JP5621398B2 - Smokeless flux for hot dip galvanizing and hot dip galvanizing method using the flux - Google Patents

Description

  The present invention relates to a smokeless flux for hot dip galvanizing and a hot dip galvanizing method using the flux, and more particularly, a smokeless flux for hot dip galvanizing that does not emit smoke when the flux-treated steel material is immersed in a hot dip galvanizing bath. And a hot dip galvanizing method using the flux.

  Hot-dip galvanization, which has been performed for a long time for the purpose of improving corrosion resistance and adding new functions, is a surface treatment method for steel materials. It has a wide range of uses in fields such as architecture and civil engineering. A general dry hot dip galvanizing process is degreasing → washing → acid washing → water washing → flux treatment → drying → plating → water cooling. The flux treatment is a treatment in which a steel material is immersed in a heated flux aqueous solution to form a flux film on the surface of the steel material, and has the following four roles. (1) Suppress rust generation between washing and plating process. (2) Remove substances that obstruct iron-zinc alloying and surface foreign substances to promote alloying. (3) Zinc oxide on the molten zinc is removed so that zinc oxide is not caught in the plating. (4) The flux is efficiently desorbed from the steel material base and exchanged with molten zinc. After the flux treatment, the hot dip galvanization is generally performed by dipping in a hot dip galvanizing bath at a temperature of 450 to 500 ° C.

The components conventionally used as fluxes are mainly a double salt of ZnCl ₂ and NH ₄ Cl, or a mixture having a molar ratio of 1: 1 or 1: 3, depending on the case. Only NH ₄ Cl is used. In addition to the function of the original flux, the flux composed of NH ₄ Cl is easy to dry, that is, the drying process is completed in a short time, so that the efficiency is good. In addition, even when the drying process is insufficient, there is an advantage that a splash phenomenon in which the molten zinc is scattered due to the residual moisture and the molten zinc is scattered is hardly caused.

However, the flux treatment mainly composed of NH ₄ Cl has a problem that a large amount of white smoke is generated during plating. Here, white smoke at the time of plating, mainly NH ₄ Cl sublimation, or are those caused by decomposing volatile, white smoke as long as using NH ₄ Cl are considered out. Although there is a commercially available smokeless flux, since it must play the original role of the flux, at least NH ₄ Cl is blended, and it is not possible to completely smokeless. As for the flux used in hot dip galvanizing, no smokeless flux has been developed so far using an alternative material that does not completely use NH ₄ Cl.

  In recent years, environmental protection has been regarded as important, and it is desired that the process of making a product is not limited to consideration for the environment of the product itself, and that the process for producing the product should be friendly to the environment and the earth. The conventional hot dip galvanizing process generates a lot of harmful white smoke in the plating process, which causes problems such as adverse effects on the human body, poor scenery in the factory, and increased costs due to the installation and operation of smoke exhausters. There is a need for process improvements.

By the way, in the molten Al—Zn alloy plating, a flux not using NH ₄ Cl is used. The reason is that non-plating occurs due to reaction between Al and NH ₄ Cl in the plating bath. Therefore, a flux as described in Patent Documents 1 and 2 is used. Patent Document 1 discloses a flux composed of SnCl ₂ : 0.3 to 8% by weight, a mixture of an organic acid and an acid fluoride: 1 to 20% by weight, alkali chloride: 5 to 30% by weight, and residual ZnCl _2. Has been. Here, it is disclosed that sodium hydrogen fluoride (NaHF ₂ ) is used as the acidic fluoride and NaCl is used as the alkali chloride.

In Patent Document 2, in mol%, (a) 65 to 85% of ZnCl ₂ and (b) one or more of NaF, KF, MgF ₂ , and Na ₂ SiF ₆ are added in a total of 0.5 to 3%. (C) 5-25% in total of any one or more of chlorides of alkali metal elements or alkaline earth metal elements, (d) one or more of chlorides of Sn, In, Tl, Sb, Bi A flux characterized by containing a total of more than 5% and 20% or less is disclosed.

