JPH028374A - Production of corrosion resistant chromate treated electrogalvanized steel sheet excellent in resistance to blackening - Google Patents

Abstract

PURPOSE:To produce a corrosion resistant chromate treated electrogalvanized steel sheet excellent in resistance to blackening by performing electrogalvanizing with plating liquid little in the contents of impurities such as Pb and Ni and thereafter performing chromate treatment with chromate treatment liquid having specified composition in the case of producing the electrogalvanized steel sheet. CONSTITUTION:The surface of a steel sheet is degreased and purified and thereafter electrogalvanizing is performed by utilizing electrogalvanizing liquid incorporating <=0.5ppm Pb<2+>, 100-300ppm Ni<2+> and >=500 Ni<2+>/Pb<2+> as the impurities at 30-200A/dm<2> current density at 1-2m/sec flow velocity of plating liquid. Then chromate liquid which incorporates both reduced chromic acid having (3/7-5/5) Cr<3+>/Cr<6+> at 5-50g/l expressed in terms of chromic anhydride and silica sol at 10-50g/l expressed in terms of SiO2, or furthermore is added with 1-80g/l phosphoric acid is applied at 30-100mg/m<2> expressed in terms of Cr and thereafter the steel sheet is forcedly dried at 60-150 deg.C. The chromate processing electrogalvanized steel sheet which is excellent in resistance to corrosion and resistance to blackening even in the high-temp. wetted environment is obtained.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a method for producing corrosion-resistant chromate-treated electrogalvanized steel sheets that prevents discoloration over time that occurs in chromate-treated electrogalvanized steel sheets stored in a high-temperature and humid environment. It is something.

(Conventional technology) Electrogalvanized steel sheets prevent white rust (improves corrosion resistance),
The majority of products are chromate treated to improve paint adhesion. Among them, for the purpose of omitting painting, chromate treatment (which has corrosion resistance comparable to painted steel sheets) has been applied.
Corrosion-resistant chromate-treated electrogalvanized steel sheets (hereinafter referred to as corrosion-resistant chromate treatment) exhibit a beautiful and uniform golden color and are used in a wide range of applications. When a corrosion-resistant chromate-treated electroplated steel sheet is stored in a high-temperature, humid environment, a portion or the entire surface of the plating may sometimes turn dark gray or brown (blackening phenomenon).

This blackening phenomenon is thought to occur when the outermost layer of zinc plating is etched by unavoidable impurities in the chromate coating, and is likely to occur due to increases in turbidity and humidity during product storage.

As a known technique related to the present invention, Pb2 in a zinc plating bath shown in Japanese Patent Publication No. 63385-1985 is
There is a technique for controlling the concentration of + impurities to 0.2 ppm or less to preferentially precipitate the chemically stable 002 plane of υ zinc crystals, suppressing the reaction with chromate, and preventing blackening.

In addition, in the published patent publication 1988-77988, the concentration of Pb''+ and Ni2+ in the galvanizing bath was determined as N12+/Pb2+.
5-500 Ni''/Zn2+ ratio is 1/25 or less and N12+ is 1 Oy/ or less, which makes it stable against chromate even in the presence of p b 2+.
It is stated that crystals on the 02 plane are preferentially precipitated.

(Problem to be solved by the invention) Although these disclosed techniques are excellent techniques, it has been found that they do not necessarily provide satisfactory results for the highly corrosion-resistant chromate treatment that is required in recent years. . In other words, conventional technology was aimed at the blackening phenomenon that occurs during transportation and inventory, but the composition of chromate is mainly hexavalent chromium (Cr6+), which has excellent corrosion resistance, and is a chromate solution with minimal anionic impurities. It can be obtained using

In recent years, the target has been products used as substitutes for painted products, as mentioned above. Therefore, discoloration is common for a certain period of time, not only during transportation and inventory, but also after being made into parts. Furthermore, in order to ensure a high degree of corrosion resistance, the amount of chromate film deposited is increased (30-100m9/m2 as Cr).
The required amount and the chromate solution are trivalent chromium (Cr
It is essential to have a bake-hardenable composition in which chromium (Cr6+) and hexavalent chromium (Cr6+) coexist. Furthermore, it is well known that anionic impurities in the chromate solution are undesirable factors that promote blackening. However, with the conventional strict concentration control, there is a lot of liquid dumping, which causes problems in terms of productivity and wastewater treatment, and there is a need for a technology that can suppress the occurrence of blackening in chromate liquids containing anionic components. Furthermore, the quality of chromate can be improved by adding anions.

