JPS5920491A - Improvement of secondary adhesion of paint film on zinc plated steel sheet - Google Patents

Abstract

PURPOSE:To form a dense, fine and uniform phosphate film and to improve remarkably the secondary adhesion of a paint coated film, by applying plating of a limited thickness on zinc plating with an iron plating soln. contg. a specific amt. of tin. CONSTITUTION:Fe and Sn are electrodeposited at 0.01-10g/m<2> total on a zinc plated steel sheet from an aq. soln. contg. Fe<2+> and Sn<2+> within a range of <=0.2 (Sn<2+>/Fe<2+>+Sn<2+>). Or Fe is electroplated at 0.01-10g/m<2> on said steel sheet and in succession Sn is electroplated or electroless plated at about <=1g/m<2>. Then, Sn deposits on the part where the Fe plating layer is imperfect so as to coat thoroughly the base zinc, and since a slight amt. of deposited Sn increases the number of nuclei for generating a phosphate crystal, the purpose is presumably achieved. Thus the slight amt. of the deposited Sn is effective and if said amt. is too large, the nonuniform phosphate treatment is resulted.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for improving the secondary adhesion of a coating film on a zinc-based plated copper plate.C When applied to a zinc-based plated steel plate, the adhesion of the coating film is good after the coating is applied. However, the deterioration over time is more significant than that of cold-rolled steel sheets, and generally occurs as paint film blisters and corrosion on the paint film (underfilm fusion), which diminishes the original anti-corrosion effect of the paint film. This will significantly impair the durability of the steel plate.The adhesion of the coating after it has been applied is called primary adhesion, and the adhesion after aging is called secondary adhesion.Secondary adhesion is called hot water immersion. This phenomenon of secondary adhesion deterioration, which can be easily evaluated by an accelerated test using Therefore, in order to improve this secondary adhesion, there is a method of passivating the zinc-plated surface by chromate treatment, and a method of making the phosphate film as a coating base dense and poorly water-soluble. Attempts have been made to date.

Ta. In particular, in recent years, zinc-plated steel sheets have come into use for automobile bodies, and at the same time cationic electrodeposition coatings that provide an alkaline atmosphere have become widespread. The problem of poor secondary adhesion has been attracting attention.Measures to improve secondary adhesion include passivation of zinc through chromate treatment, Fe plating, and coating of zinc-based plating surfaces with Fe-Zn alloy plating. However, chromate treatment requires primary adhesion (especially surface J scratch resistance).
, chromium pollution problems, Fe plating, Fe-Zn
In plating, there are problems such as variations in effectiveness due to incomplete coating, and a sufficient improvement technology for secondary coating adhesion has not yet been established.

The present invention aims to provide a method for further improving the poor secondary adhesion of coating films on galvanized steel sheets. Note that the zinc-based plated steel sheets include bath-dip galvanized steel sheets, hot-dip Fe-Zn alloyed steel sheets, electrogalvanized steel sheets, (2), Ni-Zn alloy plated steel sheets, and other 7. Tamping 11 (generic term for sheets with plating containing n) 6゜When iron plating is applied on zinc-based plating, the secondary adhesion of the coating film is improved, but there is some variation, and it is the same as cold-rolled steel sheet. Performance cannot be obtained.The reasons for this are: (1) the iron plating layer is uneven and there are exposed areas where the base zinc plating is not covered, (2) the phosphate crystals are coarse.
As a result of researching methods to improve these two points, we found that instead of Fe plating alone, we should eutectoid Fe and S11, or we could use Sn after Fe plating. It is extremely effective to precipitate a small amount of
It was found that this resulted in the formation of a dense, fine, and homogeneous phosphate film, and that the secondary adhesion of the coating film was significantly improved and stabilized.