  The fluxes described in Patent Documents 1 and 2 contain at least Sn, In, Tl, Sb, and Bi chlorides. Among these elements, even the cheapest Sn is as expensive as about 10 times that of Zn. For this reason, the cost of the flux is increased, and the hot dip galvanizing, which often processes large steel materials, significantly increases the cost.

Japanese Examined Patent Publication No. 4-59387 Japanese Patent No. 3588452

  In order to satisfy the role as a flux for hot dip galvanizing, the following points are required. (1) Low melting point, (2) Sufficient chemical detergency, (3) High lubricity and fluidity, (4) No smoke generated when immersed in hot dip galvanizing bath That is. The conventional flux satisfies the requirements (1) to (3), and the alternative flux of the present invention must naturally satisfy these requirements, and the present invention also imposes the requirement (4).

Therefore, in view of the above-mentioned situation, the present invention intends to solve the problem that a plating quality equivalent to that of a conventional hot dip galvanizing flux using NH ₄ Cl can be obtained and a hot dip galvanizing bath can be used. The object of the present invention is to provide a novel smokeless flux for hot dip galvanizing that does not generate smoke when immersed, or can reduce the amount of generated smoke to a negligible level, and a hot dip galvanizing method using the flux.

In order to solve the above-mentioned problems, the present invention provides (a) 55 to 86% by weight of ZnCl ₂ , (b) 5 to 38% by weight of NaF (excluding 10% by weight or less) , (c) NaCl, It is a flux composed of 0 to 35% by weight of any one or more of KCl , and the ZnCl ₂ mol concentration is set to be higher than 1/3 of the NaF mol concentration, and the flux is used in water. Dissolve to make an aqueous solution, add HCl to adjust the pH to 0.9 to 2.0, dissolve the NaZnF ₃ precipitate produced during the reaction, and immerse the steel treated with this flux in a hot dip galvanizing bath The smokeless flux for hot dip galvanizing is characterized in that no smoke is generated or the amount of smoke generated can be reduced to a negligible level .

Furthermore, the flux solution, it is also preferably made by adding a surfactant (Claim 2).

And the hot dip galvanization using the smokeless flux characterized by immersing the steel material processed with the smokeless flux for hot dip galvanization mentioned above in the hot dip galvanization bath of 450-500 degreeC for a predetermined time, and performing hot dip galvanization. A method was constructed (claim 3 ).

  When hot dip galvanizing is applied to a steel material treated with the smokeless flux for hot dip galvanizing of the present invention, plating quality equivalent to the case of using a flux mainly composed of conventional ammonium chloride is obtained, and it is also caused by the flux component. Since the generation of white smoke can be suppressed, the working environment of the plating plant can be improved and the minimum smoke exhaust equipment can be used, or the operating rate of the smoke exhaust equipment can be reduced. Contribute.

By XRD analysis (a) precipitation material is a graph showing the results of measurement of the (b) NaZnF _3.

The smokeless flux for hot dip galvanizing of the present invention does not use NH ₄ Cl, which is the cause of white smoke generation, so that a plating quality equivalent to the flux using conventional NH ₄ Cl can be obtained. The component composition of was determined. That is, the smokeless flux for hot dip galvanizing of the present invention is a total of (a) 55 to 86% by weight of ZnCl ₂ and (b) NaF, KF, MgF ₂ , ZnF ₂ , Na ₂ SiF _6. 5 to 38% by weight, and (c) one or more of chlorides of alkali metal elements or alkaline earth metal elements in total, 0 to 35% by weight.

Below, the composition of the smokeless flux for hot dip galvanizing of this invention is demonstrated in detail. The component composition of the alternative flux was specifically determined from the role of the flux and the reaction mechanism of the conventional flux. First, ZnCl ₂ which has been used and has a proven record is used as the main composition. In addition, the flux is required to have a low melting point and high fluidity. By adding a salt to ZnCl ₂ , the melting point is lowered and the fluidity is improved. Therefore, NaCl is added. NaCl is an extremely inexpensive chloride and is convenient because it is common to ZnCl ₂ . KCl was also used for the same purpose. Further, when both NaCl and KCl are mixed with ZnCl ₂ , the melting point becomes lower. In general, chlorides of alkali metal elements or alkaline earth metal elements can be used for this purpose.

