JPS62202097A - Galvanized steel sheet and surface treatment thereof - Google Patents

Abstract

PURPOSE:To form a film which contains Cr and P at specified ratios and has high corrosion resistance by coating a cationic or anionic org. material on the surface of a galvanized steel sheet then impressing AC to the coating in an electrolyzing bath contg. hexad Cr and phosphate ions. CONSTITUTION:The org. material having cations or anions and the salt thereof, for example, oil amine acetate, etc., are coated on the surface of the steel sheet plated with zinc or zinc alloy contg. >=60wt% zinc. Electrolysis is then executed by the AC in the electrolyzing bath contg. the hexad chromium and phosphate ions. The mixed film made of the compsn. consisting of 10-500mg/m<2> as the content of the org. material and chromium and <=500mg/m<2> as content of phosphorus is formed by such treatment. The film is highly fixed with the hexad chromium by the effect of the org. ion group and contains the org. material and therefore, the film exhibits water repellency and has good corrosion resistance.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a galvanized steel sheet and a surface treatment method thereof.

[Prior art]

Galvanized or zinc-based alloy coated steel sheets are widely used in automobiles, home appliances, outdoor equipment, building materials, etc. due to their excellent corrosion resistance. However, its corrosion resistance is not sufficient, and treatments such as chromate are generally performed. These chromates are electrolytic chromates using a direct current cathode depending on the method of forming the film.

It is broadly classified into reactive chromate and applied chromate.

Among these, the first two chromates have a film component of trivalent C.
This is a Cr''/Cr type film whose main component is r, which does not have sufficient corrosion resistance compared to coated chromate.The later coated chromate has a coating composed of cr''+/Cr'+, but The disadvantages of this are that hexavalent chromium is easily eluted into water, so it is easy to discolor, and corrosion resistance cannot be sufficiently improved.

[Problem to be solved by the invention]

As a solution to these problems, coating type chromate Cr
In order to increase the fixation rate of I'+, there are known methods such as adding additives to the treatment solution (Japanese Unexamined Patent Publication No. 128268/1982) and converting Cr" and 0r34' with reducing agents. However, both have the drawback that the reaction solution tends to deteriorate due to the reaction between the additive and chromic acid.

The present invention is a galvanized steel plate that provides excellent corrosion resistance by forming a film in which Cr is fixed on the galvanized steel plate without degrading the chromate-1 treatment solution, and a method for manufacturing the same.

[Means for solving problems]

The features of the present invention are as follows: (1) Anionic or cationic group-containing organic substances and their salts are added to the surface of zinc plating or zinc alloy plated steel sheet containing 60% by weight or more of zinc, and the amount of chromium is 10 to 500+o.
g/rn, 500mg/n as phosphorus amount? A galvanized steel sheet on which a surface film consisting of the following is formed.

■ Organic substances and their salts having anionic or cationic groups on the surface of zinc plating or zinc alloy plated steel sheet containing 60% by weight or more of zinc and 10 to 500 mg of chromium.
/ m, when forming a surface film with a phosphorus content of 500 mB/rn' or less, the surface of the galvanized steel sheet is coated with an organic substance having an anion or cation group and its salt, and then hexavalent chromium and phosphate ions are coated. The present invention relates to a method for surface treatment of plated steel sheets, characterized by applying alternating current in an electrolytic bath solution containing the present invention.

According to research conducted by the present inventors, a film formed by applying an alternating current to a zinc-based plated steel sheet in an electrolytic solution containing chromic acid and phosphoric acid is formed by a combination of Zn2+ and chromate ions eluted from the coating layer by the adjacent flow of the alternating current at the interface. Volume is rate-limiting. In order to carry out this reaction more efficiently, the plating surface layer is coated with cationic or anionic organic substances, and electrolysis is performed with alternating dε in an electrolyte containing hexavalent chromium and phosphate ions. Ions are concentrated at the interface and form a mixed film of organic matter, chromium, and phosphoric acid.

This film has high fixation of hexavalent chromium due to the effect of organic ionic groups, and since it contains organic substances, it exhibits water repellency and has very good corrosion resistance.

The galvanized steel material that can be treated in this way is zinc 60%.
It is necessary to contain at least % by weight. It can be applied to electrogalvanized steel sheets, hot-dip galvanized steel sheets, zinc sprayed steel sheets, zinc vapor-deposited steel sheets, etc. regardless of the plating method.

