CN109750244B - Method for reducing reactivity of malleable cast iron hot dip galvanizing silicon - Google Patents
Method for reducing reactivity of malleable cast iron hot dip galvanizing silicon Download PDFInfo
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Abstract
The invention belongs to the field of metal surface engineering, and particularly relates to a method for reducing the reactivity of malleable cast iron hot-dip galvanized silicon. According to the method, firstly, 25-35% hydrogen peroxide aqueous solution is adopted to carry out surface treatment on the malleable iron casting, so that the activity of silicon is effectively reduced, and then hot-dip galvanizing operation is carried out in Zn-0.05wt.% Al alloy liquid at the temperature of 450-570 ℃. The invention can reduce the galvanizing temperature, effectively reduce the thickness of the coating of the malleable iron casting and obviously increase the uniformity of the thickness of the coating of the malleable iron casting.
Description
Technical Field
The invention belongs to the field of metal surface engineering, and particularly relates to a method for reducing the reactivity of malleable cast iron hot-dip galvanized silicon.
Background
The hot-galvanized malleable cast iron is mainly applied to hot-galvanized pipe fittings which comprise galvanized elbows, galvanized flanges, galvanized bent pipes, galvanized reducing pipes and galvanized pipe cap end sockets. In the hot dip coating process, the pipe fitting substrate and the molten plating solution perform complex physical and chemical reactions to form a corrosion-resistant zinc-iron alloy layer with a compact structure. The alloy layer is integrated with the pure zinc layer and the iron matrix, so that the corrosion resistance is strong. The hot galvanizing pipe fitting is mainly used for pipeline projects of conveying gas, heating and the like, and if a steel pipe body needs to be galvanized and subjected to anticorrosion treatment, a connecting pipe fitting needs to be galvanized.
Research shows that due to the fact that the silicon content of the malleable iron casting is high (1.4-1.8%), obvious silicon reactivity exists during hot dip galvanizing, and the thickness of a coating is increased and uneven. At present, the malleable cast iron hot galvanizing process and the foreign galvanizing process have obvious difference, firstly, the temperature of hot galvanizing liquid is higher, generally about 600 ℃, and the foreign temperature is about 460 ℃; secondly, the zinc coating of the malleable iron casting is thicker, the uniformity of the thickness of the coating is poor, and the zinc consumption is obviously increased. Generally, the lower the carbon-silicon content of the plated malleable cast iron part and the better the surface quality of the casting, the easier the zinc and iron will bond and lower the galvanization temperature. On the contrary, a higher galvanizing temperature is adopted, at the moment, the zinc-iron reaction is intensified, the iron content in the galvanizing solution is increased, zinc slag is formed, the viscosity of the galvanizing solution is increased, and thus the galvanizing layer is thicker. Therefore, it is necessary to perform surface pretreatment of the wrought iron castings to mitigate silicon reactivity during the galvanization process, and finally achieve the objectives of lowering the galvanization temperature, reducing the thickness of the plating layer, and increasing the uniformity of the thickness of the plating layer.
Disclosure of Invention
The invention designs a method for reducing the reactivity of hot dip galvanized silicon of malleable cast iron, which adopts aqueous hydrogen peroxide solution with certain concentration to carry out surface pretreatment on a malleable cast iron piece and reduce the activity of silicon, aiming at reducing the galvanizing temperature, effectively reducing the thickness of a coating of the malleable cast iron piece and obviously increasing the uniformity of the thickness of the coating of the malleable cast piece.
The technical scheme of the invention is as follows:
the method comprises the steps of performing surface pretreatment on a malleable cast iron piece for 50-70 minutes by using a 25-35% hydrogen peroxide aqueous solution, performing surface treatment on the pretreated cast iron piece by using alkali washing, water washing, acid washing, water washing, plating assistant and drying, and then soaking the pretreated cast iron piece into a Zn-0.05wt.% Al alloy pool at 450-570 ℃ for hot dip galvanizing for 3-5 minutes to obtain a thin and uniform malleable cast iron coating. The invention aims to reduce the galvanizing temperature, reduce the thickness of a coating and increase the uniformity of the thickness of the coating.
