JPS6056436B2 - Surface-treated steel sheet with excellent corrosion resistance and phosphate treatment properties - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a surface-treated steel sheet with excellent bare corrosion resistance and phosphate treatment properties.

In general, steel sheets for automobiles are either used as they are after undergoing processing such as press forming during actual use, or they are treated with phosphate and painted before being used for the final purpose, but in the former case, bare corrosion resistance However, in the latter case, phosphate treatment properties and corrosion resistance after painting are very serious problems, and the object of the present invention is to provide a surface-treated steel sheet that is resistant to both of these problems.

As a recent trend, especially for automotive steel sheets, for example, in regions with heavy snowfall such as North America and Canada, rock salt has been sprinkled on roads to melt snow, but as a result, automotive steel sheets are exposed to severe corrosive environments. I ended up being put down.

In contrast, electrogalvanized and hot-dip galvanized steel sheets have been mainly used so far, but they are not resistant to bare corrosion, suffer severe corrosion in a relatively short period of time, and pose various safety problems. It has produced points.

Under these circumstances, there was a strong desire to develop surface-treated steel sheets with excellent corrosion resistance both bare and after painting, which could withstand salt damage, and steel manufacturers made great efforts to develop many surface-treated steel sheets. , one copy is commercially available.

For example, these include Zn-Ni series, Zn-Ni-Cr series,
Fe-Zn series, Fe-Zn-Ni series, Zn-Al series, Z
There are various surface-treated steel sheets, such as electroplated steel sheets such as n-Mn type and Zn-Ti type, synchrometal, and steel sheets further treated with chromate.

When surface-treated steel sheets are used as steel sheets for automobiles, they go through a process of press forming, degreasing or press forming, degreasing and monophosphate treatment, and painting. However, even with the new surface-treated steel sheets mentioned above, there are cases where bare corrosion resistance or post-painting corrosion resistance is poor. There are many products that are sufficient, and it can be said that there is no surface-treated steel sheet that satisfies both properties at the same time. As a method of improving phosphate treatment properties, a method of spraying a suspension of sodium phosphate immediately before phosphate treatment has been known.

There is also a known method of adding heavy metal salts to increase the reactivity of the phosphate treatment solution itself.

However, for example, the method of spraying a suspension onto a molded product after processing requires an additional step to be added to the series of phosphate treatment steps, which is a heavy burden in terms of equipment and cost, and does not require the specifications of the existing line. In some cases it may not be possible. The method of adding heavy metal salts to the treatment liquid itself accelerates the reaction of other parts when treating articles made of parts with various surface textures, such as automobiles, so it may be necessary to add heavy metal salts to some areas. There is a concern that film deposition may occur, and the cost of the chemical conversion treatment solution itself increases considerably.

Although all of the above measures improve the phosphate treatment properties of the surface-treated steel sheet to some extent, no improvement in bare corrosion resistance or the like is observed.

In response to this, the inventors of the present invention have conducted various studies and have discovered a technique that can be easily and reliably applied to the steel plate manufacturing process, and which has an extremely large processing effect.

It has been found that this method can significantly improve the bare corrosion resistance, phosphate treatment properties, and post-painting corrosion resistance of general surface-treated steel sheets.

Generally, the corrosion reaction rate in a salt spray test is determined by the diffusion of oxygen in the thin salt water film supplied to the metal surface, and does not depend on the metal type, unless a protective film is formed on the metal surface.

Salt spray tests were actually conducted on metal zinc plates and cold-rolled steel plates, and the results showed that the initial corrosion rates were almost the same. However, in long-term salt spray tests, salt water immersion tests, and exposure of galvanized iron sheets, a protective film of zinc hydroxide or basic zinc carbonate is formed, and the corrosion rate of zinc is lower than that of steel.

The protective effect of the zinc corrosion product film can be considered as follows.

That is, the corrosion reaction of zinc in a neutral atmosphere is expressed by the following equation. Anodic reaction: Cathode reaction: The generated Zn(0H)2 is easily dehydrated and converted into Zn.

In this way, the zinc corrosion product film contains ZnO. Zn
(0H)2 is included, but since ZrO is a P-type semiconductor and has electronic conductivity, it does not suppress the oxygen reduction reaction (2).

Zn(0H)2 has low electronic conductivity, and if Zn(0H)2 is formed on the surface, the oxygen reduction reaction (2) is suppressed and the corrosion rate is reduced. Therefore, it is considered that if a stable Zn(0H)2 film can be formed on the surface of a Zn or Zn-based alloy plated steel sheet, it will be effective in improving the corrosion resistance of the plated layer.