Further, alloyed hot-dip galvanized steel sheets and electrically alloyed steel sheets 5 having a surface layer containing 60% by weight or more of zinc, such as zinc-iron and zinc-nickel alloy-plated steel sheets, can also be treated.

Other elements, such as Mn and Cr.

Sn, Pb, A1. Mo, J Co, Ti,
P, Si, Na, Ca, Mg.

Even if 0, C, etc. are contained, it is applicable as long as zinc is contained in an amount of 60% by weight or more.

If necessary, pretreatments such as pickling and surface grinding can also be applied.

The organic substances that coat the surface of zinc-based plated steel sheets are anionic or cationic surfactants such as the following: 0S0311
It is necessary to have one or more functional groups that can become anions or cations in an aqueous solution, such as -0PO(O)l), -0PO-OH-OR groups, and the number of carbon atoms in the main chain. is 8 or more and 22 or less.

The main chain and side chain of this organic compound can be either saturated or unsaturated, and aromatic ones such as laurylbenzene groups are also effective. Moreover, the effect is greater when the above functional group is located at the terminal of the compound.

By coating the surface of a zinc-based plated steel sheet with the organic compound described above, the organic compound becomes an anion or a cation on the surface of the steel sheet during electrolysis in a chromate solution, which will be described later, and becomes a zinc ion.

chromate ions, etc. are concentrated at the interface to make the chromate film more dense. If the number of carbon atoms in the main chain is 7 or less, adsorption to the plating surface is insufficient, and the coated organic compound is likely to dissolve into the electrolyte afterward, which is less effective. If the number of carbon atoms in the main chain is 23 or more, , difficult to obtain.

Moreover, the above organic compounds are also effective in the form of salts of ammonia, sodium, potassium, acetic acid, formic acid, sulfuric acid, and the like. The method for coating the above organic compounds is to dissolve them and their salts in alcohol, acetone, water, etc., and then coat them with a roll coater.
Any method, such as a method, a dipping method, a spray method, etc., may be employed.

The coating amount of these organic compounds may range from a trace level to 1 g/M or less. It is effective enough to form a monomolecular film.
If the concentration exceeds Ig/m, organic substances may be leached into the chromate electrolytic bath and the deterioration of the chromate solution may be stopped. After the above pretreatment, chromate treatment is applied.
In the present invention, electrolysis is performed using alternating current in a treatment solution containing hexavalent chromium and phosphoric acid.

The amount of hexavalent chromium in the treatment liquid is Cry, converted to 5g/Q~2
00gIQ, phosphoric acid amount is 0.1g/Q in terms of PO43-
~100g/Q, pl+ can be treated with 1 or more and 12 or less. As a particularly preferable range, the amount of chromium is Cry
, in terms of Log/Q~100g/Q, the amount of phosphorus is po
, 10g/Q~50g/Q, pt+ is 3 in 3-conversion
~8.

If the amount of chromium is less than 5g/(1, it will be sufficient for the film and the amount of chromium will not be obtained, and if it is more than 200gIQ), it will not affect performance improvement such as corrosion resistance, and the cost of preparing the treatment solution will increase, so it is not a good idea.Amount of phosphorus If it is less than 0.1g/Q in terms of PO43', phosphorus will be absorbed into the film and it will have good corrosion resistance and weldability.If it is more than 100g/Q, it will have little effect on the performance and This is not a good idea as it increases the production cost of 1.If the pH is less than 1, the pretreated amine-based organic matter may dissolve into the treatment solution, and the effect of the pretreatment may be lost.Also, if the pn is 12 Even with the above, the pretreated organic substances will be leached out, and the effect will be diminished.

Hexavalent chromium ions and phosphate ions can be prepared as salts or acids with NH, ", alkali metals, etc., and are not necessarily Nl+4+ as in US Pat. No. 3,011,958.
Ions are not essential, and free alkali or phosphoric acid, chromic anhydride, or other acids may be added to adjust the pH. Fluorine ion.

Addition of halogen ions such as chlorine ions, sulfate ions, etc. is sometimes effective in accelerating the reaction of the plating layer.

In addition, Si2. TiO2, ZnO, 5b2
It is sometimes effective to add colloids such as O, 54102, A140i, molybutenic acid, vanadic acid (and salts thereof), etc. Addition of metal ions such as Zn2+ ions is sometimes effective in preventing unnecessary dissolution of metals such as zinc in the plating layer.

