Aluminum alloy coating hot dipping method

Abstract

The invention relates to the technical field of hot dipping, in particular to an aluminum alloy coating hot dipping method which comprises the following steps: step 1, pretreating steel; secondly, the pretreated steel is placed in a nitriding furnace to be subjected to nitriding treatment; thirdly, the steel is immersed in a plating assisting solution to be subjected to plating assisting treatment, the plating assisting treatment temperature ranges from 80 DEG C to 85 DEG C, and the plating assisting treatment time ranges from 2 min to 3 min; fourthly, the steel is soaked in an aluminum alloy solution for hot dipping, the hot dipping temperature ranges from 660 DEG C to 690 DEG C, and the hot dipping time ranges from 1 min to 1.5 min; and fifthly, after hot dipping is completed, the steel is naturally cooled. According to the method, the steel is subjected to nitriding treatment and plating assisting treatment in sequence before the steel is subjected to aluminum alloy hot dipping, the adhesive force of an aluminum alloy coating on the surface of the steel is improved, and the corrosion resistance of the aluminum alloy coating is improved.

Description

Technical Field

[0001] The invention relates to the technical field of hot-dip plating, and in particular to a hot-dip aluminum alloy plating method. Background Art

[0002] Hot-dip galvanizing is a process in which the plated part is immersed in molten metal for a certain period of time, so that the metal in the molten metal penetrates into the plated part and forms a protective metal film on the surface of the plated part. Hot-dip galvanizing and hot-dip aluminizing are two common steel surface treatment processes. The hot-dip galvanizing process can form a galvanized layer on the steel surface. The galvanized layer can not only isolate the steel matrix from the outside air, moisture and corrosive media, acting as a physical barrier, but also provide electrochemical protection for steel as a sacrificial anode. The hot-dip galvanizing process has a low cost, but due to the high density of zinc, forming a galvanized layer on the steel surface will significantly increase the weight; the hot-dip aluminizing process can form an aluminum layer on the steel surface. The density of aluminum is low, and it is easy to react with oxygen in the air to form a dense aluminum oxide layer. The aluminum oxide layer has good chemical stability and corrosion resistance, which can make the aluminum layer in a passivated state and improve the oxidation resistance and high temperature resistance of steel. In recent years, hot-dip aluminum coating technology has been widely used in many fields such as petroleum, chemical industry, metallurgy, machinery, aviation, etc. due to its excellent corrosion resistance, high temperature oxidation resistance, carburization resistance, wear resistance and heat resistance.

[0003] In order to improve the adhesion of the aluminum coating on the steel surface, the existing technology usually uses electrolytic degreasing, electrolytic polishing and plasma cleaning to activate the steel surface before hot-dip aluminum coating. Although the above methods can increase the roughness of the steel surface to a certain extent, if the electrolytic parameters are not properly controlled, it may cause excessive corrosion of the steel surface, resulting in defects such as pits, affecting the uniformity of hot-dip aluminum coating. Summary of the invention

[0004] In view of the problem that the prior art uses an electrolytic method to perform activation pretreatment on the steel surface, which easily causes uneven hot-dip aluminum plating, the present invention provides a hot-dip aluminum alloy coating method, which sequentially performs nitriding treatment and plating-assisting treatment on the steel before hot-dip aluminum plating, thereby improving the adhesion of the aluminum alloy coating on the steel surface; after the hot-dip aluminum plating is completed, the steel is spray-cooled, thereby significantly reducing the cooling rate difference of the aluminum alloy coating on the steel surface.

[0005] The present invention provides a hot-dip aluminum alloy coating method, comprising the following steps: Step 1: Pre-treat the steel; Step 2: placing the pretreated steel in a nitriding furnace, the nitriding temperature is 550-650°C, the nitriding time is 15-35h, and the nitrogen supply agent introduction rate is 1-1.5L / min; Step 3: Immerse the steel in a plating solution for plating treatment. The plating treatment temperature is 80-85°C and the plating treatment time is 2-3 minutes. Step 4: Immerse the steel in the aluminum alloy solution for hot-dip plating. The hot-dip plating temperature is 660-690°C and the hot-dip plating time is 1-1.5 minutes. Step 5: After hot dip coating, the steel is cooled naturally.