The specific treatment process of the surface treatment comprises the following steps: using 15% NaOH solution to carry out alkali washing at 70-80 ℃ for 3 minutes, washing with clean water, washing with 15% hydrochloric acid for 3 minutes, washing with clean water, and washing with clean water, wherein the component of the clean water at 70-80 ℃ is 100ml water containing 27g of NH4Cl、23g ZnCl2、5g SnCl2、2g K2ZrF6The plating assistant solution of (1) was subjected to plating assistant treatment for 5 minutes, and then dried at 100 ℃ for 10 minutes.
Further preferably, the concentration of the aqueous hydrogen peroxide solution for pretreating the malleable cast iron is 30%, and the pretreatment time is 60 minutes.
The surface of the malleable cast iron is pretreated by aqueous hydrogen peroxide for a long time (preferably 60 minutes), so that silicon in the surface layer is selectively oxidized into silicon oxide, a concentration gradient from inside to outside is formed, the surface layer is poor in silicon, active silicon in a dissolved state is obviously reduced, and the purpose of reducing the activity and activity of silicon is achieved, so that the generation of silicon reaction is greatly reduced, the coating is thin, and the coating is uniform. And then the conventional surface treatment is carried out, the oxidation layer is washed by alkali and degreasing and acid, and the silicon oxide in the oxidation layer is removed, so that the galvanizing is more favorably carried out, the plating leakage is avoided, the plating layer is thinner and the plating layer is uniform.
Further optimally, the temperature for hot galvanizing treatment of the malleable cast iron is 460-560 ℃, and the hot dip galvanizing time is 4 minutes. Greatly reduces the galvanizing temperature and reduces the galvanizing time.
The principle of the invention is as follows:
generally speaking, the content of silicon in malleable cast iron is generally 1.4-1.8%, and obvious silicon reactivity exists in hot dip galvanizing, so that the reduction of the activity of silicon on the surface of the malleable cast iron is beneficial to obtaining a thin and uniform hot dip galvanized layer. When the surface of the malleable iron casting is pretreated by adopting the aqueous hydrogen peroxide solution, silicon element in the surface layer of the malleable iron casting is oxidized to form SiO with lower activity due to the strong oxidizing property of the hydrogen peroxide2The activity of silicon on the surface of the malleable iron casting can be effectively reduced, so that the silicon reactivity of the malleable iron casting in the hot-dip galvanizing process can be effectively inhibited, the galvanizing temperature can be reduced, the thickness of a galvanized layer can be effectively reduced, and the uniformity of a plating layer can be increased.
The invention has the technical effects that:
after the surface pretreatment is carried out on the malleable cast iron by adopting the aqueous solution of hydrogen peroxide with certain concentration, the hot-dip galvanizing temperature can be reduced, the thickness of the coating of the malleable cast iron can be effectively reduced, and the uniformity of the thickness of the coating of the malleable cast can be obviously increased.
Drawings
FIG. 1 is a photograph of the microstructure of a plated layer of a wrought cast iron sample that has not been treated with an aqueous solution of hydrogen peroxide for 60 minutes after hot galvanizing at 460 ℃.
FIG. 2 is a photograph of the microstructure of the plated layer after hot galvanizing at 460 ℃ of a wrought cast iron sample treated with an aqueous solution of hydrogen oxide for 60 minutes.
FIG. 3 is a photograph of the microstructure of a plated layer of a wrought cast iron sample that has not been treated with an aqueous solution of hydrogen peroxide for 60 minutes after hot galvanizing at 510 ℃.
FIG. 4 is a photograph of the microstructure of the plated layer after hot galvanizing at 510 ℃ of a wrought cast iron sample treated with an aqueous hydrogen oxide solution for 60 minutes.
FIG. 5 is a photograph of the microstructure of a plated layer of a wrought cast iron sample that has not been treated with an aqueous solution of hydrogen peroxide for 60 minutes after hot galvanizing at 560 ℃.
FIG. 6 is a photograph of the microstructure of the plated layer after hot galvanizing at 560 ℃ of a wrought cast iron sample treated with an aqueous hydrogen oxide solution for 60 minutes.
FIG. 7 is a photograph of the microstructure of the plated layer after hot galvanizing at 460 ℃ of a wrought cast iron sample treated with an aqueous solution of hydrogen peroxide for 20 minutes.
