JPH02104672A - Method for chromating galvanized steel material - Google Patents

Abstract

PURPOSE:To render superior corrosion resistance to galvanized steel material by coating the surface of the steel material with an aq. chromating soln. contg. Cr and silica sol in a specified ratio and having a specified Cr<6+> content and specified particle sizes of the silica sol and drying the coated steel material. CONSTITUTION:An aq. chromating soln. contg. 10-50g/l (expressed in terms of chromic anhydride) total Cr compds. and silica sol in 0.1-6.0 ratio of SiO2 to Cr and having superior stability over a long period of time is prepd. In the soln., the ratio of Cr<6+> to total Cr is 0.3-0.7 and the ratio of silica sol of 40-50mmum particle size (SiO2-L) to silica sol of 5-20mmu particle size (SiO2-S) is 0.3-5.0. The surface of a Zn or Zn alloy plated steel material is coated with the chromating soln. and the coated steel material is dried without washing. A surface-treated steel material having superior corrosion resistance is obtd.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention is directed to the production of zinc-based plated steel (plated with Zn or Zn-based alloy) using a treatment solution with excellent long-term stability. The present invention relates to a chromate treatment method for imparting high corrosion resistance to steel materials.

<Prior art> In recent years, there has been a growing trend to reduce costs by means such as omitting the painting process, mainly in the fields of home appliances, OA equipment, copying machines, audio equipment, etc. There is an increasing demand for further improvement in corrosion resistance (white rust resistance) of chromate treatment, which has traditionally been carried out mainly for primary rust prevention of zinc-plated steel, and various research efforts are being carried out to meet this demand. has been done.

By the way, the chromate treatment methods that have traditionally been adopted as a measure to improve the corrosion resistance of zinc-plated steel materials are (a) reactive chromate treatment.

(bl　Electrolytic chromate treatment.

fc) It can be roughly divided into three types of coating type chromate treatment. However, among these “
There has been interest in ``spray-type chromate treatment'' as a method that can meet the recent demands for improved corrosion resistance as described above, and many proposals have been made for improving this technology.This ``spray-type chromate treatment'' Adding an etching agent and a film forming aid to an aqueous solution having the composition as a component, SO4''-,
This is a surface treatment method for metal materials in which a treatment liquid containing P O4''-1N O5-9F-, etc. is applied to the surface and then dried.

Among the proposals related to the above-mentioned chromate treatment method aimed at further improving corrosion resistance, 5iO
Adding z (Special Publication No. 1405, 1973).

Noteworthy methods such as Japanese Patent Publication No. 52-2851 (Japanese Patent Publication No. 52-2851) and adding a silane compound in addition to SiO□ (Japanese Patent Publication No. 59-35685) have been found, and this has improved the corrosion resistance of zinc-based plated steel materials. became possible.

Measures to Solve the Problems> However, the recent demand for corrosion resistance of zinc-plated steel materials has become higher and higher, and in order to meet this demand, JIS
Salt spray test specified in Z-2371 (S S T
), ``a certain level of high corrosion resistance with a white rust occurrence rate of 5% or less in 240 hours'' is now required.

Therefore, when considering conventional chromate treatment technology from the above perspective, it is true that adding silica sol to the treatment solution to improve corrosion resistance is an effective means, but chromate treatment solution with a high chromium concentration to increase corrosion resistance When silica sol is added, the stability of the solution is poor, leading to the formation of precipitates and gelling of the solution, making chromate treatment impossible.On the other hand, if the chromium concentration is reduced, The problem was pointed out that it was difficult to obtain a sufficient amount of Cr to provide sufficient corrosion resistance.

Furthermore, if the amount of silica sol added is increased, the deterioration of the solution becomes significant, and if the chromate treatment solution is applied after a long period of time, there is a problem that this results in a significant deterioration of corrosion resistance.

Of course, it is not acceptable to frequently prepare the treatment liquid from the viewpoint of cost and sanitary aspects such as disposal of waste liquid.

Therefore, considering the cost, etc., the stability of the liquid (including preventing precipitation and preventing deterioration of corrosion resistance even after application over time) needs to be maintained for at least 3 months. It was considered.

Therefore, the main object of the present invention is to provide a chromate treatment method that can impart high corrosion resistance to galvanized steel using a highly stable treatment solution.

<Means for Solving the Problems> The present invention has been made to achieve the above-mentioned object. value).

Silica sol: SiO,/Cr ratio of 0.1 to 6.
0, and the chromium is Cr &
The ratio of + is within the range of Crb*7 total Cr amount = 0.3 to 0.7, and the silica sol has a particle size of 40 to 50 mμ (referred to as SiOz-L) and a particle size of 5 to 20 mμ.
The proportion of silica sol (referred to as StO□-8) of μ is Si
By applying an aqueous chromate treatment solution with Oz-L/SrO□-5 in the range of 0.3 to 5.0 and drying it without washing with water, it will be good regardless of the conditions over time. It is characterized by the ability to impart excellent corrosion resistance to galvanized steel under easy working conditions.

Next, the configuration of the present invention will be explained in detail together with its operation.

Effects> In the present invention, silica sol is added to the chromate treatment solution in order to significantly improve the corrosion resistance of treated steel materials. The reasons why this silica sol improves corrosion resistance are as follows: a) Silica sol acts as a film forming agent.

b) Since Cr" is adsorbed on the surface of the silica particles, Cr"+ is retained in the coating film for a long time, and depending on the corrosive environment, the adsorbed Cr"+ gradually dissolves, causing damage to the galvanized steel sheet. Prevents corrosion.

It is possible that Therefore, the smaller the silica particle size, the larger the surface area of the silica, which increases the adsorption amount of Cr and improves corrosion resistance.In addition, the smaller the silica particle size, the more dense the chromate film itself becomes, resulting in better corrosion resistance. It is thought that.

On the other hand, when silica sol is added to a chromate treatment solution containing multivalent ions such as Cr'' and Cr'', the ion balance is disrupted, causing silica particles to aggregate and form precipitates, as described above. may occur, or a gelation phenomenon may occur. This tendency is due to Cr adsorbed on the surface.
``+'' is more noticeable for small particle size silica sol where the amount is large.

Therefore, in order to improve corrosion resistance, it is advantageous to add small particle size silica sol, but adding small particle size silica sol deteriorates the stability of the solution, making it difficult to achieve both corrosion resistance and stability of the processing solution. Very difficult.

Therefore, the inventors of the present invention conducted research through numerous experiments and studies, and found that ``If a small particle size silica sol, which is advantageous for corrosion resistance, and a large particle size silica sol, which is advantageous for processing liquid stability, are added in a predetermined ratio, We obtained new knowledge that "a chromate treatment solution can be obtained that provides excellent corrosion resistance to zinc-based plated steel materials and has good liquid stability." However, if you dry it without washing with water after applying it, it will stabilize the desired high corrosion resistance to zinc-based plated steel. The present invention was completed by investigating the fact that it can be applied by

Now, in the present invention, the total amount of chromium in the treatment liquid is 10 to 50 g/j in terms of chromic anhydride! In addition to adjusting the concentration so that SiOt
A chromate treatment liquid added to give an act ratio of 0.1 to 6.0 is used, but the total amount of chromium in the treatment liquid is limited to the above range for the following reasons. That is, the total amount of chromium in the treatment liquid is 10 g/j in terms of chromic anhydride?
If the content is less than 50g/l, the liquid concentration will be low and it will be difficult to form a romate film that is necessary to ensure sufficient corrosion resistance on the zinc-plated steel sheet after treatment.On the other hand, if the content exceeds 50g/l, the liquid concentration will be low. This is because not only does it become difficult to control the amount of chromate deposited because the amount of chromate becomes too high, but also the liquid stability deteriorates even if the silica sol added has a large particle size.

In addition, the amount of silica sol added is from 0.1 to 0.1 in terms of SiO2/Cr ratio.
6.0 because if the value is less than 0.1, it will be necessary to increase the amount of Cr deposited in order to achieve the desired corrosion resistance, which will only lead to processing difficulties. The amount of Cr'+ eluted from the finished material increases, which poses a hygiene problem.On the other hand, if silica sol is added at a SiOg/Cr ratio exceeding 6.0, the resulting chromate film becomes brittle, making the steel material This is because it becomes easy to peel off due to mechanical processing, etc., and also causes deterioration of corrosion resistance and extreme deterioration of processing solution stability.

By the way, chromium is contained in an oxidized state in the chromate treatment solution, and the oxidation state of chromium in the chromate treatment solution according to the present invention is Cr'' and Cr3+, but the proportion is Cr6+. /Total Cr amount (Cr ” ”
The sum of Cr and Cr) is adjusted to 0.3 to 0.7. This is because if the ratio of Cr6+/total Cr is less than 0.3, the self-healing effect of the chromate film will be impaired and the scratched area will be damaged. On the other hand, if the ratio of Cr6+/total Crfi exceeds 0.7, chromium elution occurs when the chromate film is exposed to water, which deteriorates the corrosion resistance of the film and causes problems such as environmental pollution. This is because it occurs.

The adjustment of the ratio of Cr6+/total Cr amount in the treatment solution is
This is done by adjusting the reduction rate (the amount of reducing agent added) when r'+ is reduced with ethylene glycol or the like.

The compound form of Cr''+ used in bath preparation is generally chromic anhydride (Crys), and in some cases dichromate or chromate.