[0006] Furthermore, in step 1, the pretreatment includes alkali washing and acid washing, and water washing is performed after the alkali washing and the acid washing. The temperature of the alkali washing is 45-55°C, the time of the alkali washing is 1-3h, and the alkali washing is performed under ultrasound. The temperature of the acid washing is 50-70°C, the time of the acid washing is 1-2h. The temperature of the water washing is 50-60°C, and the time of the water washing is 1-2h.

[0007] Furthermore, the alkaline treatment liquid used in the alkaline washing process is a sodium hydroxide aqueous solution, and the acidic treatment liquid used in the acid washing process is a hexamethylenetetramine-hydrochloric acid buffer solution. In the hexamethylenetetramine-hydrochloric acid buffer solution, the mass percentage of hexamethylenetetramine is 1%, and the mass percentage of hydrochloric acid is 25%.

[0008] Furthermore, in step 2, the nitrogen supply agent used in the nitriding treatment is ammonia.

[0009] Furthermore, in step 2, the nitriding treatment method comprises the following steps: Step a: introducing ammonia into the nitriding furnace to remove oxygen until the decomposition rate of ammonia is less than 10%, wherein the introduction rate of ammonia is 1.2-1.5 L / min; Step b: Raise the temperature of the nitriding furnace to 550-650°C and reduce the flow rate of ammonia to 1-1.2 L / min until the decomposition rate of ammonia reaches 30%-45%; Step c: Control the temperature of the nitriding furnace at 550-650°C, and keep the temperature for 15-35 hours. During the temperature reaction, the decomposition rate of ammonia is maintained at 30%-50%; Step d: Control the temperature of the nitriding furnace at 550-650°C, reduce the flow of ammonia, denitrify for 1-2 hours, and remove the residual ammonia on the surface of the steel.

[0010] Furthermore, in step 2, after the nitriding treatment is completed, the steel is cleaned, and the cleaning treatment includes water washing and drying.

[0011] Furthermore, in step 3, the composition and mass percentage of the plating solution are as follows: NaCl 20%-25%, CaCl 2 5%-10%, maleic anhydride 0.1%-0.5%, SnCl 210%-15%, the rest is water. Maleic anhydride contains active double bonds and carboxyl groups, which helps to reduce the surface energy of the nitriding layer on the steel surface and can effectively improve the uniformity and bonding strength of the aluminum alloy coating.

[0012] Furthermore, in step 4, the composition and mass percentage of the aluminum alloy solution are as follows: 1.5%-5% cobalt, 0.3%-1% rare earth yttrium, and the rest is aluminum. The cobalt element in the aluminum alloy solution can strengthen the steel, and the rare earth yttrium in the aluminum alloy solution can enhance the corrosion resistance of the aluminum alloy coating, so that the aluminum alloy solution can better adhere to the surface of the nitriding layer of the steel, and improve the bonding force between the aluminum alloy coating and the nitriding layer.

[0013] The beneficial effects of the present invention are: The present invention provides a hot-dip aluminum alloy coating method, which comprises the following steps: firstly, pre-treating steel to remove grease and rust layer on the surface of the steel; then, nitriding the pre-treated steel under nitrogen protection to form a nitride layer on the surface of the steel, which is beneficial to improving the hardness and corrosion resistance of the steel; performing a plating-aiding treatment on the nitrided steel, and adjusting the composition and proportion of the plating-aiding solution to achieve a synergistic effect, thereby improving the conductivity, enhancing the adhesion, and increasing the stability of the plating-aiding solution and improving the coating quality; after the plating-aiding treatment, immersing the steel in an aluminum alloy solution for hot-dip plating to form an aluminum alloy layer on the surface of the nitride layer, thereby further improving the corrosion resistance of the steel; after the hot-dip plating, the present invention first spray-cools the steel and then cools it naturally, and the spray cooling method is used to improve the cooling rate, thereby improving the bonding strength between the aluminum alloy layer and the steel and improving the corrosion resistance of the aluminum alloy layer. DETAILED DESCRIPTION

