JPH0352546B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Fields) Hot dip galvanized steel sheets, zinc alloy coated steel sheets such as zinc-aluminum alloy coated steel sheets, zinc-nickel alloy coated steel sheets, and zinc-iron alloy coated steel sheets can be used for a wide range of purposes. However, in order to prevent white rust during transportation or storage of these galvanized steel sheets, chromate treatment is often carried out within the plating line, and products subjected to this treatment are called chromate materials. . As a phenomenon peculiar to this chromate material, a blackening phenomenon in which the plated surface turns gray or black may occur, which is a problem because it impairs commercial value. The ease with which this blackening phenomenon occurs is influenced by the environmental conditions during transportation or storage, and tends to occur more easily in a packaged state than in an open state, and in a hot and humid environment. It also varies depending on the type of plating and plating method, such as hot-dip galvanized steel sheets and hot-dip zinc-aluminum alloy plated steel sheets.
It is much more susceptible to blackening than electrogalvanized steel sheets. Furthermore, it is known that blackening is sometimes promoted when leveler treatment or skin pass rolling is performed after plating. Regarding a surface treatment method for preventing blackening of molten zinc or hot-dip zinc alloy coated steel sheets without causing any adverse effects, this specification describes the usefulness of treatment using an alkaline solution containing iron ions together with a precipitation control agent. This paper describes the results of developmental research on gender. (Prior art) JP-A-55-131178 discloses that since the solidified galvanized layer has irregularities, when mechanical processing such as leveler treatment and skin pass rolling is applied to the solidified galvanized layer, the irregularities occur. The oxide film on the parts may be destroyed,
Recognizing that the removal of active zinc surfaces and exposing them is the cause of blackening, a method for preventing blackening has been disclosed in which a galvanized steel sheet is heated after the mechanical treatment and prior to chromate treatment. ing. Furthermore, JP-A No. 57-114695 discloses a method of completely dissolving oxides on the melt-plated surface of a zinc-based alloy with an alkali metal carbonate solution and then applying electrogalvanizing to prevent blackening. is disclosed. Furthermore, Japanese Patent Application Laid-Open No. 59-177381 discloses that after applying zinc or zinc alloy hot-dip plating to the surface of a steel plate,
It has been described that treatment with Ni ions, Co ions, or a solution containing a mixture of both is used to prevent the occurrence of blackening. The former two methods, such as heat treatment to improve the surface properties of the hot-dip galvanized layer and redundant treatment by electrogalvanizing, involve considerable costs and equipment disadvantages, while the latter It is difficult to control the time, processing solution temperature, processing solution concentration, etc. If the amount of Ni or Co precipitated is insufficient due to improper control, it will not be effective in preventing blackening, and if the amount is excessive, the surface will become blackish. It begins to show itself. (Problem to be solved by the invention) Accurately and stably with simple management of the processing liquid,
It is an object of the present invention to prevent the blackening phenomenon that occurs on the plated surface of molten zinc or molten zinc alloy plated steel sheets without causing any adverse effects. (Means for Solving the Problems) The inventors have conducted intensive research to prevent blackening and found that, as a treatment for preventing blackening, an alkaline solution containing iron ions is applied before chromate treatment. It has been found that it is particularly effective to precipitate Fe on the plated surface by treating the plated steel plate surface. However, this treatment with an alkaline solution containing iron ions is difficult to control, and depending on the treatment conditions, the plated surface after treatment may turn brown due to excessive precipitation of Fe, which reduces the commercial value as well as blackening. This left a negative impact. Therefore, the gist of the present invention is as follows, in which the alkaline solution containing iron ions further contains an Fe precipitation controlling agent. Aluminate ions and/or zincate ions to control the precipitation of iron ions and Fe on the surface of galvanized steel sheets that are subjected to chromate treatment after hot-dip galvanizing or hot-dip zinc alloy plating. 1. A method for surface treatment of a molten zinc or molten zinc alloy plated steel sheet, the method comprising applying an alkaline solution treatment containing the following. The iron ion may be either a ferrous ion or a ferric ion. As a compound that supplies iron ions,
Ferrous chloride, ferrous sulfate, ferrous ammonium sulfate,
It is possible to use compounds that are easily soluble in water, such as ferric chloride, ferric sulfate, ferric nitrate, ferric oxalate, ferric ammonium oxalate, ferric citrate, and ferric ammonium citrate. Advantageously, it is also possible to use iron oxide dissolved in acid. As a complexing agent, sodium gluconate, sodium gluconepeptate EDTA, NTA, DTPA (diethertriaminepentaacetic acid), TTHA (triethylene tetramine N′N′,N″N″-hexaacetic acid), CDTA
(cyclohexanediaminetetraacetic acid), trietacoolamine, tartaric acid, citric acid, oxalic acid, malonic acid,
Acetylacetone, etc. can be used, and although the complexing agent is not limited to those mentioned above, the concentration of the complexing agent should be at least the amount necessary to complex iron ions on the alkali side. Needless to say, it is necessary. In the treatment liquid, it is necessary that iron ions coexist with aluminate ions and/or zincate ions. This is because with alkaline solution treatment using only iron ions, it is difficult to properly control the precipitation of Fe on the plated surface, which often tends to cause brownish discoloration on the plated surface. When/or zincate ions are contained, appropriate and effective prevention of blackening is achieved for the first time without causing such brownish discoloration. The concentration of aluminate ion and/or zincate ion in the alkaline treatment solution is Al and/or Zn relative to Fe.
A wide range of 0.1 to 10 in terms of weight ratio is suitable for the purpose of this invention. In addition, the processing temperature can range from room temperature to the boiling point, but from the perspective of energy costs,
℃ is preferable. In particular, when this invention is applied to a zinc-aluminum alloy coated steel sheet, when painting is performed after chromate treatment, when phosphate treatment is applied as a base treatment, phosphates are removed due to the accumulation of Al ions in the phosphate treatment solution. There is also the advantage that there is no disadvantage of deterioration in processability and the life of the phosphoric acid treatment solution is extended. (Function) In the treatment of molten zinc or molten zinc alloy plated surfaces with an alkaline solution containing aluminate ions and/or zincate ions in addition to iron ions,
Under a wide range of processing conditions, a nearly constant amount of
Fe can be precipitated on the surface of the steel sheet, and Al on the plated surface can be removed, without causing discoloration to brown. Al or
When the ratio of Zn to Fe exceeds 10, the processing time becomes longer.
Processing within the plating line is somewhat difficult, and
On the other hand, when it is less than 0.1, the effect of suppressing Fe precipitation is lost and iron precipitation cannot be controlled. Therefore, due to fluctuations in processing time, processing temperature, etc., excessive Fe precipitation occurs and the appearance deteriorates significantly. There is. The pH of the alkaline treatment solution is preferably in the range of 9 to 14. If the pH is less than 9, the dissolving action of impurities such as aluminum present on the surface is insufficient, and the zinc dissolving power is also poor, so the amount of iron substitution precipitation is small, making it difficult to expect a sufficient blackening prevention effect. The amount of iron precipitation required to prevent blackening was determined by an experiment to be 0.2.
It was ^about 10 mg/m2. If it is less than 0.2 mg/m ² , there is no effect of preventing blackening, while if it exceeds 10 mg/m ² , the appearance becomes reddish brown and the product value is lost. (Example) A skin pass material made of a 5% Al-Zn alloy plated steel plate was used as the material, and the alkaline solution treatment shown in Table 1 was performed.