In addition, since chemical cleaning power is required for the flux, NH ₄ Cl itself and the role of hydrogen chloride generated from NH ₄ Cl were replaced with fluoride. Since it is convenient to use the same elements as NaCl and KCl without complicating the system, NaF and KF were listed as candidates. However, KF is difficult to handle because of its relatively deliquescent nature. NaF has a high melting point of 992 ° C. and does not decompose by heat, so it does not produce smoke by itself. MgF ₂ , ZnF ₂ , and Na ₂ SiF ₆ can also be used as fluorides other than NaF and KF.

The main component composition of the smokeless flux for hot dip galvanizing of this embodiment was ZnCl ₂ , NaF, NaCl and / or KCl. Specifically, (a) 55 to 86% by weight of ZnCl ₂ , (b) 5 to 38% by weight of NaF (except 10% by weight or less) , (c) one or more of NaCl and KCl A flux consisting of 0 to 35% by weight was prepared.

The basic properties of the component composition of the flux used in the examples and comparative examples of the present invention are summarized below.
NH ₄ Cl is one of the main components of the conventional flux and plays a major role in the flux. Reaction occurs at 335 ° C or higher, producing white smoke. The solubility in water is 37.2 g / 100 mL (20 ° C.).
ZnCl ₂ is one of the main components of conventional flux and plays a role of flux. Heating produces zinc oxide and hydrogen chloride smoke. The melting point (MP) is 275 ° C. and the solubility is 432 g / 100 mL (25 ° C.).
NaCl and KCl are used to lower the melting point of the flux and contribute to the effect of suppressing volatilization of ZnCl ₂ . This action is effective for alkali metal and alkaline earth metal chlorides. In the present embodiment, these two substances are used.
NaF is a colorless solid that has a fluoride cleaning action and is used as an alternative to NH ₄ Cl in the present invention. In hot dip aluminum plating, a fluoride system has been conventionally blended in a flux. M.M. P is 992 ° C. and the solubility is 4 g / 100 mL (20 ° C.).
ZnF ₂ is nonflammable, but generates irritating / corrosive fumes (solid fine particles) by heating. Slightly soluble in hydrochloric acid, nitric acid and ammonia. M.M. P is 872 ° C., and the solubility is 0.000052 g / 100 mL (20 ° C.) for anhydride and 1.52 g / 100 mL (20 ° C.) for tetrahydrate.

Incidentally, NaF used as a main component of the flux in the present invention is used in various applications as a source of fluoride ions. Compared with KF, it is inexpensive and has low hygroscopicity. NaF is often used as a fluorination treatment for dental caries prevention in dentistry and may be added to toothpaste. Similarly, for the purpose of preventing caries, in the United States and Australia, fluoride may be added to tap water. Sodium fluoride is used in the initial stage, and then hexafluorosilicic acid (H ₂ SiF ₆ ) and its sodium are used. It was replaced by the salt (Na ₂ SiF ₆ ). Thus, fluoride is used in everyday life and is a familiar substance.

  When the steel material is flux-treated, it is usually performed by immersing the steel material in a flux aqueous solution, but it may be treated by a coating method or a spray method. In any case, the flux is used in the form of an aqueous solution. And after a flux process, after making it dry and removing a water | moisture content, it is immersed in the hot dip galvanizing bath. Table 1 shows component compositions of the reference flux (comparative example) and the flux of the present invention (example), respectively. The reference flux includes a flux of only zinc chloride in addition to a conventional flux that is generally used, and is a flux for comparison with the flux of the present invention. Here, Comparative Example 1 (JIS accessory) is a flux defined in JIS suitable for processing small steel materials such as screws. Comparative example 2 (JIS large) is a flux defined in JIS suitable for processing large steel materials such as steel frames. Comparative Example 3 (general commercially available) is a commercially available flux that can be widely used for ordinary general steel materials. Comparative Example 4 (commercial smokeless) is a commercially available flux that can suppress the amount of smoke generated.