As a result of repeated extensive experiments over a long period of time under various corrosive environments, the inventors of the present invention have found that when specific elements are attached to the surface of Zn and Zn-based alloy plated steel sheets, the Zn(0H)2 film is stabilized. It was found that the corrosion resistance of the L-shaped steel sheet could be significantly improved, and not only the red rust resistance but also the hole resistance could be overwhelmingly improved. As a result of many studies, it was found that CO has the greatest effect on stabilizing the Zn(0H)2 film.

The next most effective one is NI. ．． C.U. ．． M.O. ．． W. ．． S
ilSn, which was found to be quite effective, although not as effective as CO.

Next is Mn, which varies considerably depending on the corrosive environment and may be effective or completely ineffective.
Overall, it was found that although it is relatively effective in a dry environment, it is not so effective when left in a wet environment for a long time.

The present invention will be explained in detail below.

From the above study results, the present invention provides a method for applying Ni. ．． C.O. ．． Cu. ．． M.O. ．． Wl
Sl. ．． The purpose is to allow one or more metals selected from Sn to exist in an amount of 3 to 500 m9/d.

When the present invention is actually applied, for example, after completion of plating in various plating lines, it is sufficient to wash with water and immediately enter the main treatment. For example, the case of CO will be explained.

A Zn-Nl surface treated steel sheet with a basis weight of 20 y/d was electroplated with CO in various proportions, and the subsequent evaluation of bare corrosion resistance, phosphate treatability, and post-painting corrosion resistance was reported. It is shown in Fig. 1, Fig. 2, and Fig. 3.

Here, the CO plating conditions are bath: COSO4・7H2010y/e (Nlll)2・SO42Oy/EPH: 6.8
~7.0 Bath temperature: 50°C DK = 1A/d.

As is clear from the figure, the bare corrosion resistance of Zn-Ni plating improves depending on the amount of CO plating;
It is extremely noteworthy that even the extent of adhesion of /771' is considerably improved.

In addition, Zn-Ni galvanized steel sheets usually do not have sufficient phosphate treatment properties, but CO 3ffi9/
In the adhesion region of d to 500 m9/d, phosphate crystals become fine and uniform crystals are formed.

If it is less than 3mg/Wt, the crystals will become coarser and 500m9/Wt.
Similarly, when the temperature exceeds d, the crystal becomes coarse.

Next, after the phosphate treatment, a commercially available ED coating was applied, followed by an intermediate coating and a top coating, and the corrosion resistance after coating was examined. As an investigation method, a cross cut was made on the painted material, a salt water spray test was conducted for 1000 hours, and the corrosion width of the cross cut portion was measured.

As is clear from Figure 3, CO is 3m9/d~500T
rL9/A has extremely good paint corrosion resistance of 3mg/j
It was found that there is a slight decrease when the distance is less than 1' or more than 500 m9/i.

As is clear from the above results, CO is 3m9/d~5
00m9/77! In addition, the bare corrosion resistance, phosphate treatment properties, and post-painting corrosion resistance of Zn--Nl alloy plating are greatly improved.

I explained about CO, but although it is not as good as CO, Ni. ．．
Cu. , M.O. ．． W,. Si. Almost similar results were obtained for sSn.

Furthermore, almost the same results were obtained by plating with, for example, a Ni--CO alloy.

In addition, although various treatments have been described above for Zn-Ni alloy plating, other Zn alloy platings such as F
e-Zn series, Fe-Zn-Ni series, Fe-Zn-Ni-
CO-based, Zn-Mn-based, Zn-Al-based, Zn-Ni-C
Approximately similar results were obtained for various surface-treated steel sheets such as O-based, Zn-Nj-Cr-based, and Zn single-layer steel sheets.

The various surface-treated steel sheets plated with Zn and various Zn-based alloys mentioned above include electroplating and hot-dip plating. Further, various types of chromate-treated steel sheets are typical examples of surface-treated steel sheets, but it is generally said that it is extremely difficult to form a phosphate film on a chromate-treated film.

However, when the treatment according to the present invention is carried out, the corrosion resistance is further improved, as in the case of the steel plate mentioned above, and when the phosphate treatment is carried out, dense and uniform phosphate crystals are formed, and after painting, It was found that corrosion resistance was significantly improved.

From the above results, in the present invention, CO,
．． Ni. ．． Cu. ．． M.O. ．． W,. One or more metals selected from Si are present in an amount of 3 to 500 mg/d. The above treatments can greatly improve the bare corrosion resistance, phosphate treatment properties, and post-painting corrosion resistance of various surface-treated steel sheets.

The reason why the bare corrosion resistance, phosphate treatment properties, and post-painting corrosion resistance are greatly improved when the above-mentioned metals are present in a specific amount in various surface-treated steel sheets is considered to be as follows.