[Table] After washing with water, the blackening prevention effect was evaluated using a blackening acceleration test on specimens treated with chromate under the following conditions. Chromate treatment conditions Treatment liquid composition: chromic anhydride 10g/, sodium fluoride: 1g/, treatment temperature: 50°C, treatment time;
2 seconds, Processing method: Spray skies. Blackening acceleration test method: Place a 60 x 150 mm test piece with the test surfaces facing each other, wrap it in paper coated with vinyl on one side, tighten bolts at 4 points with a torque of 70 kg/fcm, and heat at 50℃95%.
After testing for 10 days in the RH humidity testing machine, the brightness (L value) before and after the test was measured with a color difference meter, and ΔL = (L value before the test)
Value) - (L value after the test) was graded and evaluated as follows:

[Table] In addition, discoloration due to the alkaline solution treatment itself was also evaluated according to Table 2 from the measured value ΔL of the color difference meter before and after the treatment. The test results are shown in Table-3.

【table】

[Table] (Effects of the invention) As the surface treatment method for hot-dip galvanized steel sheets or hot-dip zinc alloy-coated steel sheets, it is only necessary to treat them with an alkaline aqueous solution containing iron ions together with aluminate ions and/or zincate ions. It is a very simple procedure and does not require special equipment.
The quality of the chromate material can be advantageously improved by reliably preventing black spots without causing the problem of browning of the surface.

Claims (1)

[Claims] 1. Aluminate ions and/or zinc acid to control the precipitation of iron ions and Fe on the surface of the galvanized steel sheet that is subjected to chromate treatment after hot-dip galvanizing or hot-dip zinc alloy plating. applying an alkaline solution treatment containing ions;
A method for surface treatment of molten zinc or molten zinc alloy coated steel sheet, characterized by: