KR20120133534A - surface treatment composition for pickling and bleaching of stainless steel, surface treatment method using the same - Google Patents

Abstract

PURPOSE: A surface treatment composition for pickling and bleaching stainless steel and a surface treatment method using the same are provided to prevent the production of effluent containing odor, toxic gas, and nitric nitrogen by using sulfuric acid and sodium fluoride instead of nitric acid and hydrofluoric acid. CONSTITUTION: A surface treatment composition for pickling and bleaching stainless steel comprises 60-80wt.% of water, 10-30wt.% of sulfuric acid, 3-8wt.% of sodium fluoride, and 3-5wt.% of sodium chlorate. The surface treatment composition also includes polyaluminum chloride for accelerating reaction.

Description

Surface treatment composition for pickling and bleaching of stainless steel, surface treatment method using the same

The present invention relates to a surface treatment composition for pickling and bleaching stainless steel and a surface treatment method using the same, and more particularly, to pickling of stainless steel, which does not contain nitric acid and hydrofluoric acid to prevent odor (air pollution) and generation of wastewater. And a surface treatment composition for bleaching and a surface treatment method using the same.

Stainless steel is a product that greatly improves corrosion resistance by forming various alloys such as Cr, Ni, Mo, and Cu in general steel, and these alloying components form a passivation film on the surface of metal to prevent oxygen from being exposed to air. It does not progress oxidation, and it is excellent in corrosion resistance.

However, stainless steel is subjected to heat treatment during processing of the product, and the surface of the product is not good because there is a black oxide scale (Fe ₂ O ₃ , Cr ₂ O _3, etc.) on the surface.

Therefore, conventionally, the oxide scale layer is chemically dissolved and commercialized by washing with a solution containing nitric acid and hydrofluoric acid, and this process is called a pickling process. After pickling the surface is chemically activated, which means that the surface is once again coated with an optically interfering surface layer in air. This can be prevented by passivating the pickled surface during or after pickling.

In the conventional process, it is necessary to use nitric acid so that the product is beautiful and the passivation film can be evenly formed, and since stainless steel has been developed, it has been pickling continuously using a mixture of nitric acid and hydrofluoric acid.

However, as nitric acid is used, high concentrations of NOx gas are generated, and waste acid and product wash water also contain high concentrations of nitrate nitrogen (NO ₃ -N), causing problems in the air and water.

Exhaust gas purification facilities and wastewater treatment facilities have been used together to prevent this, but due to the strengthening of environmental regulations around the world and the significant increase in environmental costs, it is difficult to stably comply with environmental standards. In addition, environmental problems are caused even when phosphoric acid is used instead of nitric acid. In addition, when using hydrochloric acid it is difficult to control the reaction rate, there is a problem that the generation of toxic gas is severe.

Hydrofluoric acid, mixed with nitric acid, has a strong odor, causing work environment and air pollution problems.

The present invention has been made to improve the above problems, and can be used for various types of stainless steel and does not contain nitric acid and hydrofluoric acid to prevent pickling and bleaching of stainless steel that prevents the generation of odor and wastewater containing nitrate nitrogen. It is an object of the present invention to provide a surface treatment composition and a surface treatment method using the same.

Surface treatment composition for pickling and bleaching of stainless steel of the present invention for achieving the above object is 60 to 80% by weight of water, 10 to 30% by weight of sulfuric acid, 3 to 8% by weight of sodium fluoride, 3 to 5% by weight of sodium chlorate It is characterized by containing.

It is characterized by further containing polyaluminum chloride to promote the reaction.

And the surface treatment method for pickling and bleaching the stainless steel of the present invention for achieving the above object is characterized in that the stainless steel pickling or bleaching treatment using the surface treatment composition.

As described above, according to the present invention, sulfuric acid and sodium fluoride are used instead of the conventionally used nitric acid and hydrofluoric acid to prevent the occurrence of odor, harmful gas and nitrate-containing waste water, thereby improving the working environment and improving air and water quality. There is no fear of contamination.

Hereinafter, a surface treatment composition for pickling and bleaching stainless steel according to a preferred embodiment of the present invention and a surface treatment method using the same will be described in detail.

Surface treatment compositions for pickling and bleaching stainless steels of the present invention can be applied to the manufacture of stainless steels of any shape, such as wires, rods, tubes, plates, coils, and final products.

The surface treatment composition according to an embodiment of the present invention may be formed by mixing sulfuric acid, sodium fluoride, and sodium chlorate in water at a predetermined ratio.

In the surface treatment composition of the present invention, sulfuric acid and hydrofluoric acid are applied instead of a mixed acid of nitric acid and hydrofluoric acid which are commonly used.

Surface treatment compositions may typically contain one or more strong acids for high pickling and bleaching treatments. Such strong acids preferably comprise at least one strong acid in order to have a pH value of 2.5 or less, particularly preferably 1 or less.