Detailed description of the invention
The invention is described in more detail below with reference to the following examples:
example 1:
as a reference hot-dip galvanized sample, malleable cast iron with the size of 10mm multiplied by 10mm is subjected to a series of treatments of alkali washing, water washing, acid washing, water washing, plating assistance and drying, the hot-dip galvanized alloy liquid comprises Zn and 0.05wt.% of Al, the temperature of the zinc liquid is controlled at 460 ℃, and the malleable cast iron sample is immersed in the zinc liquid for 4 minutes. Quantitative metallographic analysis showed that the average thickness of the coating of the wrought cast iron sample was 102 μm, and the difference between the thickness of the coating at the thickest part and the thickness of the coating at the thinnest part was 148.6 μm.
TABLE 1 malleable cast iron after 4 minutes immersion plating in 460 ℃ Zinc bath the thickness of the galvanized layer (. mu.m)
In the invention, 30% hydrogen peroxide aqueous solution is adopted to carry out surface pretreatment on the malleable cast iron with the thickness of 10mm × 10mm for 60 minutes, and after being taken out, the malleable cast iron is dried to carry out a series of treatments of alkali washing, water washing, acid washing, water washing, plating assistant and drying, wherein the concrete treatment processes comprise the steps of alkali washing for 3 minutes at the temperature of 70-80 ℃ by using 15% NaOH solution, acid washing for 3 minutes in 15% hydrochloric acid after washing with clean water, and 27g NH in 100ml water with the component of 70-80 ℃ after washing with clean water4Cl、23g ZnCl2、5g SnCl2、2g K2ZrF6The plating assistant solution of (1) was subjected to plating assistant treatment for 5 minutes, and then dried at 100 ℃ for 10 minutes. The hot dip galvanizing alloy liquid comprises Zn and 0.05wt.% Al, the temperature of the zinc liquid is controlled at 460 ℃, and then the malleable cast iron sample is immersed in the zinc liquid for 4 minutes. Quantitative metallographic analysis showed that the average thickness of the wrought cast iron coating was 65.6 μm, which was 35.7% less than the average thickness of the coating without the pre-treated sample. The thickness difference of the coating between the thickest part and the thinnest part is 83.2 mu m, which is reduced by 44 percent compared with the thickness difference of the coating between the thickest part and the thinnest part of the sample without pretreatment, and the uniformity of the coating is obviously improved.
Example 2:
as a reference hot-dip galvanized sample, malleable cast iron having a size of 10mm × 10mm was subjected to a series of treatments of alkali washing, water washing, acid washing, water washing, plating assistance, drying (the treatment steps are the same as in example 1) hot-dip galvanized alloy bath composition of Zn — 0.05wt.% Al, the temperature of the zinc bath was controlled at 510 ℃, and the malleable cast iron sample was immersed in the zinc bath for 4 minutes. Quantitative metallographic analysis shows that the average thickness of the coatings of the malleable cast iron samples is 108.6 mu m, and the difference between the thicknesses of the coatings at the thickest part and the thinnest part is 101.7 mu m.
TABLE 2 malleable cast iron is dip-plated in zinc bath at 510 deg.C for 4 minutes, and then the thickness of the galvanized layer (. mu.m)
In the present invention, a surface pretreatment of 10mm × 10mm wrought cast iron was carried out for 60 minutes using 30% aqueous hydrogen peroxide, and after taking out, it was dried to carry out a series of treatments of alkali washing, water washing, acid washing, water washing, plating assistant, and drying (the treatment steps were the same as in example 1). The hot dip galvanized alloy liquid also contains Zn-0.05wt.% Al, the temperature of the zinc liquid is controlled at 510 ℃, and then the malleable cast iron sample is immersed in the zinc liquid for 4 minutes. Quantitative metallographic analysis showed that the average thickness of the wrought cast iron coating was 64.8 μm, which is 40% less than the average thickness of the coating of the non-pretreated specimens. The thickness difference of the coating between the thickest part and the thinnest part is 44.4 mu m, which is reduced by 56.3 percent compared with the thickness difference of the coating between the thickest part and the thinnest part of the sample without pretreatment, and the uniformity of the coating is improved more obviously.
Example 3:
as a reference hot-dip galvanized sample, malleable cast iron having a size of 10mm X10 mm was subjected to a series of treatments of alkali washing, water washing, acid washing, water washing, plating assistant and drying (the treatment procedure was the same as in example 1). The hot dip galvanizing alloy liquid comprises Zn and 0.05wt.% Al, the temperature of the zinc liquid is controlled at 560 ℃, and the malleable cast iron sample is immersed into the zinc liquid for 4 minutes for dip galvanizing. Quantitative metallographic analysis showed that the average thickness of the coating of the wrought cast iron sample was 163.8 μm, and the difference between the thickness of the coating at the thickest and thinnest parts was 267.6 μm.