The present invention provides a method for manufacturing a laminated steel sheet that suppresses the blackening of corrosion-resistant chromate, which requires high appearance quality and corrosion resistance.

(Means for Solving the Problems) Therefore, the present invention provides: (1) Ni"/Pb" in a tightening bath containing Pb2+ impurities of 0.5 ppm or less and N12+ of 100-300 pl)m. After galvanizing in an electrogalvanizing bath with a + ratio of over 500, a Cr+/Cr6+ ratio of 3
/7-515 reduced chromic acid converted to Cry3 5-50
9/L, silica sol 10-50 in terms of SiO2
1 to 80% of phosphoric acid to the chromate solution containing y/ or the chromate solution. 30 in terms of Cr
30-1O0/rn” After coating, plate temperature 60-150℃
A method for producing a corrosion-resistant chromate-treated electrogalvanized steel sheet with excellent blackening resistance, characterized by forced drying in a
) In claim 1, the current density is 30 to 200.
A/dn? , a method for producing a corrosion-resistant chromate-treated electrogalvanized steel sheet with excellent blackening resistance, characterized by electroplating at a flow rate of 1 to 2 m/sec. It concerns galvanized steel sheets, and assumes the combined effect of galvanizing and chromating methods. First, the galvanizing method will be described.

The key point in the zinc plating bath of the present invention is to control Pb2+ and Ni2+ among metal ions other than zinc contained as impurities. Pb2+ needs to be 0.5 ppm or less. If Pb exceeds 0.5 ppm, it is difficult to completely suppress black discoloration when using the corrosion-resistant chromate used in the present invention.Furthermore, even if only Pb2+ is suppressed, black discoloration still occurs, and the impurity old 2+ 100-300
This is achieved by controlling ppm, N12+/Pb2+ to over 500. Ni''+ is added as a Ni compound from the outside if necessary, and is removed if it is too much.

Below the lower limit of Ni2+, p eutectoids as described later.
b The black change cannot be suppressed and the color changes.

If the Ni eutectoid rate during mating exceeds the upper limit value,
Since the color tone of zinc plating without chromate produced in the same bath is slightly darkened, when untreated material is subjected to phosphate treatment, etching type chromate post-treatment, etc., it tends to become darkish, which is not preferable. As will be described later, N12+ is required to have an N12+/Pb2+ ratio of over 500 in order to suppress surface concentration of Pb over time. If it is less than 500, the concentration of PB in the plating layer becomes higher at the surface, causing discoloration.

As a method for reducing the Pb''+ concentration in the plating bath, it is possible to adopt a coprecipitation method by adding strontium carbonate or barium carbonate, a displacement precipitation removal method using metal zinc powder, an ion exchange resin method, etc., which are carried out using known techniques. can.

Ni2+ is an impurity ion contained in a hard member of plating equipment, such as a conductor roll, or in a normal plating solution when mixed with an alloy plating solution.

The concentration of impurities varies depending on the production volume of the line and product composition.In the case of the present invention, external additions and ion exchange,
Control the concentration by adding zinc dust.

Further, the plating in which the effects of the present invention are most clearly exhibited is produced on a so-called high-speed plating line with excellent productivity.

Preferred plating conditions are a current density of 30 to 200 A/dt7
? , the flow rate is 1 to 2 m/sec. If the current density is too low, the rate of pbO eutectoid in the plating film is high and there is a high risk of blackening. On the other hand, if the current density is too high, the deposition rate of the plating exceeds the crystal growth rate of the plating, and powdery plating called salmon is likely to occur. In order to prevent plating burns and gas unevenness in high-speed plating as in the present invention, a flow rate of 1 m/set or higher is preferable.

