JPS61204392A - Production of chromium coated stainless steel strip - Google Patents

Abstract

PURPOSE:To prevent the repassivation of a stainless steel strip and to improve the adhesion by thinly plating the steel strip with Ni in a nickel chloride bath before Ni electroplating and by carrying out the Ni electroplating adn the following Cr electroplating. CONSTITUTION:A stainless steel strip is immersed in an electroplating soln. contg. 200-300g/l nickel chloride and 30-45g/l hydrochloric acid, and it is thinly plated with Ni by electrolysis at 2-20A/dm<2> cathode current density and 20-40 deg.C bath temp. for 1-10min. The steel strip is then electroplated with Ni as usual, further electroplated with Cr as usual and subjected to diffu sion processing in a hydrogen atmosphere to form a Cr alloy layer on the sur face of the steel strip.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention provides a chromium-coated stainless steel strip in which the surface of the stainless steel strip is coated with chromium and the chromium coating layer is made ductile by performing a diffusion treatment. This relates to a manufacturing method.

<Prior Art> There are various methods for improving the corrosion resistance of steel, but especially when high corrosion resistance is required, a surface treatment method of coating with chromium is the most effective. However, if chromium is simply coated on the surface, since chromium is inherently poor in ductility, even a slight deformation will cause the coating to break, exposing the base metal, and it will no longer have the properties of a corrosion-resistant coating. It will stop happening.

Therefore, such surface treatments have been applied to final products, but not to intermediate products such as copper strips and steel plates.

As a method to compensate for this drawback, there is a method of coating the surface of a steel sheet with chromium and then performing a diffusion treatment to form a chromium alloy layer, thereby making it possible to process and form the steel sheet to a certain extent. In this case, by coating with nickel prior to chromium plating, diffusion of iron from the steel material to the surface can be prevented, and high corrosion resistance can be maintained after the diffusion treatment.

<Problems to be Solved by the Invention> However, even if stainless steel is coated with nickel by electroplating, a plated coating with good adhesion cannot be obtained due to the passive film on the surface of the stainless steel. Therefore, pretreatment is performed using the steps shown below to obtain a nickel plating layer with good adhesion.

(1) Electrolytic degreasing process Current density 1-3 in lθ% sodium orthosilicate bath at 50℃
Treatment at A/da2 for 3 minutes (2) Water washing process (3) Electrolytic activation process Cathode treatment for 2 minutes with a 5% sulfuric acid solution at 30°C at a current density of 1 to 5 A/da2 (4) Water washing process (5) Nickel plating process Current density 1~IOA/dff in Watts bath at 40~50℃
However, when continuous nickel plating is performed on a stainless steel strip using this pretreatment, a nickel coating layer is formed, but in the subsequent chromium plating process, due to the plating stress caused by chromium plating, nickel is removed. There was a problem in that the layer peeled off from the base stainless steel strip, making subsequent diffusion treatment impossible.

When continuous nickel plating is performed on a stainless steel strip, the reason for the deterioration of the adhesion of the nickel plating layer is that there are unavoidable separation spaces between each process, and the surface of the stainless steel strip comes into contact with the atmosphere while passing through these spaces. It turned out that this was due to repassivation. Therefore, after electrolytically activating the stainless steel strip, we used a water shower containing a corrosion inhibitor, an inert gas, etc. in order to prevent the copper strip from coming into contact with the atmosphere, but all results were positive. No results were obtained.

As a result of various experiments, the present inventors have found that before performing nickel plating in a Watts bath, thinning of nickel can be carried out by cathodic electrolytic treatment in a hydrochloric acid solution of nickel chloride. For example, it has been found that even if the nickel plating layer is subsequently plated with nickel and then chromium, the nickel plating layer does not peel off from the base metal of the stainless steel strip.

<Structure and operation of the invention> According to the present invention, the stainless steel strip is coated with nickel chloride 20
0 to 300 g/l, 30 to 45 g of hydrochloric acid, IQ solution, cathode current density of 2 to 20 A/da2, bath temperature of 20 to 40°C, depending on the cathode current density, cathodic electrolytic treatment for 1 to 10 minutes to apply nickel thinning. A chromium coating characterized in that a chromium alloy layer is formed on the stainless steel surface by subsequently electroplating nickel using a conventional method, and then performing a diffusion treatment in a hydrogen atmosphere after electroplating chromium using a conventional method. A method of manufacturing stainless steel strip is provided.