The flux of Examples 1 to 13 of the present invention shown in Table 1 is an alternative flux to replace the commercial smokeless flux prepared with a unique composition. Specifically, (a) 55 to 86% by weight of ZnCl ₂ ( b) 5 to 38% by weight of NaF, (c) 0 to 35% by weight of at least one of NaCl and KCl. When converted to mol%, NaF becomes 11 to 66 mol%. When the flux of the present invention is prepared, a precipitate is formed when the flux composition components shown in Table 1 are dissolved in water. This precipitate was identified as NaZnF ₃ as shown in FIG. 1 as a result of substance identification using XRD-6000 manufactured by Shimadzu Corporation. That is, the following reaction occurs.
ZnCl ₂ + 3NaF → NaZnF ₃ + 2NaCl

1 mol of ZnCl ₂ and 3 mol of NaF react to produce 1 mol of NaZnF ₃ and 2 mol of NaCl. Since ZnCl ₂ is required in the flux solution, be varied NaF is consumed by the reaction to NaZnF _3, it is necessary to ZnCl ₂ remain. That is, the molar concentration of ZnCl ₂ must be greater than 1/3 of the molar concentration of NaF. Examples 1 to 13 all satisfy this condition. Further, since NaCl is generated by the reaction of ZnCl ₂ and NaF, it may not be included in the flux component composition at the time of preparation, but it is desirable to prepare an appropriate amount from the beginning. As described above, ZnCl ₂ , NaZnF ₃ , and NaCl are the main compositions in the flux aqueous solution. For example, the flux of Example 3 is ZnCl ₂ : 26% by weight, NaZnF ₃ : 29% by weight, NaCl: 45% by weight in the aqueous solution, and Example 6 is ZnCl ₂ : 21% by weight, NaZnF ₃ : 44% by weight. %, NaCl: 35% by weight.

Substances having respective component compositions of Comparative Examples and Examples shown in Table 1 were dissolved in water to obtain a flux aqueous solution having a concentration of 20% by weight. As described above, a precipitate (NaZnF ₃ ) is produced when the flux of the present invention is prepared. However, if the precipitate does not dissolve, it cannot be a flux that can be used in the hot dip galvanizing process. Therefore, an appropriate amount of hydrochloric acid was added to the aqueous flux solution of the present invention. Then, the pH value gradually decreased from pH 5.0 to 5.8, and all precipitates were dissolved at pH 0.9 to 2.0, and a colorless and transparent solution was obtained. The acid added to dissolve the precipitate (NaZnF ₃ ) may be sulfuric acid in addition to hydrochloric acid.

  First, in order to investigate the volatility of the flux of the comparative example and the example, using an electric crucible furnace (MAX1150 ℃), Auto Tuning Control System (AT-E58) of Isuzu Manufacturing Co., Ltd., heating to a temperature of 480 ℃ and measuring the volatilization ratio Went. The volatilization ratio is a ratio representing how much a certain amount of flux has volatilized in a certain time. Specific test methods performed are shown below.

  Each component was crushed with a mortar to form a fine uniform particle, and a flux was prepared so that the total amount was 10 g, and the mixture was placed in a heat-resistant beaker. The weight of the flux was measured together with the heat-resistant beaker. A heat-resistant beaker containing a flux was placed in a furnace crucible having a furnace temperature setting of 585 ° C. and a crucible internal temperature of about 480 ° C. for 5 minutes. Meanwhile, it was observed whether the flux was heated and smoke came out of the heat-resistant beaker. Then, the heat-resistant beaker was taken out from the furnace, the state of the flux was confirmed, and left in air at room temperature for 10 minutes. Thereafter, the flux weight was measured together with the heat-resistant beaker. The amount of decrease in flux due to heating was calculated from the change in weight of the beaker before and after heating, and the ratio of the amount of decrease in flux after heating to 10 g before heating was defined as the volatilization ratio.