TABLE 3 thickness (. mu.m) of galvanized layer after immersion plating of malleable cast iron in zinc bath at 560 ℃ for 4 minutes
In the present invention, a surface pretreatment of 10mm × 10mm wrought cast iron was carried out for 60 minutes using 30% aqueous hydrogen peroxide, and after taking out, it was dried to carry out a series of treatments of alkali washing, water washing, acid washing, water washing, plating assistant, and drying (the treatment steps were the same as in example 1). The hot dip galvanized alloy liquid also contains Zn-0.05wt.% Al, the temperature of the zinc liquid is controlled at 560 ℃, and then the malleable cast iron sample is immersed in the zinc liquid for 4 minutes. Quantitative metallographic analysis showed that the average thickness of the wrought cast iron coating was 122.7 μm, which is a 25% reduction over the average thickness of the coating of the non-pretreated specimens. The thickness difference of the coating between the thickest part and the thinnest part is 222.8 μm, which is reduced by 16.7% compared with the thickness difference of the coating between the thickest part and the thinnest part of the sample without pretreatment, and the uniformity of the coating is improved to a certain extent.
Comparative example 1:
as a reference hot-dip galvanized sample, malleable cast iron having a size of 10mm X10 mm was subjected to a series of treatments of alkali washing, water washing, acid washing, water washing, plating assistant and drying (the procedure was the same as in example 1). The hot-dip galvanizing alloy liquid comprises Zn and 0.05wt.% of Al, the temperature of the zinc liquid is controlled at 460 ℃, and the malleable cast iron sample is immersed in the zinc liquid for 4 minutes for dip galvanizing. Quantitative metallographic analysis showed that the average thickness of the coating of the wrought cast iron sample was 102 μm, and the difference between the thickness of the coating at the thickest part and the thickness of the coating at the thinnest part was 148.6 μm.
In the present invention, after the surface of the wrought cast iron was pretreated with 30% aqueous hydrogen peroxide for 20 minutes, it was taken out and dried to perform a series of treatments of alkali washing, water washing, acid washing, water washing, plating assistant and drying, in the same manner as in example 1. The hot dip galvanized alloy liquid also contains Zn-0.05wt.% Al, the temperature of the zinc liquid is controlled at 460 ℃, and then the malleable cast iron sample is immersed in the zinc liquid for 4 minutes. The average thickness of the wrought cast iron coating was 90.5 μm, which is 11.3% less than the average thickness of the coating of the non-pretreated specimens. The difference of the thickness of the plating layer between the thickest part and the thinnest part is 131.4 μm, which is reduced by 11.6% compared with the difference of the thickness of the plating layer between the thickest part and the thinnest part of the sample without pretreatment, and the uniformity of the plating layer is slightly improved, but is obviously lower than that of the embodiment 1.
TABLE 4 thickness (μm) of galvanized layer after immersion plating of malleable cast iron in zinc bath at 460 ℃ for 4 minutes
Claims (4)
1. A method for reducing the reactivity of malleable cast iron hot dip galvanised silicon, characterised by: the method comprises the steps of pretreating a malleable cast iron piece for 50-70 minutes by using 25-35% hydrogen peroxide water solution, carrying out alkali washing, water washing, acid washing, water washing, plating assistant and drying surface treatment on the pretreated cast piece, removing silicon oxide in an oxide layer through acid washing, immersing the surface treated cast piece into a zinc alloy plating bath at the temperature of 450-570 ℃ for hot dip galvanizing treatment for 3-5 minutes, and obtaining a thin and uniform malleable cast iron coating.
2. The method for mitigating the reactivity of hot dip galvanized silicon of malleable cast iron as claimed in claim 1 wherein the aqueous hydrogen peroxide solution has a concentration of 30%.
3. The method of mitigating hot dip galvanized silicon reactivity of malleable cast iron, according to claim 1, wherein the malleable cast iron article is pretreated in an aqueous hydrogen peroxide solution for a period of 60 minutes.
4. The method for mitigating hot dip galvanization silicon reactivity of malleable cast iron, according to claim 1, where the surface treated casting is hot dip galvanized in a Zn-0.05wt.% Al bath at 460-560 ℃ for 4 minutes.
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