[0014] In order to enable those skilled in the art to better understand the technical solutions in the present invention, the technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0015] Example 1 A hot-dip aluminum alloy coating method comprises the following steps: Step 1: (1) Place the steel in a 25% by mass NaOH solution and perform alkaline washing under ultrasound. The alkaline washing temperature is 45°C and the alkaline washing time is 2h. (2) After the alkaline washing is completed, the steel is washed with water for the first time to remove the residual alkali solution on the surface of the steel with distilled water. The water washing temperature is 60°C and the water washing time is 1h. (3) Use hexamethylenetetramine-hydrochloric acid buffer solution to pickle the steel to remove the oxide layer on the surface of the steel. The pickling temperature is 60°C and the pickling time is 1h. In the hexamethylenetetramine-hydrochloric acid buffer solution, the mass percentage of hexamethylenetetramine is 1% and the mass percentage of hydrochloric acid is 25%. (4) After the pickling is completed, the steel is washed with water for the second time to remove the residual buffer solution and metal ions on the surface of the steel with distilled water. The water washing temperature is 50°C and the water washing time is 2h.

[0016] Step 2: placing the pretreated steel in a nitriding furnace for nitriding treatment. The nitriding treatment method includes the following steps: Step a: introducing ammonia into the nitriding furnace to remove oxygen until the decomposition rate of ammonia is less than 10%, wherein the introduction rate of ammonia is 1.2 L / min; Step b: Raise the temperature of the nitriding furnace to 550°C and reduce the flow rate of ammonia to 1 L / min until the decomposition rate of ammonia reaches 30%; Step c: Control the temperature of the nitriding furnace at 550°C and keep the temperature for 35 hours. During the temperature reaction, the decomposition rate of ammonia is maintained at 30%; Step d: Control the temperature of the nitriding furnace at 550°C and denitrify for 1 hour to remove the residual ammonia on the surface of the steel. After the nitriding treatment is completed, the steel is washed and dried.

[0017] Step 3: Immerse the steel in a plating solution for plating treatment. The plating temperature is 80°C and the plating time is 2 minutes. The composition and mass percentage of the plating solution are as follows: NaCl 20%, CaCl 2 5%, maleic anhydride 0.5%, SnCl 2 15% and the rest is water.

[0018] Step 4: Immerse the steel in an aluminum alloy solution for hot dip plating. The hot dip plating temperature is 660°C and the hot dip plating time is 1 minute. The composition and mass percentage of the aluminum alloy solution are as follows: 1.5% cobalt, 1% rare earth yttrium, and the rest is aluminum.

[0019] Step 5: After hot-dip galvanizing, place the steel in the air to cool naturally.

[0020] Example 2 A hot-dip aluminum alloy coating method comprises the following steps: Step 1: (1) Place the steel in a 25% by mass NaOH solution and perform alkaline washing under ultrasound. The alkaline washing temperature is 55°C and the alkaline washing time is 1h. (2) After the alkaline washing is completed, the steel is washed with water for the first time to remove the residual alkali solution on the surface of the steel with distilled water. The water washing temperature is 50°C and the water washing time is 2h. (3) Use hexamethylenetetramine-hydrochloric acid buffer solution to pickle the steel to remove the oxide layer on the surface of the steel. The pickling temperature is 50°C and the pickling time is 2h. In the hexamethylenetetramine-hydrochloric acid buffer solution, the mass percentage of hexamethylenetetramine is 1% and the mass percentage of hydrochloric acid is 25%. (4) After the pickling is completed, the steel is washed with water for the second time to remove the residual buffer solution and metal ions on the surface of the steel with distilled water. The water washing temperature is 60°C and the water washing time is 1h.

[0021] Step 2: placing the pretreated steel in a nitriding furnace for nitriding treatment. The nitriding treatment method includes the following steps: Step a: introducing ammonia into the nitriding furnace to remove oxygen until the decomposition rate of ammonia is less than 10%, wherein the introduction rate of ammonia is 1.3 L / min; Step b: Raise the temperature of the nitriding furnace to 650°C and reduce the flow rate of ammonia to 1L / min until the decomposition rate of ammonia reaches 45%; Step c: Control the temperature of the nitriding furnace at 650°C and keep the temperature for 15 hours. During the reaction, the decomposition rate of ammonia is maintained at 45%; Step d: Control the temperature of the nitriding furnace at 650°C and denitrify for 2 hours to remove the residual ammonia on the surface of the steel.