Although there is no quality problem with a flow rate of more than 2 m/sec, a flow rate of 2 m/sec or less is appropriate due to equipment constraints.

As described in Section 1, the chromate treatment of the present invention
This is a bake hardening type chromate treatment containing reduced chromic acid and silica sol that coexists with ``+'' and Cr'+.

Furthermore, a chromate solution containing phosphoric acid can be applied for the purpose of improving corrosion resistance, uniformity, and color tone. From the viewpoint of corrosion resistance, a thick chromate film with a thickness of 30 to 100~/m'' is selected as Cr, and the plate temperature is 60 to 150°C to suppress moisture absorption of Cr'+ and outflow of Cr6+. The film is hardened by applying it.

The amount of chromate deposited is 30% as Cr from the viewpoint of corrosion resistance.
m9/rr? 100m9 for reasons of adhesion and workability.
/m'' or less, preferably 40-60 m9/rr?, which provides excellent overall quality.The chromium compound in the chromate solution has a Cr3+/Cr6+ ratio of v7 to 51.
Control to 5. Although Cr6+ has a strong anticorrosion effect, it is easily soluble in water, and when the amount of adhesion is low, it is reduced by zinc and becomes difficult to dissolve. However, when the amount of adhesion is thick, which is the target of the present invention, it is difficult to maintain effective Cr6+ in the film, and it is difficult to maintain Cr6+ in the film due to moisture absorption. This causes uneven appearance. Cr3+ has a small anticorrosion effect, but is poorly soluble in water and hardens the coating. By forming a composite hydrated oxide of Cr6+ and Cr3+, a chromate film that is sparingly soluble in water can be obtained. For the above reasons, the Cr''+/Cr6+ ratio in the chromate solution is preferably 3/7 to 515.The chromate solution with this composition is produced by adding a reducing agent such as starch or sucrose to a concentrated solution of chromic anhydride. By sufficiently reacting, it can be obtained without leaving any unreacted substances.

Thereafter, the solution is diluted to a bath concentration of 5 to 5 oy7 t in terms of CrO3, at which the bath is stable and a predetermined amount of chromate attached is easily obtained. If the chromic acid content is less than 52%, chromium hydroxide tends to precipitate due to the accumulation of zinc ions, etc., and is easily repelled by the zinc surface. If it exceeds 50 t/, it will be difficult to obtain a uniform appearance due to problems such as bath viscosity. The chromate solution used in the present invention includes silica sol. Silica sol is a completely dispersed sol with a particle size of 5 to Loomμ. Furthermore, it is an acidic sol with the content of alkali metal ions, which are soluble components, kept to a minimum.

Is the concentration of silica sol 10-50? /L is desirable.

The purpose of adding silica sol is to improve corrosion resistance, adhesion with paints such as silk printing, and wettability with galvanized surfaces. Silica sol improves the above-mentioned quality by adsorbing Cr"+ and Cr6+ and coagulating silica particles during the formation process of the chromate film. At a high concentration of more than 50 f/l, the stability of the liquid This is not preferable because it tends to cause unevenness due to the increase in viscosity.

The chromate solution used in the present invention can contain pIJ acid in an amount of l to soy/, if necessary. The addition of phosphoric acid improves the uniformity of the appearance and makes the chromate film pale yellow in color. Furthermore, corrosion resistance is significantly improved. In the conventional technology, it was necessary to suppress anionic components such as phosphoric acid as much as possible because they cause blackening. There is no problem with the present invention. The concentration of phosphoric acid is determined in relation to the external color of chromate. That is, for gold color, it is 1 to 10 t/, and for pale yellow, it is 1 O-50.
9/l, and 50 to B 0 f/l for colorless.

If it exceeds 80 f/, the reaction with the plating metal will be intense, the resulting chromate film will be brittle, and free, unreacted phosphoric acid will remain, resulting in poor water resistance.

It is preferable to perform forced drying at a temperature of 60-150' as the final board temperature for baking. If it is less than 60°C, the chromate film will not be sufficiently hardened, resulting in insufficient moisture absorption and water resistance. On the other hand, if it exceeds 150°C, Cracks occur in the chromate film,
Corrosion resistance deteriorates. The preferred baking plate temperature is 80-100℃
It is.