<Function> When thinning nickel using the nickel chloride plating solution according to the present invention, the current efficiency of nickel is about 10%, and the remainder is used to generate hydrogen. The hydrogen generated here acts on the repassivated stainless steel surface and removes it. Therefore, by cathodic electrolytic treatment in a nickel chloride plating solution, it is possible to activate the stainless steel and coat it with nickel at the same time. Subsequently, a plated layer with good adhesion can be obtained by thick nickel plating and chromium plating.

In nickel chloride plating solution, nickel chloride is 200
~300g/Ω, but nickel chloride is 200g/Q
If it is less than that, the current efficiency of nickel will be low and the plating layer will partially peel off after chromium plating. Also, nickel chloride is 30
Since the effect of improving the adhesion of the plating layer is observed even when the amount exceeds 0 g/Q, the upper limit was set at 300 g/Q. The concentration of hydrochloric acid is preferably 30 to 45g/Q.If the hydrochloric acid is less than 30g/Q, the removal of the passive film will be incomplete, and if it exceeds 45g/Q, pitting may occur depending on the stainless steel.The treatment temperature should be 40℃. If the temperature exceeds the temperature, pitting may occur, and the temperature was set at 20 to 40°C in consideration of workability. If the current density is less than 2A/da2, there is no adhesion improvement effect, and if the current density is less than 20A/da2,
If the current density exceeds da2, the smoothness of the stainless steel after treatment deteriorates, so the current density was set at 2 to 20 A/da2. The processing time corresponds to the current density, and the amount of electricity varies from 1O to 30c/
It is sufficient to choose within the range that satisfies cxl.

In the method of the invention, applying on top of the nickel dilution,
Nickel and chromium electroplating can be carried out by any known method, but nickel plating is advantageously carried out by a standard method using a so-called Watt bath, and chromium plating is advantageously carried out by a standard method using a so-called Sargent bath.

The thickness of the nickel coating layer in the Watt bath is 4 to 40 μm.
, the thickness of the chromium coating layer by Sargent bath is 0.2
If the thickness of the nickel coating layer is less than 4 μm and/or the thickness of the chromium coating is less than 0.2 μm, even if the diffusion treatment is performed, the effect of improving corrosion resistance will be poor.
On the contrary, even if the thickness of the nickel coating layer exceeds 40 .mu.m or the thickness of the chromium coating layer exceeds 10 .mu.m, the corrosion resistance will not be further improved.

After plating, diffusion treatment is carried out at a temperature of 800~800℃ in a hydrogen atmosphere.
Conducted at 1200°C.

The reason why the diffusion treatment is performed in a hydrogen atmosphere is as follows.

Since chromium is a metal that is very easily oxidized and nitrided, oxide Cr203 is formed on the surface even if the diffusion treatment is performed in a vacuum, for example. Also, hydrogen 75%-
When the diffusion process is performed in an atmosphere containing 25% nitrogen, nitride Cr2N is formed on the surface.

Since the Cr203 and Cr2N compounds formed here are hard, the chromium coating layer and the alloy layer are destroyed by processing and forming after the diffusion treatment, exposing the base metal, so that it no longer has the characteristics as a corrosion-resistant coating. The conditions for this diffusion treatment in a hydrogen atmosphere are a temperature range of 800 to 1200°C, and the thickness of the alloy layer formed by the diffusion treatment is 15 μm.
■The processing temperature and time should be selected so that the above results are achieved. In this case, if the formed alloy layer has a thickness of less than 15 μm, the corrosion resistance after the diffusion treatment will not be sufficient.

<Specific Description of the Invention> The present invention will be described in detail below with reference to Reference Examples and Examples.

Reference Example 1 A commercially available Sυ5410 stainless steel strip was electroplated with nickel and chromium in various steps shown in Table 1, and then the adhesion of the plating layer was investigated. As a result.

After diluting the nickel in a nickel chloride bath.

It can be seen that a plated coating layer with good adhesion can be obtained by electroplating nickel and chromium.

The processing conditions for each step are as follows.

Material: SO34102B steel strip Electrolytic degreasing: 10% sodium orthosilicate, 50'C current density 2A/dm, 2.3 minute treatment Pickling: 5% H2So4. Immersion at 50°C for 3 minutes Electrolytic activity: 5% H2So4. 30℃ Current density 3A/dm2.2 minute treatment Ni chloride plating: 250g#! N1CQ2・6H2
0,t(+,1240gIQ.

