JPS6112896A - Chemical treatment of tinned steel sheet - Google Patents

Abstract

PURPOSE:To improve the adhesion of a tinned steel sheet to a paint film and the corrosion resistance by treating the steel sheet witha weakly alkaline bath to activate the surfaces, subjecting the steel sheet to electrolysis as a cathode in an aqueous soln. contg. CrO3 and H2SO4 in a prescribed ratio, and immersing the steel sheet in an aqueous soln. contg. CrO3. CONSTITUTION:A tinned steel sheet is treated with a weakly alkaline bath to activate the surfaces, and it is subjected to electrolysis as a cathode in a treating bath contg. 5-30g/l CrO3 in 1/100-1/300 ratio of H2SO4/CrO3. The steel sheet is then immersed in an aqueous soln. contg. 50-300g/l CrO3.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for forming a chemically treated film with excellent coating adhesion and corrosion resistance on tin-based plated steel (including copper strips, the same applies hereinafter).

Tin-plated steel sheets are widely used as a material for various metal containers such as cans and reports, but in recent years, tin-plated steel sheets have been used to reduce the amount of tin adhesion and to coat the inside and outside surfaces of cans to ensure corrosion resistance. is increasing.

In this case, the quality of paint film adhesion is an important factor that determines the corrosion resistance after painting.

As a method to improve the adhesion of tin-plated steel sheets, metallic chromium (hereinafter referred to as C
A method of forming a film consisting of hydrated chromium oxide (hereinafter referred to as Cr-) and chromium oxide (hereinafter referred to as Cr-) is known, but simply performing cathodic electrolytic treatment in an anhydrous chromic acid bath results in the following problems: When the amount of Cr necessary to obtain the effect of improving film adhesion is removed, the amount of Cr that forms at the same time tends to be too large, impairing solder wettability and seam weldability, and reducing the amount of Cr'''. Non-uniformity in outer II (color tone) is likely to occur due to non-uniform production.

In order to solve these problems, we investigated various treatment conditions using chromic anhydride baths, and found that following cathodic electrolysis treatment in a low-concentration chromic anhydride aqueous solution with a lower sulfuric acid content than usual, we found that Immersion treatment in a concentrated chromic anhydride aqueous solution not only reduces the amount of Cr and improves solderability, weldability, and uniform appearance, but also improves paint film adhesion. The present invention was based on the discovery that a significant improvement effect can be brought about even when the method is used.

In other words, the present invention aims to reduce the amount of Crx and thereby improve solderability, weldability, and appearance uniformity, and to improve the chemistry of tin-based plated steel sheets, which also significantly improves coating adhesion. The purpose is to provide a processing method.

According to the first aspect of the present invention, the tin-based plated steel sheet has 5 to 3
0 g'/l of CrO3 and 1/1 weight ratio with CrO3
Cathodic electrolytic treatment is performed in a treatment bath containing 100 to 1/300 H2SO4, followed by 0r0350 to 300
An object of the present invention is to provide a method for chemically treating a tin-based plated steel sheet, which is characterized by immersing the tin-based plated steel sheet in an aqueous solution of g/l.

According to the second aspect of the present invention, the tin-plated steel sheet is subjected to a surface activation treatment using a weak alkaline bath in advance.
Cathodic electrolytic treatment is carried out in a treatment bath containing ~30 g/l of CrO3 and I2SO4 (in a weight ratio of 1/100 to 1/300 with Gro 3), followed by CrO350~
An object of the present invention is to provide a method for chemically treating a tin-based plated steel sheet, which is characterized by immersion treatment in an aqueous solution of 300 g/fL.

The present invention will be explained in more detail below.

In this specification, the term "tin-plated steel sheet" refers to ordinary tin-plated steel sheet generally called tin-plated steel sheet, as well as steel sheet that is used for so-called welded cans, etc., and is used in some way to compensate for the decrease in corrosion resistance due to a reduction in the amount of tin coating. Various materials (i.e., tin-based alloy plated steel sheets containing metals other than tin such as ?li, tin with thin appendix plating on a base layer that has been subjected to base plating treatment such as old or further heat diffusion treatment) laminated plated steel sheets, etc.), and the method of the present invention can be applied to any of these.

After applying tin-based plating, it is preferable to perform surface treatment such as cathodic electrolysis treatment or immersion treatment in a weak alkaline bath such as sodium carbonate or sodium bicarbonate as an activation treatment for the plating surface. However, no reflow treatment is required. For active surfaces such as finishing plating, it is not necessary to perform this kind of surface activation treatment, and the treatment conditions may be the same as those used for normal dusting, and specific conditions may be required. For bath concentration 5-20 g/l, bath temperature 30-6
It may be carried out under the conditions of 0° C. and cathode charge density θ˜3 C/d layer 2.

After the above surface activation treatment and water washing, a film consisting of Cr" and Cr- is formed by cathodic electrolysis treatment in a low concentration chromic anhydride bath and immersion treatment in a high concentration chromic anhydride aqueous solution. The amount of adhesion is 2 to 20-
g/go and 1 to 101 g/m'' (both per side)
is desirable.

Cr' 2mg/rr? less than or Cr-1ymg/
If the amount is less than Or″20mg/rn″, a sufficient effect of improving paint film adhesion cannot be obtained, and as the amount of Cr″ and CraX increases, the adhesion of the paint film will improve.
This is because if it exceeds -10 mg/g, the effect will be saturated and it will have an adverse effect on solderability and seam weldability.