(effect) The mechanism of action regarding plating of the present invention will be described below.

FIG. 1 is an example showing the relationship between black discoloration and Pb2+ and Ni2+ according to the present invention. Ni”, Pb2+ controlled zinc sulfate 3 o t/l and sodium sulfate s o y/l
I) Using a zinc plating bath with H = 1.2, zinc plating (2097 m'') was performed at a current density of 8 oA/d-liquid flow rate of 1.2 m/sec, then washed with water and immediately coated with Cr.
"+/Cr'+ is reduced chromic acid of 4/6 l 2 y/
After spraying a chromate solution containing 30 t/l of silica sol with an average particle size of 20 mμ, the amount of Cr deposited was approximately 50 m9/rr using an air knife. After applying, use hot air to bring the plate temperature to 8.
Corrosion-resistant chromate was applied by forcibly heating and drying to 0°C. The rating for black discoloration is 72 in an atmosphere with a temperature of 70°C and a humidity of 98%.
Exposure for a period of time, measure the brightness (L value) before and after that, and measure the difference (
ΔL) is shown as a scale.

The broken line (- to -) in the figure represents p b 2 + in the plating solution.
Concentration is 0.3 ppm Ni”/Pb”+ from 330 to
1300, the solid line (-〇-) indicates Pb2
+0, 5 ppm Ni”/Pb2+ 200-80
It is changed up to 0. The dotted line (...×...) indicates the N12+/Zn2+ ratio of 12 at Pb" 0.8 ppm.
The value was varied from 5 to 500.

As is clear from the figure, the ΔL value of the plating bath containing 0.8 ppm Pb decreases due to the addition of Ni2+, but it is not as significant as in other low lead plating baths, compared to the plating baths containing 0.3 and 0.5 ppm Pb2+. The obtained example of the present invention is Ni
”/Pb2+ ratio was 500, there was almost no change in the ΔL value and no blackening occurred.

Figure 2 shows pb”+ as 0.3 ppm Nl/pb
Corrosion-resistant chromate galvanized steel sheets using plating baths with ratios of 167 and 1000 were exposed in the above-mentioned 70' (:9B%) humid atmosphere for 7 days, and the surface of the sample was analyzed in the depth direction using SIMS (secondary ion mass spectrometry). surface analyzed pb.

The intensity of Nl, the profile of Pb-b, N1~
b is a blackened one with an N12+/Pb2+ ratio of 167;
Pb-a.

N1-a are those in which black discoloration of 1000 did not occur.

In the pb-b curve where Ni2+ is insufficient, pb is concentrated on the surface and the slope in the depth direction is steep. On the other hand, Pb-a has a small slope of Pb intensity and depth, is smooth, and is very similar to the profile of N1-a.

As described above, the PB eutectoided by the presence of Ni2+ changes over time, and the blackened material has a high PB concentration on the surface.

Zinc crystal orientation (00
Regarding the relationship between 2 intensity/sum of other intensities) and blackening, many P
Fig. 3 shows the results of the investigation on the target materials below b''+0.8 ppm.The chromate material of the present invention could not be explained in terms of its relationship with the crystal orientation of zinc.

From these results, the mechanism of action of Pb" and Ni2+ in the corrosion-resistant chromate treated electrogalvanizing of the present invention can be explained as follows.

The influence of the PB itself in the blackened plating layer is strong, and the blackened material has a high PB content in the surface layer. Therefore, Pb2+ is 0.5p
PbO into the plating layer is suppressed to below pm.
It can be assumed that the amount of eutectoid is kept as low as possible, and that Ni2+ uniformly disperses and fixes the eutectoid PB in the plating layer, thereby suppressing surface concentration of PB over time.

(Example) In order to clarify the effects of the method of the present invention, the following example is shown.

A cold-rolled steel plate with a thickness of 0.8 m was degreased by spraying for 2 minutes using a commercially available alkaline degreaser solution with a consistency of 60°C, and after washing with water, it was pickled for 5 seconds at normal humidity using a 5% H□S04 bath. Processing was carried out. Plating and chromate treatment were performed under the standard conditions shown below unless otherwise specified.

