peeled off dries, iron oxide rust will form.

As described above, with conventional painting methods, it was not possible to obtain a good coating film that was sufficiently resistant to corrosion caused by salt water, etc.
The present inventor has discovered the use of a conductive paint as a method for painting joints between dissimilar metals.

Until now, conductive paints have been made by coating a conductive paint on plastic solder, which cannot be electrodeposited because it is not conductive, to form a conductive coating, and then applying electrodeposition coating.
When electrocoating is used to form a smooth coating film, or when electrocoating is applied to areas where brass brazing is applied to joints between steel plates, uneven steps are formed on the surface of the electrocoat coating, so conductive coating is not recommended. This method was used to form a uniform, smooth coating by pre-forming a coating on the same area and then applying electrodeposition.

However, in either method, the purpose and effect of the present invention are different.

The method according to the present invention involves pre-treating the joints of dissimilar metals and their surroundings for use as a base for painting, coating this with a paint containing a conductive pigment to form a conductive paint film, and then forming the conductive paint film. It is characterized by an electrically conductive coating applied on top.

That is, in order to prevent the electrodeposition coating film on the iron side from swelling and peeling as described above, the method according to the present invention uses a resin as a main component which has better chemical resistance, especially alkali resistance, than electrodeposition coating. It is characterized by applying a paint containing a conductive pigment to the joint of dissimilar metals and its surroundings.

The conductive paint is prepared by pre-treating the base for painting, specifically spray degreasing with 2 parts of a weak alkaline degreaser at a temperature of 60'C for 2 minutes, then washing with water, then applying 2 parts of a zinc phosphate treatment solution, Total acidity 17 points, free acidity 0.9 points, accelerator value 1
．． After spraying for 2 minutes at a 5-point temperature of 50°C, washing with tap water and then with deionized water, then applying after drying, drying thoroughly, and then applying electrodeposition coating, it is rich in alkalinity generated in the cathodic reaction. Swelling of the coating film caused by the aqueous solution and subsequent peeling of the coating film from the steel plate can be prevented.

Drying after applying the conductive paint can be done either naturally or by baking, but if drying is insufficient, components in the conductive paint film may be eluted into the electrodeposition paint during the next step of electrodeposition painting. It is desirable that the conductive coating film be thoroughly dried, since it may be mixed in and have an adverse effect on the electrodeposition paint.

As the pigment contained in the conductive paint used in the present invention, conductive materials such as graphite powder, aluminum powder, zinc powder, copper powder, or iron powder can be used, and if necessary, these pigments can be used. Type 1 or 2
Use a combination of more than one species.

Further, as the vehicle for the coating composition used, any one can be used as long as it is a cured type and has better chemical resistance, especially alkali resistance, than an anionic electrodeposition coating.

Specifically, acrylic resin and epoxy resin can be used.

The amount of conductive pigment to be blended is preferably about 20 to 200 parts by weight based on 100 parts by weight of the solid content of the vehicle.

The rust prevention method according to the present invention can be applied to all cases involving joints of dissimilar metals.

In addition, since the method according to the present invention is a conductive coating, it is possible to obtain a smooth coating surface even if electrodeposition is applied later, and electrodeposition can also be applied to areas coated with conductive paint in a duct shape. Since a film is formed, the appearance quality and rust prevention properties are not inferior in any way to those of electrodeposition coating alone.

The present invention will be explained in more detail using Examples, but the present invention is not limited to these Examples.

Reference Example 1 This example is intended to explain the corrosion state of a product in which the flat joint portion of dissimilar metal steel plates is painted by the conventional method.

As shown in FIG. 1, there are a steel plate 1 and a steel plate plated with a metal that has a higher ionization tendency than iron, such as a galvanized steel plate 2.
After pre-treatment for the base of coating is applied to the joined portion, electrodeposition coating is performed, and the resulting electrodeposition coating film 4 is coated with an intermediate coat on one side and then with a top coat.