[0022] Step 3: Immerse the steel in a plating solution for plating treatment. The plating temperature is 85°C and the plating time is 3 minutes. The composition and mass percentage of the plating solution are as follows: NaCl 25%, CaCl 2 9%, maleic anhydride 0.5%, SnCl 2 15% and the rest is water.

[0023] Step 4: Immerse the steel in an aluminum alloy solution for hot dip plating. The hot dip plating temperature is 680°C and the hot dip plating time is 1.5 minutes. The composition and mass percentage of the aluminum alloy solution are as follows: 5% cobalt, 0.3% rare earth yttrium, and the rest is aluminum.

[0024] Step 5: After hot-dip galvanizing, place the steel in the air to cool naturally.

[0025] Example 3 A hot-dip aluminum alloy coating method comprises the following steps: Step 1: (1) Place the steel in a 25% by mass NaOH solution and perform alkaline washing under ultrasound. The alkaline washing temperature is 48°C and the alkaline washing time is 2.5 hours. (2) After the alkaline washing is completed, the steel is washed with water for the first time to remove the residual alkali solution on the surface of the steel with distilled water. The water washing temperature is 54°C and the water washing time is 1.5 hours. (3) The steel is pickled with a hexamethylenetetramine-hydrochloric acid buffer solution to remove the oxide layer on the surface of the steel. The pickling temperature is 60°C and the pickling time is 1.5 hours. In the hexamethylenetetramine-hydrochloric acid buffer solution, the mass percentage of hexamethylenetetramine is 1% and the mass percentage of hydrochloric acid is 25%. (4) After the pickling is completed, the steel is washed with water for the second time to remove the residual buffer solution and metal ions on the surface of the steel with distilled water. The water washing temperature is 58°C and the water washing time is 1 hour.

[0026] Step 2: placing the pretreated steel in a nitriding furnace for nitriding treatment. The nitriding treatment method includes the following steps: Step a: introducing ammonia into the nitriding furnace to remove oxygen until the decomposition rate of ammonia is less than 10%, wherein the introduction rate of ammonia is 1.5 L / min; Step b: Raise the temperature of the nitriding furnace to 600°C and reduce the flow rate of ammonia to 1.2L / min until the decomposition rate of ammonia reaches 40%; Step c: Control the temperature of the nitriding furnace at 600°C and keep the temperature for 25 hours. During the temperature reaction, the decomposition rate of ammonia is maintained at 40%; Step d: Control the temperature of the nitriding furnace at 600°C and denitrify for 1.5 hours to remove the residual ammonia on the surface of the steel. After the nitriding treatment is completed, the steel is washed and dried.

[0027] Step 3: Immerse the steel in a plating solution for plating treatment. The plating temperature is 82°C and the plating time is 2 minutes. The composition and mass percentage of the plating solution are as follows: NaCl 25%, CaCl 2 7%, maleic anhydride 0.5%, SnCl 2 15% and the rest is water.

[0028] Step 4: Immerse the steel in an aluminum alloy solution for hot dip plating. The hot dip plating temperature is 690°C and the hot dip plating time is 1 minute. The composition and mass percentage of the aluminum alloy solution are as follows: 3.8% cobalt, 2.2% rare earth yttrium, and the rest is aluminum.

[0029] Step 5: After hot-dip galvanizing, place the steel in the air to cool naturally.

[0030] Comparative Example 1 A hot-dip aluminum alloy coating method comprises the following steps: Step 1: (1) Place the steel in a 25% by mass NaOH solution and perform alkaline washing under ultrasound. The alkaline washing temperature is 45°C and the alkaline washing time is 2h. (2) After the alkaline washing is completed, the steel is washed with water for the first time to remove the residual alkali solution on the surface of the steel with distilled water. The water washing temperature is 60°C and the water washing time is 1h. (3) Use hexamethylenetetramine-hydrochloric acid buffer solution to pickle the steel to remove the oxide layer on the surface of the steel. The pickling temperature is 60°C and the pickling time is 1h. In the hexamethylenetetramine-hydrochloric acid buffer solution, the mass percentage of hexamethylenetetramine is 1% and the mass percentage of hydrochloric acid is 25%. (4) After the pickling is completed, the steel is washed with water for the second time to remove the residual buffer solution and metal ions on the surface of the steel with distilled water. The water washing temperature is 50°C and the water washing time is 1h.