The evaluation method was also conducted under the following conditions.

[Metsuki standard conditions] Raku composition: Zinc sulfate: 300 f
// Sodium sulfate: sog/Noah N12+lPb2+: Mixed with the addition of strontium carbonate and nickel carbonate PH: 1.2 [Chromate side semi-standard] Liquid composition 5y: cr3"/cr"
: 4/6Cry3: 12 t/L SiO□ :3Of/Current density: Plating liquid flow rate: Plating amount: Electrode.

80 A/d 1.2 m/5 ec 20 y 7 m2 (one side) Pb-5 Sn-based insoluble electrode treatment method: After spraying the above treatment liquid, high-pressure air
Using (1,2!(y/CJ)), the adhesion amount is 50■/?
? After adjusting to +2 (one side), dry for 3 seconds at a board temperature of 90℃ [blackening acceleration test conditions] ■Leave it in the refrigerator for 60 minutes, take it out, and after condensation, test it in a stacked state at 49℃'-98% humidity for 20 days ■Condensation After that, test in a stacked state at 70℃ 100% humidity for 2 days ■Exposed state test at 70℃ 9B humidity for 7 days Evaluation of black discoloration...Visual evaluation and hunter's own seat (L value) using a color difference meter specified in JIS Z8730. Difference before and after test (ΔL) ◎: Very good with no surface color ・・・・・・ΔL≦10
: Occurrence of black discoloration is not observed, but slight change in color tone is observed...ΔL≦3△: Slight black discoloration occurs ・・・・・・・・・・・・・・・・・・・・・・・・ΔL≦5×: Black discoloration Occurrence ・・・・・・・・・・・・・・・・・・ΔL
≧10 [Other qualities] (1) Initial plating appearance Visually evaluate the color tone of the untreated zinc plating without chromate.

(2) Appearance of untreated galvanized steel sheet after phosphate treatment (commercially available spray sunlay treatment)
Visually evaluate appearance after acid zinc treatment.

○・Normal white appearance △・・Darkened appearance (3) Corrosion resistance evaluation Corrosion resistant chromate treated galvanized steel sheet JISZ 237
It is expressed as the time (HrS) required for the area ratio of white rust to reach 5% in a 1-normal salt spray test.

(4) Hygroscopic chromate galvanized steel sheet was immersed in ion-exchanged water at 20-25°C for 5 minutes, and the eluted chromium was expressed as a percentage of the total chromium in the chromate film.

(5) Appearance of plating after corrosion-resistant chromating The plating appearance after corrosion-resistant chromate was visually evaluated.

◎...Very uniform〇...Various uniformity (average to standard) △...Unevenness is observed ×...A lot of color unevenness Example 1 Corrosion resistance of zinc plating and chromate using treatment method under standard conditions Chromate and untreated materials were prepared and evaluated. The results are shown in Tables 1 and 2. Samples Nu■～■ are Pb”0.
05 ppm N12+/Pb2+ The inventive examples of 2000 or more showed good blackening resistance. Sample Nα■~0 is P
At b2+0.1 ppm, the Ni ”+/Pb”+ ratio is 1
000 or more of the present invention examples showed good results. In addition, samples Nl@~@ were Ni''/P at a Pb2+ concentration of 0.2 ppm.
In the case of a b2+ ratio of 600 or more, [phase] showed good results except that the appearance of the untreated zinc plating became black. Corner 0~■
is an example of Pb" 0.3 ppm and [phase] is Ni"+
Black discoloration occurred due to the lack of water. ■The appearance became black after fitting.

[Phase] Those within the range of the present invention of 0 to 0 were good. Nα
[Phase] to [Phase] are oi in the example of Pb"" 0.5 ppm
-o turned black because the Ni''+ concentration became insufficient compared to Pb''+. [Phase] - O was good in the present invention example. [
Phase] Plating caused poor appearance.

Nα0~[phase] is a comparative example containing 0.8 ppm of Pb, and the black discoloration and plating appearance are insufficient.