For example, it has been conventionally used in the form of nitric acid, hydrochloric acid and phosphoric acid alone or mixed acid mixed with hydrofluoric acid as a strong acid, but nitric acid and phosphoric acid are difficult to treat wastewater, hydrochloric acid is difficult to control the reaction rate, toxic gas There is a problem that the occurrence of is severe. In addition, the mixed acid mixed with hydrofluoric acid is also a problem of odor and air pollution by the hydrofluoric acid gas generated from hydrofluoric acid.

Accordingly, the present invention is to improve the problems of nitric acid, forestry, hydrochloric acid, hydrofluoric acid by using sulfuric acid and hydrofluoric acid. The preferred content of sulfuric acid is 10 to 30% by weight. If the content of sulfuric acid is less than 10% by weight, the reactivity is remarkably decreased, and if it exceeds 30% by weight, sodium chlorate, an oxidizing agent, may be decomposed.

And the preferred content of hydrochloride is 3 to 8% by weight. If the content of fluorate is less than 3% by weight, the reactivity is lowered, and if it exceeds 8% by weight, salt may be eluted on the surface of the steel. The most preferred form of fluorate is sodium fluoride. In addition to sodium fluoride, ammonium fluoride generates ammonia fluoride gas which is similar to hydrogen fluoride. Magnesium fluoride and calcium fluoride are almost insoluble in water, and potassium fluoride has good solubility but is too expensive. In addition, boric acid or salts thereof do not have an odor but are difficult to treat wastewater due to difficult decomposition.

As described above, the present invention containing sulfuric acid alone as a strong acid may reduce the processing speed compared with a conventional mixed acid of nitric acid and hydrofluoric acid, and thus an accelerator may be added to compensate for this. Small amounts of chlorine compounds can be added as promoters. Polyaluminum chloride, which is a polymer stabilized with a preferred promoter. Such promoters may be contained from 0.01 to 0.3% by weight. If no accelerator is added or less than 0.01% by weight, the reaction is very slow until a suitable amount of Fe ions are contained in the liquid. In addition, accelerators exceeding 0.3% by weight may cause pitting.

Chloride or hydrochloric acid other than polyaluminum chloride may also be used as an accelerator to accelerate the treatment speed, but in this case, it is difficult to control the temperature rise. Rapid temperature rise can cause problems such as gas scattering, volume expansion of the treatment liquid, pitting, and the like.

Meanwhile, sodium chlorate is applied as the oxidizing agent in the present invention. Sodium chlorate may contain 3 to 5% by weight. If the content of sodium chlorate is less than 3% by weight causes a problem of poor bleaching. However, in the case of sodium chlorate, even if it is added in excess, there is no problem in the treatment process, but in consideration of economical efficiency, it is preferable to be limited to the range of 5% by weight or less.

These sodium chlorates replace nitric acid or peroxides, in particular hydrogen peroxide, which are conventionally used oxidizing agents. Nitric acid causes problems in the atmosphere and water environment with odor, and hydrogen peroxide is unstable, so that there is a problem that hydroquinoline, sodium stearate, phosphoric acid, salicylic acid, pyridine carboxylic acid, glycol ether and the like must be used together with a stabilizer.

Sodium chlorate (NaClO ₃ ) applied in the present invention acts as a colorless, odorless relatively stable oxidizing agent. However, sodium chlorate may be unstable and decompose at high temperature or high concentration of strong acid. Severe chlorine odor at 50 ~ 60 ℃ and chlorine odor occurs when sulfuric acid concentration is over 30% by weight even at room temperature.

One embodiment of the surface treatment composition of the present invention described above is in the form of a liquid, but may be formed in the form of a gel (gel) or paste by adding a thickener.

Hereinafter, a surface treatment method for pickling and bleaching stainless steel using the surface treatment composition of the present invention described above will be described. Both ferrite and austenite can be treated by the same surface treatment composition of the present invention, but the composition can be adjusted according to the type.

The temperature of the surface treatment composition for pickling treatment is preferably maintained below 40 ℃. Preferably the surface treatment composition is at a temperature of 20 to 40 ° C. The stainless steel may be immersed in the surface treatment composition, or the surface treatment composition may be sprayed on the surface of the stainless steel for pickling treatment. In the case of immersion, the surface treatment composition is stirred by mechanical stirring means. The treatment time can be appropriately adjusted according to the type of steel, the shape of the steel and the pretreatment between rolling or annealing and pickling.

During the surface treatment for pickling and bleaching stainless steel, the consumption of some materials slows down the processing speed. To this end, it is necessary to supplement the supplement periodically.