[0031] Step 2: placing the pretreated steel in a nitriding furnace for nitriding treatment. The nitriding treatment method includes the following steps: Step a: introducing ammonia into the nitriding furnace to remove oxygen until the decomposition rate of ammonia is less than 10%, wherein the introduction rate of ammonia is 1 L / min; Step b: Raise the temperature of the nitriding furnace to 550°C and reduce the flow rate of ammonia to 0.7 L / min until the decomposition rate of ammonia reaches 40%; Step c: Control the temperature of the nitriding furnace at 550°C and keep the temperature for 5 hours. During the temperature reaction, the decomposition rate of ammonia is maintained at 40%; Step d: Control the temperature of the nitriding furnace at 550°C and denitrify for 0.5h to remove the residual ammonia on the surface of the steel. After the nitriding treatment is completed, the steel is washed and dried.

[0032] Step 3: Immerse the steel in a plating solution for plating treatment. The plating temperature is 95°C and the plating time is 2 minutes. The composition and mass percentage of the plating solution are as follows: NaCl 25%, CaCl 2 9%, maleic anhydride 0.5%, SnCl 2 15% and the rest is water.

[0033] Step 4: Immerse the steel in pure aluminum solution for hot dip coating. The hot dip coating temperature is 680°C and the hot dip coating time is 1.5 minutes.

[0034] Step 5: After hot-dip galvanizing, place the steel in the air to cool naturally.

[0035] Comparative Example 2 A hot-dip aluminum alloy coating method comprises the following steps: Step 1: (1) Place the steel in a 25% by mass NaOH solution and perform alkaline washing under ultrasound. The alkaline washing temperature is 48°C and the alkaline washing time is 2.5 hours. (2) After the alkaline washing is completed, the steel is washed with water for the first time to remove the residual alkali solution on the surface of the steel with distilled water. The water washing temperature is 54°C and the water washing time is 1.5 hours. (3) The steel is pickled with a hexamethylenetetramine-hydrochloric acid buffer solution to remove the oxide layer on the surface of the steel. The pickling temperature is 60°C and the pickling time is 1.5 hours. In the hexamethylenetetramine-hydrochloric acid buffer solution, the mass percentage of hexamethylenetetramine is 1% and the mass percentage of hydrochloric acid is 25%. (4) After the pickling is completed, the steel is washed with water for the second time to remove the residual buffer solution and metal ions on the surface of the steel with distilled water. The water washing temperature is 58°C and the water washing time is 1 hour.

[0036] Step 2: placing the pretreated steel in a nitriding furnace for nitriding treatment. The nitriding treatment method includes the following steps: Step a: introducing ammonia into the nitriding furnace to remove oxygen until the decomposition rate of ammonia is less than 10%, wherein the introduction rate of ammonia is 1.5 L / min; Step b: Raise the temperature of the nitriding furnace to 650°C and reduce the flow rate of ammonia to 0.5L / min until the decomposition rate of ammonia reaches 40%; Step c: Control the temperature of the nitriding furnace at 650°C and keep the temperature for 25 hours. During the reaction, the decomposition rate of ammonia is maintained at 40%; Step d: Control the temperature of the nitriding furnace at 650°C and denitrify for 1.5 hours to remove the residual ammonia on the surface of the steel.

[0037] Step 3: Immerse the steel in a plating solution for plating treatment. The plating temperature is 82°C and the plating time is 2 minutes. The composition and mass percentage of the plating solution are as follows: NaCl 25%, CaCl 2 9%, maleic anhydride 0.5%, SnCl 2 15% and the rest is water.

[0038] Step 4: Immerse the steel in pure aluminum solution for hot dip coating. The hot dip coating temperature is 680°C and the hot dip coating time is 1.5 minutes.

[0039] Step 5: After hot-dip galvanizing, place the steel in the air to cool naturally.