Example 2 Using plating as standard conditions, the conditions of chromate treatment were varied to evaluate black discoloration, corrosion resistance, hygroscopicity, and appearance after chromate treatment.

Nα0 ~ @) is the reduction of chromic acid as Cr ” +/Cr
In the example where the 6+ ratio was changed from 0/10 to 7/3, ■ is a comparative example that does not contain Cr"@ is Cr" where Cr+/Cr6+ is 7/3
In the comparative examples with a high + ratio, ■ indicates moisture absorption, and @ indicates poor corrosion resistance. Q
, t5@ is good. N [l o.

＠ is a comparative example in which the drying plate temperature was 40° C. and 200″C, respectively, and was non-luminous in terms of moisture absorption and corrosion resistance.Nα＠ showed uneven color of chromium in the blackening test.

N11 (d~@ are those with different amounts of chromate deposited, @ is poor corrosion resistance, (c) is black discoloration and the appearance after chromate is insufficient. @ and @ are examples of the present invention and have excellent quality. ing.

NIl@-[phase] is an example in which phosphoric acid is added to the chromate solution, and the appearance after chromate is beautiful and both blackening and corrosion resistance are good.

Example 3 Based on standard conditions, Pb” 0.3 ppm N
After plating was performed using a plating bath controlled at i2''200 ppm and varying the flow rate (y) and current density (DK) of the plating bath, chromate treatment was performed under standard conditions.

Evaluation was made by observing black discoloration (rating of ■) and plating appearance after chromate.

y = t3.5m/sec, DK = 100
The plating of the A7dm sample itself turned black and stopped. Also, y
== 1.2 m/sec In the case of DK 400A/d-, plating turned black and stopped as well. Vl, 2 m1s
DK 50 A/dm2, (sample Nα51), D
K 1ooA/am' (Nα52), D K l 5
0 A/dtn'' (Nn53) had no black discoloration at all and was ``◎'' and the appearance after chromate was also good ``○''.

(Effects of the Invention) As described above, the corrosion-resistant chromate-treated steel sheet manufactured by the method of the present invention has excellent corrosion resistance, a beautiful appearance, and virtually no blackening phenomenon on the surface in a high-temperature, humid atmosphere. It can be used as a substitute for painted products.

Furthermore, since the quality of plating is improved, the dependence on chromate solution is alleviated, and a wide range of chromate solutions can be applied.

[Brief explanation of the drawing]

Figure 1 shows the effect of the N12+/Pb2+ ratio in the plating solution on the blackening resistance (ΔL) of a corrosion-resistant chromate-treated electrogalvanized steel sheet. FIG. 2 shows the Pb and Ni concentration profile in the depth direction of the plating layer after the blackening test investigated by SIMS (secondary ion mass spectrometer). Figure 3 shows the Pb2+ concentration range (08~
This figure shows the relationship between the crystal orientation of the black discoloration-produced plate and the good material produced by 0.05 ppm).

Claims (2)

[Claims] (1) The content of Pb^2^+ impurities is 0.5 ppm or less, and the Ni^2^+/Pb^2^+ ratio in the tightening bath containing 100 to 300 ppm of Ni^2^+ is over 500. After galvanizing in an electrogalvanizing bath, Cr^3^+/
Reduced chromic acid with a ratio of 3/7 to 5/5 of Cr^6^+
5 to 50 g/l in terms of O_3, silica sol to SiO_2
After applying a chromate solution containing 10 to 50 g/l in terms of Cr, or a solution obtained by adding 1 to 80 g/l of phosphoric acid to the chromate solution at a rate of 30 to 100 mg/m^2 in terms of Cr, the plate temperature was 60 to 150°C. A method for manufacturing a corrosion-resistant chromate-treated electrogalvanized steel sheet with excellent blackening resistance, which is characterized by forced drying. (2) In claim 1, the current density is 30 to 2.
A method for producing a corrosion-resistant chromate-treated electrogalvanized steel sheet with excellent blackening resistance, characterized by electroplating under conditions of 00 A/dm^2 and a flow rate of 1 to 2 m/sec.