For example, when a specific amount of CO is present in a Fe-Zn-Ni alloy plating, the CO adheres to active sites on the alloy plating surface.

In other words, this means that CO adheres to points where CO is likely to react, that is, points that are likely to corrode. It seems that it will.

Furthermore, as mentioned above, Zn(0H)2 formed on the surface due to the presence of CO does not change to ZnO, and Zn(0H)2, which does not have conductivity, stably exists as a barrier. Corrosion is significantly suppressed.

It is thought that overwhelming corrosion resistance can be obtained by the combined action of both. On the other hand, the amount of CO deposited in the region shown in the present invention is not uniformly deposited, and CO is dispersed as nuclei on the surface of the surface-treated steel sheet.

When the metal is dispersed in such a state, when the metal is further treated with phosphate, phosphate crystals grow on the nucleating base in which -CO is dispersed and develop uniformly.

That is, since phosphate crystals develop using CO, which is already dispersed, as a nucleus, phosphate crystals become uniform and dense depending on the state of CO.

On the other hand, when coating is applied on a phosphate coating formed under the above conditions, an excellent coating is formed due to the excellent phosphate crystals, and the corrosion resistance after coating is greatly improved.

As explained above, the properties of the surface-treated steel sheet are greatly improved by the present invention, but the Ni. ．． C.O.
．． Cu,. M.O. ．． W. ．． Si. ．． Fe,. The method for causing the presence of Zn is not particularly limited, and electroplating or chemical plating may be used.

Hereinafter, the present invention will be described in detail with reference to specific examples.

Example 1 The bare corrosion resistance of a surface-treated steel sheet plated with Zn-N1 alloy at 20 da/Rrl and electroplated with CO at 25 m9/d was examined by SST. As a result, 5 to 10% of red rust was observed after 14 days.

Further, when phosphate treatment was performed, a uniformly dense phosphate film was formed.

Furthermore, after applying ED coating, intermediate coating, and top coating, the coating was cut and left in SST for 1000 hours, and the corrosion resistance of the cut portion was examined. As a result, there was no evidence of corrosion.

On the other hand, the bare corrosion resistance in SST of the surface-treated steel sheet plated with Zn-Ni alloy at 20y/Rrl showed that 20 to 30% of red rust was observed after 7 days.

In addition, when phosphate treatment was performed, the phosphate crystals grew considerably larger and were not dense crystals.

In addition, after applying ED coating, intermediate coating, and top coating, the coating film was cut and left in SST for 1000 hours, and the corrosion resistance of the cut portion was examined. It was corroded and the paint film was peeling off.

Example 2 The bare corrosion resistance of a steel plate electroplated with 50 m9/Rll of MO on a surface-treated steel plate plated with zn-Ae alloy at 20q/7T (') was found to be 10 to 1% red rust after 14 days.
5% were accepted.

Further, when phosphate treatment was performed, a dense phosphate film was uniformly formed.

Furthermore, after applying ED coating, intermediate coating, and top coating, the coating was cut and left in SST for 1000 hours, and the corrosion resistance of the cut portion was examined. As a result, there was no evidence of corrosion.

On the other hand, the bare corrosion resistance in SST of a surface-treated steel sheet plated with Zn-A' alloy at 20y/d shows that the red rust level is 1 after 7 days.
0-15% was observed.

Furthermore, when lysotic acid salt treatment was performed, phosphate crystals were hardly observed, and only a few phosphate crystals were observed in a mottled manner.

In addition, ED coating, intermediate coating, and top coating were applied, and the coating film was cut and left in SST for 1000 hours, and the corrosion resistance of the cut portion was examined. As a result, the coating film was peeled off almost entirely.

Example 3 The bare corrosion resistance of a surface-treated steel sheet plated with Fe-Zn alloy at 60 m9/d and Ni chemically plated at 30 m9/d was examined by SST, and red rust was observed in 50 to 70% after 14 days. Ta.

Further, when phosphate treatment was performed, a uniformly dense phosphate film was formed.

Furthermore, after applying ED coating, intermediate coating, and top coating, the coating was cut and left in SST for 1000 hours, and the corrosion resistance of the cut portion was examined. As a result, there was no evidence of corrosion.

On the other hand, the bare corrosion resistance in SST of the surface-treated steel sheet plated with Fe-Zn alloy at 60 f/d was 60 to 80% red rust after 7 days.

Furthermore, when phosphate treatment was carried out, the phosphate crystals developed somewhat larger and were not dense crystals.

In addition, we applied ED coating, intermediate coat, and top coat, cut the paint film, left it in SST for 1000 hours, and examined the corrosion resistance of the cut part.As a result, 4 to 5 spots of corrosion occurred mainly in the cut part, and the paint film peeled off. Was.

Example 4 The bare corrosion resistance of a copper plate electroplated with Cu at 15m9/d on a surface-treated steel plate plated with zn-Nl-Cr alloy at 20y/d was found to be 20-40% red rust after 14 days.
Admitted.

Further, when phosphate treatment was performed, a uniformly dense phosphate film was formed.

Furthermore, after applying ED coating, intermediate coating, and top coating, the coating was cut and left in SST for 1000 hours, and the corrosion resistance of the cut portion was examined. As a result, there was no evidence of corrosion.

On the other hand, the bare corrosion resistance in SST of the surface-treated steel sheet plated with Zn-Nl-Cr alloy for 20 y/d showed that 30 to 50% of red rust was observed after 7 days.

In addition, when phosphate treatment was performed, the phosphate crystals grew considerably larger and were not dense crystals.

In addition, after applying ED coating, intermediate coating, and top coating, the coating film was cut and left in SST for 1000 hours, and the corrosion resistance of the cut portion was examined. It was corroded and the paint film was peeling off.

Example 5 The bare corrosion resistance of a surface-treated steel sheet plated with 20y/Rrl Zn-Mn alloy and electroplated with W at 30mg/wl was examined by SST. After 14 days, red rust was 10 to 20%.
Admitted.

Further, when phosphate treatment was performed, a uniformly dense phosphate film was formed.

Furthermore, after applying ED coating, intermediate coating, and top coating, the coating was cut and left in SST for 1000 hours, and the corrosion resistance of the cut portion was examined. As a result, there was no evidence of corrosion.

On the other hand, the bare corrosion resistance in SST of the surface-treated copper plate plated with 20 g/7T1 of Zn-Mn alloy showed that 10 to 20% of red rust was observed after 7 days.

In addition, when phosphate treatment was performed, the phosphate crystals grew considerably larger and were not dense crystals.

In addition, after applying ED coating, intermediate coating, and top coating, the coating film was cut and left in SST for 1000 hours, and the corrosion resistance of the cut part was investigated.
Tt corrosion occurred and the coating film was peeled off.

Example 6 Chromate treated steel plate (TFS: Cr25m9/
A) When the bare corrosion resistance of the steel plate electroplated with Si at 10 y/d was examined by SST, 5 to 10% of red rust was observed after 3 days.

Further, when phosphate treatment was performed, a uniformly dense phosphate film was formed.

In addition, after applying ED coating, intermediate coating, and top coating, the coating was cut and left in SST for 100 hours, and the corrosion resistance of the cut portion was examined. As a result, there was no evidence of corrosion.

On the other hand, chromate treated steel plate (TFS: Cr25m
9/d) SST bare corrosion resistance is 70% red rust after 1 day.
~80% was recognized.

Furthermore, when phosphate treatment was performed, almost no phosphate crystals were observed.

In addition, after applying ED coating, intermediate coat, and top coat, the paint film was cut and left in SST for 100 hours, and the corrosion resistance of the cut part was examined. As a result, corrosion occurred for 3 to 4 wIn mainly in the cut part, and the paint film peeled off. was.

Example 8 70 mg of Ni-CO alloy was applied to a surface-treated steel plate plated with two layers of 10y/Rfl of Zn-Mn alloy.
/d As a result of examining the bare corrosion resistance of the electroplated steel sheet by SST, 30 to 40% of red rust was observed after 14 days.

Further, when phosphate treatment was performed, a uniformly dense phosphate film was formed.

Furthermore, after applying ED coating, intermediate coating, and top coating, the coating was cut and left in SST for 1000 hours, and the corrosion resistance of the cut portion was examined. As a result, there was no evidence of corrosion at all.

On the other hand, the bare corrosion resistance in SST of the surface-treated steel sheet plated with two layers of Zn--Mn of 10 y/d showed 35 to 45% red rust after 7 days. Further, when phosphate treatment was performed, phosphate crystals were slightly observed in a mottled manner.

In addition, after applying ED coating, intermediate coating, and top coating, the coating film was cut and left in SST for 1000 hours, and the corrosion resistance was investigated.
It was corroded and the paint film was peeling off. l

[Brief explanation of the drawing]

Figure 1 shows that C
A diagram showing the results of evaluating the corrosion resistance after plating with O in various proportions, Figure 2 shows the results of the phosphate treatment, and Figure 3 shows the results of the corrosion resistance of 7 after the same coating. FIG.

Claims (1)

[Claims] 1 Ni, Co on the surface of Zn and Zn-based alloy plated steel sheet
, Cu, Mo, W, Si, and Sn, the surface treatment is characterized by the presence of 3 to 500 mg/m^2 of one or more metals selected from the group consisting of Cu, Mo, W, Si, and Sn. steel plate.