The supplement may be a single component or a mixture of two or more components. For example, the supplement may be a mixture of sulfuric acid solution and sodium fluoride in a weight ratio of 4: 1. In this case, when a high concentration of sulfuric acid solution such as 98% is replenished, it is preferable to use a sulfuric acid solution having a concentration of 50% or less because it may cause an abnormality in the oxidant due to a rapid exothermic reaction.

In the case where the bleached state of the surface of the treated stainless steel is poor, or when blackening or yellowish brown coloration occurs, sodium chlorate may be replenished. In addition, it is not necessary to add a promoter to the replenishment liquid. This is because an excessive amount of Fe ions serving as an accelerator is already present in the composition through the pickling process.

Hereinafter, the present invention will be described through examples. However, the following examples are intended to illustrate the present invention in detail, and the scope of the present invention is not limited to the following examples.

(Example 1)

A surface treatment composition was prepared by mixing 71.8% by weight of water, 20% by weight of sulfuric acid, 5% by weight of sodium fluoride, 3% by weight of sodium chlorate, and 0.2% by weight of polyaluminum chloride.

(Example 2)

72 wt% of water, 20 wt% of sulfuric acid, 5 wt% of sodium fluoride, and 3 wt% of sodium chlorate were mixed to prepare a surface treatment composition.

(Comparative Example 1)

A surface treatment composition was prepared by mixing 75% by weight of water, 5% by weight of hydrofluoric acid, and 20% by weight of nitric acid.

(Comparative Example 2)

A surface treatment composition was prepared by mixing 50% by weight of water, 10% by weight of hydrofluoric acid, and 40% by weight of nitric acid.

Corrosion Rate Measurement Experiment

The corrosion rate was measured by immersing the specimens in the surface treatment compositions of Examples and Comparative Examples, respectively. As a specimen, a pipe made of austenitic stainless steel (SUS 304) (diameter 15 mm, length 30 mm, surface area 28.26 cm ² ) was used. The corrosion rate was calculated by the following equation: the weight loss per unit time (the weight of the specimen before corrosion minus the weight of the specimen after corrosion) divided by the surface area of the specimen.

Corrosion rate (mg / cm ² ? Hr) = (Weight of specimen before corrosion-Weight of specimen after corrosion) / (Sample cross section × time)

The weight change of the specimen with corrosion time for each surface treatment composition is summarized in Table 1 below.

Corrosion time                      Specimen weight (g) Example 1 Example 2 Comparative Example 1 Comparative Example 2 0hr (before corrosion) 36.07 36.82 35.32 36.34 After 3 hours 35.69 36.71 34.61 35.96 After 8 hours 35.11 36.49 34.06 35.33 18 hours later 33.87 36.15 33.30 34.54 After 28 hours 32.19 35.77 32.69 34.00 After 38 hours 29.82 35.34 32.25 33.61 48 hours later 27.50 34.81 31.82 33.27 58 hours later 25.59 34.27 31.44 32.95 68 hours later 23.69 33.55 31.06 32.63

Corrosion rate was calculated using the reduced weight of the specimen shown in Table 1. The results are shown in Table 2 below.

Corrosion time Corrosion Rate (mg / cm ² ? Hr) Example 1 Example 2 Comparative Example 1 Comparative Example 2 0-3 hours 4.48 1.30 5.02 4.48 3-8 hours 4.10 1.56 3.89 4.46 8-18 hours 4.38 1.20 2.69 2.80 18-28 hours 5.94 1.34 2.16 1.91 28 ~ 38 hours 8.39 1.52 1.56 1.38 38-48 hours 8.21 1.88 1.52 1.20 48-58 hours 6.75 1.91 1.34 1.13 58-68 hours 6.72 2.55 1.34 1.13

Referring to the results of Table 2, Example 1 was found to have a higher corrosion rate than Comparative Examples 1 and 2. This means that the reaction rate on the surface of the steel can be increased to increase the processing speed.

And it was confirmed through the results of Example 1 and 2 that the reaction rate can be increased by the addition of a small amount of polyaluminum chloride. For reference, the surface treatment composition containing 0.1% by weight of polyaluminum chloride was slightly slower than in Example 1, but it did not seem to affect the workability.

On the other hand, it can be seen from the results of the comparative examples that the higher the concentration of nitric acid, the slower the reaction rate by controlling the reaction rate.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments.

Accordingly, the true scope of protection of the present invention should be determined only by the appended claims.

Claims (3)

A surface treatment composition for pickling and bleaching stainless steel, comprising 60 to 80% by weight of water, 10 to 30% by weight of sulfuric acid, 3 to 8% by weight of sodium fluoride, and 3 to 5% by weight of sodium chlorate. The surface treatment composition for pickling and bleaching stainless steel according to claim 1, further comprising polyaluminum chloride to promote the reaction. A surface treatment method for pickling and bleaching, characterized in that the stainless steel is pickled or bleached using the surface treatment composition according to claim 1.