When producing a Cr' film with a low adhesion amount as described above,
Because it is not possible to ensure the uniformity of the formed film with high current density and short-time processing, it is desirable to process with a relatively low current density of 30 A/dm or less. In order to obtain the Cr'' precipitation efficiency, the CrO3 concentration contained in the treatment bath needs to be 30 g/cross or less. On the other hand, reducing the CrO3 concentration more than necessary has disadvantages such as difficulty in controlling the bath composition and increased power consumption due to increased liquid resistance, so the lower limit of the CrO3 concentration is 5 g/l. shall be.

Adding sulfuric acid to the treatment bath is essential for the production of Cr'.
The weight ratio to chromic anhydride (hereinafter referred to as sulfuric acid concentration ratio) needs to be 1/100 to 1/300.

In the Sargent bath, commonly known as a chromium plating bath, the sulfuric acid concentration ratio is around 1/100.

When using a low concentration chromic anhydride bath for chemical treatment of tin plating as in the present invention, if the sulfur # concentration ratio exceeds 1/100, C
It was found that the precipitation efficiency of r'' decreased rapidly.

Therefore, the upper limit of the sulfuric acid concentration ratio is set to 1/100. If the sulfuric acid concentration ratio is too low, the efficiency will drop significantly, so the lower limit of the sulfuric acid concentration ratio is set to 1/300.

The above treatment bath may contain, in addition to sulfuric acid, a fluorine compound such as sodium silicofluoride, sodium borofluoride, sodium fluoride, or ammonium fluoride as a Cr″′ precipitation aid.

Following the cathodic electrolytic treatment in the above treatment bath, an immersion treatment in an aqueous chromic acid anhydride solution is performed for the purpose of reducing the amount of Cr excessively produced in the cathodic treatment and improving coating film adhesion.Anhydrous Excess C when chromic acid concentration is less than 50g/1
A higher concentration is required because the ability to dissolve r- is significantly reduced.

Furthermore, if the concentration is higher than 300 g/l, the etching effect is too strong, which may cause bit-like erosion of the underlying tin-based plating layer at minute defects in the Or'' and CraX films. , the upper limit of the chromic anhydride concentration is 300 g/l.

In addition, even without performing the immersion treatment in a relatively high-concentration chromic anhydride aqueous solution as described above, Cr"2 to 20 mg/rn
Although it is possible to obtain a chemically treated film with a concentration of Cr-1-10mg/m", when comparing the film adhesion of the material obtained in this way and the material that was subsequently treated with high concentration chromic acid, , the latter is significantly superior, and carrying out this high-concentration chromic acid treatment is the greatest feature of the present invention.

The bath temperature and treatment time for high-strength chromic acid treatment do not need to be particularly limited, and can be adjusted appropriately within the range of room temperature to 90°C and 0.5 to 10 seconds, respectively, depending on the amount of Cr that needs to be dissolved and removed. Just do it.

Examples of the present invention will be described below.

<Example> A steel plate subjected to cold rolling 1, annealing and temper rolling was degreased and pickled, and then heated to 2.8 g/r per side in a halogen bath.
Material A subjected to n' Sn plating and heat melting treatment,
and 0.1 g/rn' Ni per side after cold rolling.
Plating is applied and heat diffusion treatment that also serves as annealing (soaking temperature 69
After heat-rolling, degreasing, pickling, tin plating at 0.8 g/m' per side, and heat-melting treatment, material B was subjected to heat-melting treatment under various conditions. Chemical treatment was applied to examine the effects on paint film adhesion, appearance uniformity, and seam weldability.

In addition, for items other than plated steel sheets with no-reflow finish,
Prior to each chemical treatment, surface activation was performed by cathodic electrolysis treatment at 0.5 C/d2 in a 15 g/l aqueous sodium carbonate solution (40°C).

Furthermore, evaluation of each characteristic was based on the following criteria.

[Paint film adhesion]

After applying epoxyphenol paint, the paint film was scratched with a cutter knife and immersed in a 5% NaCl, 2% citric acid solution (50°C) for 72 br (W)
Based on the following criteria: W≦/10
05 Δ: /1005<W≦0.2 X: W>0.2 [Appearance uniformity] Visually evaluated as follows.

0: No color tone unevenness ×: Color tone unevenness [Seam weldability] At the lower limit current at which sufficient welding strength can be obtained at a welding speed of 50 s/i+in, 0: No dust generation ×: Dust generation As shown in the results in Table 1 However, those outside the scope of the present invention have drawbacks in paint film adhesion, appearance, and seam weldability.

Claims (2)

[Claims] (1) A tin-plated steel sheet is subjected to cathodic electrolytic treatment in a treatment bath containing 5 to 30 g/l of CrO_3 and H_2SO_4 at a weight ratio of 1/100 to 1/300 to CrO_3, followed by CrO_350 to 300 g/l. 1. A method for chemically treating tin-based plated steel sheets, which comprises immersing them in an aqueous solution. (2) A tin-plated steel sheet is subjected to surface activation treatment using a weak alkaline bath, and then 5 to 30 g/l of CrO_
3 and CrO_3 weight ratio is 1/100 to 1/30
1. A method for chemically treating a tin-based plated steel sheet, comprising cathodic electrolytic treatment in a treatment bath containing 0 H_2SO_4, followed by immersion treatment in an aqueous solution of CrO_350 to 300 g/l.