Furthermore, in the present invention, silica sols with different particle sizes are added in order to improve the corrosion resistance of the chromate film and the stability of the treatment solution. Contributes to improving liquid stability,
Silica sol (St0x-S) with a thickness of 5 to 20 mμ contributes to improving corrosion resistance. Here, (St Oz-L/St
The reason why the (Oz-S) ratio was limited to 0.3 to 5.0 is as follows.

That is, when 5iOz-L/5iOt-S<0.3, the stability of the treatment liquid is poor and performance deterioration over time cannot be tolerated, whereas when StOz-L/SiO□-S>5.0, In order to obtain the desired corrosion resistance, it is necessary to increase the amount of Cr deposited, which leads to processing difficulties.

In addition, in the chromate treatment solution according to the present invention, F-, CI-,
It goes without saying that S Oa"-, P 0a3-, N08I-, etc. may be added. This is because corrosion resistance is slightly improved by adding these. Also, for the same reason, Zr0z, Ti0z, 5bzOs, etc. A combination of oxide sols may be added.

The chromate treatment liquid may be applied by any known coating method such as a roll coating method or a ringer roll drawing method.

Next, the effects of the present invention will be explained in more detail with reference to Examples.

<Example> A zinc or zinc-based alloy plated steel plate with a thickness of 0.8 m was prepared, and a chromate treatment solution having the composition shown in Table 1 was applied to it by the roll coating method, and then left as is. It was dried to produce a chromate-treated steel sheet.

Next, test pieces were taken from each of the obtained chromate-treated steel plates to examine their corrosion resistance, and the liquid stability of each of the chromate treatment liquids used was examined.

The following method was adopted for the corrosion resistance test of the chromate-treated steel sheet and the liquid stability test of the chromate treatment solution.

(A) Corrosion resistance test of chromate-treated steel plate Test pieces under the following three conditions were subjected to a salt spray test in accordance with Tsuki S Z2371 (salt spray test method), and the state of white rust formation was observed with the naked eye.

a) Flat plate part: The flat test piece was subjected to the test as it was, and the salt Water spray test was conducted for 240 hours.

b) Erichsen extrusion part: Test pieces are prepared using an Erichsen extrusion tester. The 5B extrusion was subjected to a test and Spray test was conducted for 150 hours.

C) Cross cut part: A cross cut on a flat test piece was subjected to the test, and the salt spray test was carried out at 240 Time implementation.

(B) Liquid stability test The chromate treatment solution used was kept in a constant temperature bath at 40"C, and the condition of the treatment solution was observed with the naked eye for 3 months. In addition, it was kept for 1 month to investigate the deterioration of corrosion resistance over time. The treatment solution after and after being held for 3 months was applied to a galvanized steel plate, and the same corrosion resistance test as above was conducted.

These test results are also shown in Table 1.

Here, the test results shown in Table 1 were evaluated in three stages as shown in Table 2, and each stage was indicated by ◯, △, and X marks.

As is clear from the results shown in Table 1, the zinc-based plated steel sheet treated by the chromate treatment method of the present invention exhibits extremely excellent corrosion resistance, and the chromate treatment solution of the present invention is extremely stable. It can be seen that even if this treatment liquid is used for a long period of time, the corrosion resistance does not deteriorate.

By the way, in Comparative Examples 13 and 14, the viscosity of the treatment liquid increased, and even when the coating treatment was performed after holding for 1 month and after holding for 3 months, the amount of Cr deposited increased, so the corrosion resistance improved, In Comparative Example 13, it was difficult to control the amount of Cr deposited, and the stability of the treatment solution was somewhat lacking, and in Comparative Example 14, the stability of the treatment solution was also somewhat poor, and in both cases, chromium was leached from the steel plate surface after treatment. This was not desirable as there were concerns about product quality.

Summary of Effects> As explained above, according to the present invention, it is possible to realize a romate treatment solution that is extremely stable over a long period of time, and it is also possible to use this to obtain a surface-treated steel sheet with excellent corrosion resistance. Industrially useful effects are brought about, such as the possibility of

Claims (1)

[Claims] On the surface of steel plated with zinc or an alloy containing zinc as the main component, chromium: 10 to 50 g/l in total chromium amount (value converted to chromic anhydride), silica sol: as SiO_2/Cr ratio 0.1-6.0
and the chromium is Cr^6 with respect to the total Cr amount.
Within the range of Cr^6^+/total Cr content = 0.3 to 0.7, the silica sol has a particle size of 40 to 50 mμ (referred to as SiO_2-L) and a particle size of 5 ~20
The proportion of silica sol (referred to as SiO_2-S) in mμ is S
Highly corrosion-resistant chromate treatment of zinc-based plated steel, characterized by applying an aqueous chromate treatment solution with iO_2-L/SiO_2-S=0.3 to 5.0 and drying without washing with water. Method.