[0040] The polarization curves of the cooled steel obtained in Example 1-Example 3 and Comparative Example 1-Comparative Example 2 in a 3.5wt% NaCl solution were measured using an electrochemical workstation to test the corrosion rate of the steel and to perform an adhesion test. The adhesion test was performed according to the tape visual inspection method in the national standard GB / T39130-2020 to determine the level. The test results are shown in Table 1.

[0041] Table 1 Test results of steel obtained from Example 1 to Example 3 and Comparative Example 1 to Comparative Example 2

[0042] As can be seen from Table 1, the adhesion of the aluminum alloy coating on the cooled steel obtained in Examples 1 to 3 is significantly improved compared with Comparative Examples 1 and 2, and the aluminum alloy coating has high corrosion resistance.

[0043] Although the present invention has been described in detail by way of preferred embodiments, the present invention is not limited thereto. Without departing from the spirit and essence of the present invention, a person of ordinary skill in the art may make various equivalent modifications or substitutions to the embodiments of the present invention, and such modifications or substitutions shall be within the scope of the present invention. Any person of ordinary skill in the art may easily conceive of changes or substitutions within the technical scope disclosed by the present invention, and such changes or substitutions shall be within the scope of protection of the present invention.

Claims

1. A hot-dip aluminum alloy coating method, characterized in that: The steps include: Step 1: Pre-treat the steel; Step 2: placing the pretreated steel in a nitriding furnace for nitriding treatment, the nitriding temperature is 550-650°C, the nitriding time is 15-35h, and the nitrogen supply agent introduction rate is 1-1.5L / min; Step 3: Immerse the steel in a plating solution for plating treatment. The plating treatment temperature is 80-85°C and the plating treatment time is 2-3 minutes. Step 4: Immerse the steel in the aluminum alloy solution for hot-dip plating. The hot-dip plating temperature is 660-690°C and the hot-dip plating time is 1-1.5 minutes. Step 5: After hot dip coating, the steel is cooled naturally.

2. A hot-dip aluminum alloy coating method according to claim 1, characterized in that: In step 1, the pretreatment includes alkali washing and acid washing, and water washing is performed after the alkali washing and the acid washing are completed.

3. A hot-dip aluminum alloy coating method as claimed in claim 2, characterized in that: The alkaline treatment liquid used in the alkaline washing process is a sodium hydroxide aqueous solution, and the acidic treatment liquid used in the acid washing process is a hexamethylenetetramine-hydrochloric acid buffer solution.

4. A hot-dip aluminum alloy coating method according to claim 1, characterized in that: In step 2, the nitrogen supply agent used in the nitriding treatment is ammonia.

5. A hot-dip aluminum alloy coating method as claimed in claim 4, characterized in that: In step 2, the nitriding treatment method includes the following steps: Step a: introducing ammonia into the nitriding furnace to remove oxygen until the decomposition rate of ammonia is less than 10%, wherein the introduction rate of ammonia is 1.2-1.5 L / min; Step b: Raise the temperature of the nitriding furnace to 550-650°C and reduce the flow rate of ammonia to 1-1.2 L / min until the decomposition rate of ammonia reaches 30%-45%; Step c: Control the temperature of the nitriding furnace at 550-650°C, and keep the temperature for 15-35 hours. During the temperature reaction, the decomposition rate of ammonia is maintained at 30%-50%; Step d: Control the temperature of the nitriding furnace at 550-650°C, reduce the flow of ammonia, denitrify for 1-2 hours, and remove the residual ammonia on the surface of the steel.

6. A hot-dip aluminum alloy coating method according to claim 1, characterized in that: In step 2, after the nitriding treatment is completed, the steel is cleaned.

7. A hot-dip aluminum alloy coating method as claimed in claim 1, characterized in that: In step 3, the composition and mass percentage of the plating solution are as follows: NaCl 20%-25%, CaCl2 5%-10%, maleic anhydride 0.1%-0.5%, SnCl2 10%-15%, and the rest is water.

8. A hot-dip aluminum alloy coating method as claimed in claim 1, characterized in that: In step 4, the composition and mass percentage of the aluminum alloy solution are as follows: 1.5%-5% cobalt, 0.3%-1% rare earth yttrium, and the rest aluminum.

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