Additionally, the addition of metal ions such as Cr 3 +, Co"m Ni", Mn"+, etc. is sometimes effective in improving the fixation rate of chromium and corrosion resistance. The treatment solution temperature is 10°C or higher, up to the boiling point. Effective, but preferably within 40°C to 80°C,
If the temperature is less than 10°C, the liquid resistance becomes high, and the power cost and reactivity are lost.

Electrolysis requires alternating currents of adjacent current and blocking current.

By treating with alternating current in the above treatment bath, the film formed is Cr', according to ESCA and EPMA analysis.
Cr2+, Cr3+, Cri+, Zn'', PO4
3- forms a well-mixed, dense film that increases the fixation rate of chromium while containing hexavalent chromium, improving corrosion resistance. Also, this film is free from phosphate and oxidation! T! Contains lead, which forms a resistant skin that lowers the nugget formation limit current during welding and improves continuous dotting performance.

In addition, due to this electrolysis, the metal Cr present near the plating layer suppresses metal contact with the plating mold during press processing,
Improves smoothness and prevents peeling of plating (adhesion to mold)
decrease. The waveform of the applied current is a sine wave, a triangular wave,
Although a rectangular wave etc. can be applied, the former two are more preferable in order to speed up film formation. It is also possible to change the ratio of the positive half wave and negative half wave by adding a DC bias current or the like.

The valid range of the ratio of positive half wave and negative half wave is 1 in average current ratio.
The range is from 7-9 to 9/-1, but particularly preferably from 2/-8 to 6/-4. When the negative current ratio is 1/-9, there is less etching of the plating and a sufficient film is not formed. 9/-
If the positive current is greater than 1, zinc, etc. in the plating will be eluted so strongly that no film will be formed.

Although it can be applied at a frequency of IHz or higher, it is not advisable to use a high frequency range of 10'Hz or higher because the film formation will be slow.

An electrolytic interelectrode voltage of 111 V or more and 50 V or less is effective, but 5 V to 20 V is particularly preferred. If it is less than IV, film formation will not take place, and if it is more than 50 V, discharge will occur and the plated surface and film will be destroyed.

After electrolytic treatment, washing with water and drying is usually carried out, but it is sometimes effective to remove excess treatment liquid with roll squeezing, air knife, etc. after electrolysis, and then heat drying to improve corrosion resistance.

The electrode for AC electrolytic treatment can be applied to zinc, zinc-based alloys, or the plated steel sheet itself when treating continuous strips. Also, Ni or Ni-based alloy, Sn
PT-based alloys, PT-based alloys, PT-based alloys, carbon-based electrodes, and conductive ceramic materials can also be applied.

In the chromate film obtained by the above treatment,
The effective amount of skin wX in terms of corrosion resistance, workability, and weldability is 10 to 500a+g/m'' in terms of chromium and 500mg/+a2 or less in terms of phosphorus, but especially C
r2O~20001E/ll+", P 5~200m
If Cr is less than 110mg/m'', sufficient corrosion resistance cannot be obtained, and if it is more than 500II1g/II+2, the corrosion resistance will not improve much and the chromium fixation rate will also be low, which is not a good idea. Phosphorus improves processability. For good results, it is necessary to have at least a trace, and the larger the amount, the higher the resistance and the better weldability.

If it exceeds 500 mg/+'', the resistance becomes too high and welding becomes impossible.

〔Example〕

In the examples, the steel plates to be treated, the associated treatments, treatment bath compositions, and AC application conditions are shown in Table 1, and for the examples of the present invention, the coating amount, corrosion resistance, welding limit current,
Continuous dot performance and film adhesion are shown in Table 2.

〔Effect of the invention〕

By doing so, corrosion resistance, weldability, and workability are significantly improved, and an industrially excellent rust-proof steel plate can be obtained.

Claims (2)

[Claims] (1) Organic substances and their salts having anionic or cationic groups on the surface of zinc plating or zinc alloy plated steel sheet containing 60% by weight or more of zinc, and chromium content of 10 to 500%.
A galvanized steel sheet formed with a surface film having a phosphorus content of 500 mg/m^2 or less. (2) 10 to 500 m of organic substances and their salts having anionic or cationic groups and chromium on the surface of zinc plating or zinc alloy plated steel sheet containing 60% by weight or more of zinc.
To form a surface film with a phosphorus content of less than 500 mg/m^2, the surface of the galvanized steel sheet is coated with an anionic or cationic group-containing organic substance and its salt, and then hexavalent chromium and phosphorus are coated on the surface of the galvanized steel sheet. A method for surface treatment of galvanized steel sheets, characterized by applying alternating current in an electrolytic bath solution containing acid ions.