5 represents a painted layer formed by overlapping the intermediate coating film and the top coating film, and 3 represents the plated portion.

When aqueous solution of sodium chloride of the 5th grade is sprayed onto the painted area to promote rusting (according to the method of JIS Z2371), rusting will occur on the galvanized surface 7 which will serve as the anode.
This occurs due to the formation of oxides of the plating metal.

This reaction causes the ends 8 and 9 of the steel plate 1 to become cathodes.
The corrosive aqueous solution 6 in the vicinity becomes alkaline, which causes the electrodeposited coating 4 to swell, causing the coating to peel off from the steel sheet base.

After a while, rust will form on the parts where the paint film has peeled off.

Example 1 This example is intended to explain the corrosion state of a product in which a flat joint of dissimilar metal steel plates was painted using the painting method according to the present invention.

In Fig. 2, a conductive paint is applied to the joint between a steel plate 1 and a steel plate plated with a metal that has a greater tendency to ionize than iron, such as a galvanized steel plate 2, after pretreatment for a base for painting. An electrodeposited coating 4 is further formed on the conductive coating 10, and an intermediate coating and a top coating are applied to one side.

Even if the anode part is caused to rust using a 5th group sodium chloride aqueous solution in the same manner as in the reference example, the alkaline aqueous solution 6 will rise slightly to the cathode part, but the conductive coating film 10 is resistant to alkali. Because it is durable and both steel plates are connected by the same conductive film, the battery phenomenon is less likely to occur, swelling and peeling are less likely to occur, and there is almost no rust on the steel surface, so the results are good. It is.

Embodiment 2 Fig. 3 shows an example in which dissimilar steel plates are joined by hemming, and Fig. 4 is a cross-sectional view of an example in which dissimilar steel plates are joined by flanging.

When each joint is painted in the same manner as in Example 1, no peeling of the paint film occurs even if rusting is promoted by the method of the reference example, and the results are good.

The distance 13 from the end of the steel plate base to the peeled paint film obtained in the above Reference Examples and Examples 1 and 2 was measured, and the results are summarized in the following table.

Example 3 As shown in FIG.
When electrodeposited coating is applied after forming the coating film 11 and dust 11', as is well known, no electrocoated coating film is formed on the non-conductive coating film 11 and dust 11', and therefore the dust part is extremely This results in a thin film, which not only deteriorates the smoothness of the painted surface, but also has very poor corrosion resistance.

On the other hand, FIG. 6 is an enlarged view of the end portion 12 of the conductive coating film in FIG. Since it is formed on the coating film 10 and on the dust portions 10', 10', . . . of the conductive coating film, it has smoothness and has very good corrosion resistance without any deterioration.

[Brief explanation of the drawing]

Figure 1 is a cross-sectional view showing a flat joint of dissimilar metal steel plates coated with a conventional coating method and rusted with a corrosive solution.
The figure is a cross-sectional view showing a flat joint of dissimilar metal steel plates coated with the coating method of the present invention and rusted with a corrosive solution.
The figure is a cross-sectional view of a hemming joint joint coated with the coating method of the present invention, Figure 4 is a cross-sectional view of a flange joint joint coated with the coating method of the present invention, and Figure 5 is a cross-sectional view of a hemming joint joint coated with the coating method of the present invention. Cross-sectional view of electrodeposition coating after forming a neutral coating film,
FIG. 6 shows a cross-sectional view of a steel plate after forming a conductive coating film and then applying electrodeposition coating. In the figure, 1... Steel plate, 2... Metal plated steel plate, 3... Plating layer, 4... Electrodeposition coating film, 5...・・・(Intermediate coat, 10 top coats) Coating film, 6...
- Represents a chlorine ion-containing corrosive aqueous solution, 10... conductive coating film.

Claims (1)

[Claims] 1. An anti-corrosion coating method characterized by pre-treating the joints of dissimilar metals that corrode due to battery action and their surroundings for use as a base for painting, coating them with a conductive pigment, and then performing electrodeposition coating.