  Next, after flux-processing using the flux aqueous solution of a comparative example and an Example, the hot dip galvanization was given and the quality of the plating film was investigated. A hot dip galvanizing test was performed using various flux aqueous solutions in Table 1. The test method was performed in accordance with a general hot dip galvanizing process. Specific test methods are shown below.

  The Harling cell iron plate was cut to about 2 × 3 cm so as to be immersed in a hot dip galvanizing bath in the furnace, and a hole was made in the upper center of the iron plate. An iron plate was hung on a copper wire, and the iron plate was immersed in an aqueous solution of about 15% by weight sodium hydroxide and degreased. Then, the aqueous sodium hydroxide solution was washed away with distilled water, and then immersed in about 15% by weight hydrochloric acid and pickled. Similarly, after the hydrochloric acid was washed away, it was immersed in an aqueous solution of about 20% by weight flux (40 ° C.) for flux treatment.

  The iron plate was dried immediately above the heat-resistant beaker, and when the water was almost lost, the iron plate was lowered and immersed in a hot dip galvanizing bath. At this time, the state of smoke generation was observed. After being immersed for about 1 minute, it was pulled up, cooled with water, and the degree of plating was observed. A hot dip galvanizing test was performed several times on one sample.

By measuring the volatilization ratio and the hot dip galvanizing test, it was possible to observe various smoke differences depending on the sample. Table 1 shows the state of the flux at 480 ° C. and the volatilization ratio in the volatilization ratio measurement. When the smoke generation state of each flux was observed and compared, first, it was possible to clearly observe the smoke generation of the conventional flux (Comparative Examples 1 to 3) among the reference fluxes. The measured volatilization ratio was about 10 to 20%, and it was found that the flux itself was volatilized. Although the flux of only zinc chloride (Comparative Example 5) was able to confirm smoke generation, the volatilization rate was not so high. Regarding the commercial smokeless flux (Comparative Example 4), a slight amount of smoke was seen and the volatilization rate was also low. Since the commercial smokeless flux contains a small amount of NH ₄ Cl, it is difficult to make it completely smokeless. On the other hand, in the fluxes of the present invention (Examples 1 to 13), no smoke was observed in any composition, and the measured volatilization ratios all showed a low value of 0 to 4%. In comparison, it was confirmed that the flux was not volatilized.

  Next, the degree of plating was observed and evaluated. The appearance and condition of the plating film adhering to the steel material were observed by the hot dip galvanizing test described above. As a result, in the plating process using the conventional flux (Comparative Examples 1 to 3), there was no unplating, and a clean plating condition was confirmed. By the way, when the plating process was performed without any flux treatment, many unplating and unevenness were confirmed, and it was found that the flux treatment was indispensable.

  Further, in the commercially available smokeless flux (Comparative Example 4), no obvious non-plating was found, but it was slightly uneven. In the flux containing only zinc chloride (Comparative Example 5), a slight amount of unplating was observed, but a relatively smooth appearance was exhibited.

  Regarding the flux of the present invention, when treated with the fluxes of Examples 1 to 13, no obvious unplating was observed, but various conditions could be confirmed depending on the composition. In the case of treatment with the flux of Example 3 or 4, 6, 10 of the present invention, there was a partial non-plating, and it did not show a very good condition. About the case where it processed with the flux of Example 8, although it had plated, the zinc lump was made in some places. When treated with the flux of Example 2 or 5, 11 and 12, a relatively smooth and good condition was exhibited, but a slight horizontal stripe pattern was observed. With regard to the striped pattern, it is considered that the flux is caught in a certain tone because the flux fluidity is poor and the steel material and the flux are too sticky. In the case of treatment with the flux of Example 13, good plating was observed without stripes. From the above, the flux of Example 13 showed the best plating condition, and no smoke was found.