The reason for this is not yet fully clear, but Sn precipitates in the incomplete parts of the built-in layer and completely covers the base Zn, and the small amount of Sn that precipitates somehow increases the number of nuclei of phosphate crystals. 0 Therefore, the amount of Sn precipitated is small and effective. If the amount of Sn precipitated is large, it will cause uneven phosphate treatment, and Sn is expensive, so it is not desirable. 0 Therefore, the amount of Sn precipitated is It is about I S'
/n? The following is desirable: C Effective Fe and Sn plating according to the present invention is achieved by the following method: C 4- (a) Fe and 5llO eutectoid ”
≦0.210) Approximately 0.0 l-1 in water bath liquid containing Sn
The plating of 0f/- is 1-
When 6Sn/'Fe "S11"> 0.2, the precipitation becomes inhomogeneous and the appearance becomes poor. When the plating amount is 0.01 f// or less, almost no 1-effect is observed on the secondary adhesion of the coating film, and 10 Y/n? There is no special effect of "2" after that.

(b) Fe-4Sn sequential plating: Approximately 0.01 to 102/J from normal li'6 plating bath
O) Fe plating is followed by flash cathode electrolysis in a water bath containing Sn2+ to remove a trace amount of Sn. In this case, S11! +If the bath liquid is appropriately selected, chemical displacement plating with Zll is possible, so sealing S11 plating of the iron plating layer can be performed without flash electrolysis.0 Below, the method of the present invention will be described as an example. 〇〇 (p6, 311) Plating, chemical conversion treatment-electrodeposition coating, and coating film secondary adhesion test were performed as follows in each example〇■ (Fe%Sn ) Eutectoid plating (1) Plating bath Fe50.・71(,0280f/
tSnS0410 f/t [5nlFe] Total nail ratio 0.1, pt (= 2.5
Bath temperature 30℃ (2) 'Ii, M 5A/4++/x
3.5 sec (17.3 coulombs/dtr/, all calculated as FG precipitation amount 1 - approximately 0.5 f/n/) Processing 2 minutes 0 painting (1) Cationic electrodeposition painting ED9000, black,
160-190V (2) Baking 180°C x 30-1 Film thickness 25μm Target 0 Coating film secondary adhesion test After immersing the electrodeposited test piece in 40°C warm water for 240 hours, 100 111N grid grids were applied to the substrate. After making a cut and peeling off adhesive tape, the coating film [F] (Fe, S
n) Sequential plating After performing 1i'6 plating, Sn plating is performed.

(1) , Fe plating FeSO4・71120 2
B 0 f / tPll = 2.5, bath temperature 30
℃ 5A/Mountain I
) [1=0.4, bath temperature 45±5℃30A/dn
? X 0. I See below (Flushing) [Example 1] The above (Fe s Sn ) eutectoid plating, chemical conversion treatment, and cationic electrodeposition coating were applied to a hot-dip galvanized steel sheet in this order, and a secondary coating adhesion test was conducted. However, as shown in Table 1, significantly superior results were obtained compared to the case where (Fe, Sn) eutectoid plating was not applied [Example 2] Regarding the alloyed hot-dip galvanized steel sheet, Example 2 When the same treatment as in 1 was carried out, as shown in Table 1, % (Fe
% Sn) Excellent secondary adhesion of the coating film was obtained compared to the case without eutectoid plating.

[Example 3] Electrogalvanized steel sheets were subjected to the same treatment as in Example 1, and as shown in Table 1, (Fe, SnJ
Significantly superior secondary adhesion of the coating film was obtained compared to the case without eutectoid plating.

[Example 4.5] Electrical Zn''-'N1 alloy plated copper plate (Fe N-
Sr+) eutectoid plating was sequentially plated with (Fe, 8nJ) and other conditions were the same as in Example 1.
As shown in the table, all exhibited significantly superior secondary adhesion of the coating film compared to the case where no (Fe%Sn) plating treatment was performed.

8- Table 1: Paint film secondary adhesion test results Patent applicant: Kawasaki Steel Corporation 441-

Claims (2)

[Claims] (1) Fe and Sn are added to a zinc-based plated steel sheet from a water solution containing FexSn in the range of Sn /Fe +Sn ≦0.2, with a total of 0.011/lr/ or more, 10
f/rr? A method for improving the secondary adhesion of a coating film on a zinc-based plated steel sheet, which method comprises performing the following electrodeposition. (2) 0.01 f/m' or more on a zinc-based plated copper plate,
10f/rr? Electroplating less than Fe followed by approx.
A method for improving the secondary adhesion of a coating film on a zinc-based plated steel sheet, comprising electroplating or electroless plating with Sn of y7ff/or less.