30°C Current density 5A/dm 2.5 minute treatment Ni plating: Watt bath, Ni 5 μm plating Cr plating: Sargent bath Cr 7 μm plating Experiment number A-5
This is a method in which a nickel chloride solution cathode electrolytic treatment and a nickel thin plating process are added to the conventional method A-3 which provides the best product. A-6 and A-7 were then omitted from the pickling and pickling and electrolytic activation treatments. There was no substantial difference in the quality of the nickel coating.

Example 1 A commercially available SUS 430 stainless steel strip was plated with nickel chloride under varying plating conditions.Subsequently, nickel was electroplated to a thickness of 5 μm in a Watt bath, chromium was electroplated to a thickness of 7 μm in a Sargent bath, and then a hydrogen atmosphere was applied. Inside, diffusion treatment was performed at 1100'C for 10 minutes. After the treatment, the adhesion of the plating diffusion layer was evaluated by surface observation and 2 bending tests. The results are shown in Table 2. Chloride-kke/L/
200g/Q ~300g#l, hydrochloric acid 30~45g/
fl, bath temperature 40℃ or less processing electricity amount 10-30c/a#,
After thinning with nickel, coating with nickel and chromium, and performing a diffusion treatment, the adhesion of the plating layer was good.

Example 2 A commercially available SUS 410 stainless steel strip was diluted with nickel in a nickel chloride bath, and then nickel was added in a Watts bath.
After plating chromium to various thicknesses in a Sargent bath, the diffusion treatment conditions were varied in a hydrogen atmosphere. The corrosion resistance of the steel after diffusion treatment is 11000pp C1,
An anodic polarization curve was drawn using a potentiostat in a solution at 50° C., and the potential value at which the current value was 200 μA was evaluated. The larger the value of this potential, the better the corrosion resistance of the steel. Figures 1 and 2 show the corrosion resistance after diffusion treatment with varying nickel plating thickness and chromium plating thickness, respectively. It can be seen that the corrosion resistance is poor. FIG. 3 shows the change in corrosion resistance depending on the thickness of the diffusion layer formed by changing the diffusion treatment conditions after plating with nickel and chromium. It can be seen that corrosion resistance is poor when the thickness of the diffusion layer is 15 μm or less.

<Effects of the Invention> As explained above, the present invention provides the following advantages: prior to nickel electroplating and then chromium electroplating using the conventional method,
Pre-plating a thin layer of nickel in a nickel chloride bath prevents repassivation of the stainless steel strip.
By obtaining a plating layer with good adhesion and subsequently performing a diffusion treatment in a hydrogen atmosphere, a ROM-coated diffusion-treated steel strip with good workability is provided.

In addition, according to Mizuyoshi, by cathodic treatment in a nickel chloride plating bath, the degreasing, pickling, and activation treatment steps can be omitted, simplifying the treatment method, reducing the amount of waste liquid, and reducing the amount of varnish by 1. is effective.

The chromium-coated diffusion treated steel strip according to the present invention can be used in fields that require high corrosion resistance, heat resistance and sulfur dioxide gas resistance, such as automotive mufflers, chimneys, etc., and fields that require weather resistance. For example, it can be used as a roofing material, and further as a base steel plate for painting.

[Brief explanation of drawings]

Figure 1 shows nickel plating after nickel chloride plating, followed by plating with various nickel plating thicknesses in a Watts bath. 7 μm chromium plating, 1200℃ in hydrogen atmosphere,
The results of examining corrosion resistance after performing a 2-minute diffusion treatment are shown. Figure 2 shows nickel plating after nickel chloride plating, nickel plating to 5 μm in a Watts bath, chromium plating with various thicknesses in a Sargent bath, and then plating at 1100°C in a hydrogen atmosphere.
, shows the results of examining corrosion resistance after performing a diffusion treatment for 10 minutes. FIG. 3 shows the results of investigating corrosion resistance with respect to diffusion layer thickness when various diffusion treatment conditions were changed in a hydrogen atmosphere after nickel chloride plating, 5 μm of nickel plating and 7 μm of chromium plating in a Watts bath.

Claims (1)

[Claims] Stainless steel strip with nickel chloride 200-300g/l
, cathodic current density 2-20 in hydrochloric acid 30-45 g/l solution
A/dm^2, bath temperature 20-40℃, cathode electrolytic treatment for 1-10 minutes depending on cathode current density, thin nickel plating, followed by nickel electroplating by conventional method, and further by conventional method. A method for producing a chromium-coated stainless steel strip, which comprises performing a diffusion treatment in a hydrogen atmosphere after chromium electroplating to form a chromium alloy layer on the stainless steel surface.