  In addition, tests were performed at an actual hot dip galvanizing plant using slightly larger test pieces. The test piece used is SS400 material of 150 × 70 × 3 (mm). The immersion time was 1 minute and 3 minutes. After the plating process, the plating film thickness was measured using an eddy current film thickness meter. The hot dip galvanizing test at the factory is more practical than the plating test in the laboratory, that is, the plating process is performed in a manner close to the site, so that it can be expected that a result in line with the site will be obtained. The result of this factory test was slightly different from the test result in the laboratory.

  As a result of the factory test, basically, any plating process using any flux exhibited good plating without any unplating. In the plating process excluding the flux treatment, some non-plating and pinholes were observed. Regarding smoke generation, smoke was confirmed in the conventional flux, and some smoke was also confirmed in the flux of the present invention. The reason why smoke could not be visually confirmed in the plating test in the laboratory was thought to be due to the fact that the scale of the experiment was small and the amount of smoke was very small.

In the conventional flux, uniform white smoke started to appear remarkably for a while after immersion in the steel material, and smoke was settled after about 2 to 3 minutes. In the flux of the present invention, the degree of smoke generation was similar to that of the commercial smokeless flux. Specifically, in the flux of the present invention, white smoke that starts to appear light and dark after a while after dipping the steel material, and in the commercial smokeless flux, white smoke that exhibits uniform white and light and light starts to appear, and 30 seconds to 1 Smoke disappeared after about a minute.
When the fluxes of the present invention are compared, smoke is emitted when the flux is treated with the fluxes of Examples 10 and 11 rather than when the flux is treated with the fluxes of Examples 5 and 13 where the plating is better in the laboratory. The amount and time that I had was short. Moreover, there was no difference in smoke generation due to the presence or absence of a surfactant, and it was concluded that the surfactant was not involved in smoke generation. The fluxes of Examples 5 and 13 of the present invention have a larger amount of ZnCl ₂ than the fluxes of Examples 10 and 11, and it is considered that ZnCl ₂ is involved in fuming. Also, uniform smoke from the flux of NH ₄ Cl is blended it was found that white smoke exhibiting shading occurs from the flux does not contain a NH ₄ Cl.

The film thickness of the steel material immersed in each flux for 1 minute, 3 minutes, and hot dip galvanized was measured. The film thickness is obtained by measuring the thickness from the iron substrate to the plating surface, that is, the thickness of the alloy layer and the pure zinc layer. As a result, there was no significant difference in film thickness between the conventional flux and the flux of the present invention. Further, it was found that the film thickness difference at the measurement points on the upper, middle and lower sides of the test piece was small and the plating was uniformly performed. Like the conventional flux, the flux of the present invention is considered to work to remove the alloy-inhibiting substance so that the plating adheres efficiently.

Claims (3)

(A) 55 to 86% by weight of ZnCl ₂ , (b) 5 to 38% by weight of NaF (excluding 10% by weight or less) , (c) any one or more of NaCl and KCl in total 0 to The ZnCl ₂ mol concentration is set to be higher than 1/3 of the NaF mol concentration, and the flux is dissolved in water to make an aqueous solution, and HCl is added. When the pH is adjusted to 0.9 to 2.0, the NaZnF ₃ precipitate generated during the reaction is dissolved, and the steel material treated with this flux is immersed in a hot dip galvanizing bath, or no smoke is generated. A smokeless flux for hot dip galvanizing, characterized in that the amount can be reduced to a negligible amount . Wherein the flux solution, comprising adding a surface active agent according to claim 1 for hot dip galvanizing smokeless flux according. A smokeless flux characterized in that the steel material treated with the smokeless flux for hot dip galvanizing according to claim 1 or 2 is immersed in a hot dip galvanizing bath at 450 to 500 ° C for a predetermined time to perform hot dip galvanizing. Hot dip galvanizing method.

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