阅读说明：本技术 一种超耐腐金属构件的无污染复合渗锌工艺 (Pollution-free composite zinc impregnation process for super corrosion-resistant metal component ) 是由 金新鹤 于 2020-11-13 设计创作，主要内容包括：本发明涉及一种超耐腐金属构件的无污染复合渗锌工艺,方法包括以下步骤：步骤S1：固化炉高温除油；步骤S2：抛丸机进行表面氧化层处理；步骤S3：渗锌；步骤S4：抛丸；步骤S5：超声波清洗；步骤S6：涂覆无铬锌铝涂层；步骤S7：成品,应用本发明的技术方案,全程环保无污染,无泡酸,不影响金属构件强度,在金属构件表面采用多层复合防蚀处理措施,涂层均匀,金属构件在装配后仍能保持超高耐腐蚀性等特点。(The invention relates to a pollution-free composite zinc impregnation process of a super corrosion-resistant metal component, which comprises the following steps: step S1: removing oil in the curing oven at high temperature; step S2: performing surface oxidation layer treatment by using a shot blasting machine; step S3: zinc impregnation; step S4: shot blasting; step S5: ultrasonic cleaning; step S6: coating a chromium-free zinc-aluminum coating; step S7: the finished product is environment-friendly and pollution-free in the whole process, does not have acid foam, does not influence the strength of the metal component, adopts multilayer composite anticorrosion treatment measures on the surface of the metal component, has uniform coating, and can still keep ultrahigh corrosion resistance after the metal component is assembled.)

1. A pollution-free composite zinc impregnation process of a super corrosion-resistant metal component is characterized in that: the method comprises the following steps: step S1: removing oil in the curing oven at high temperature; step S2: performing surface oxidation layer treatment by using a shot blasting machine; step S3: zinc impregnation; step S4: shot blasting; step S5: ultrasonic cleaning; step S6: coating a chromium-free zinc-aluminum coating; step S7: and (5) finishing.

2. The non-pollution composite zincification process of the ultra-corrosion-resistant metal component as claimed in claim 1, wherein: the method comprises the following steps:

step S1: putting the processed workpiece into a high-temperature curing furnace at 300 ℃, and removing oil on the surface of the workpiece at high temperature in a high-temperature environment;

step S2: putting the workpiece subjected to high-temperature oil removal in the S1 curing furnace into a shot blasting machine, and impacting the surface of the workpiece for 20-30 min by using the shot blasting machine with steel shot flow at the flow rate of 60-110 m/S;

step S3: the specific process of S3 zincizing is as follows: putting the zinc impregnation powder into a zinc impregnation furnace, putting a workpiece into the furnace body to be mixed with the zinc impregnation powder, heating the furnace body to 350-420 ℃, preserving heat, standing for 180-240 min, then cooling the furnace body to 200-300 ℃, opening the furnace and cooling to room temperature;

step S4: taking the workpiece subjected to zinc impregnation out of the furnace body, separating the workpiece from zinc powder and quartz sand through a filter screen, and performing water shot blasting on the workpiece through a high-speed water flow polishing machine to remove residual media on the surface;

step S5: putting the workpiece subjected to water blasting into an ultrasonic cleaning machine, adding a Ph neutralizing liquid, cleaning impurities on the surface of the workpiece by ultrasonic waves, neutralizing the Ph value of the surface of the workpiece to 7 +/-1, and after the workpiece is detected to be qualified, putting the workpiece into a baking furnace at 100 ℃ for baking and curing for 20-30 min;

step S6: sequentially performing dip coating on the workpiece subjected to ultrasonic cleaning with chromium-free zinc-aluminum coating solution → baking → cooling → secondary dip coating with chromium-free zinc-aluminum coating solution → baking → cooling;

step S7: and finally, checking the workpiece, thereby finishing the finished product.

3. The non-pollution composite zincizing process for the ultra corrosion-resistant metal member as claimed in claim 2, wherein the step S3 further includes: the zinc infiltration powder comprises the following components in percentage by weight: the zinc powder is 15 kg/t-45 kg/t, and the quartz sand accounts for 30-40% of the effective volume of the furnace body.

4. The non-pollution composite zincizing process of the ultra-corrosion-resistant metal component as claimed in claim 2, wherein: the thickness of the obtained zincizing layer of the workpiece after the zincizing in the step S3 is controlled to be 30-50 μm.

5. The non-pollution composite zincizing process of the ultra-corrosion-resistant metal component as claimed in claim 2, wherein: and controlling the thickness of the remaining zincizing layer of the workpiece subjected to the shot blasting in the step S4 to be 10-20 mu m.

6. The non-pollution composite zincizing process of the ultra-corrosion-resistant metal component as claimed in claim 2, wherein: the chromium-free zinc-aluminum coating liquid comprises metal slurry, organic solvent aqueous solution and tackifier, wherein the metal slurry is placed in a deionized water tank to be uniformly stirred, the organic solvent aqueous solution is added while stirring, after a certain period of time, the temperature of the solution is detected, then necessary amount of tackifier and deionized water are prepared to be stirred, the mixture is quickly poured into the water tank, the mixture is uniformly stirred for more than 12 hours at the temperature of 20-25 ℃, and then the mixture is filtered by a 80-mesh filter screen to obtain the required chromium-free zinc-aluminum coating liquid.

7. The non-pollution composite zincizing process of the ultra-corrosion-resistant metal component as claimed in claim 2, wherein: the thickness of the primary chromium-free zinc-aluminum coating obtained by baking and cooling the primary dip-coating chromium-free zinc-aluminum coating liquid in the step S6 is controlled to be 5-7 mu m, and the total thickness of the secondary chromium-free zinc-aluminum coating obtained by baking and cooling the secondary dip-coating chromium-free zinc-aluminum coating is controlled to be 10-12 mu m.

8. The non-pollution composite zincizing process of the ultra-corrosion-resistant metal component as claimed in claim 2, wherein: the step S6 further includes: the baking temperature after the primary dip coating of the chromium-free zinc-aluminum coating liquid is 120 +/-20 ℃, the pre-baking time is 10-15 min, the intermediate baking temperature is 320-345 ℃, the baking time is 20-40 min, the baked workpiece is taken out and placed at room temperature, the workpiece is blown by a blower to be cooled, and the workpiece is naturally cooled after the temperature reaches 100 ℃; and (4) repeating the steps of pre-baking, baking and cooling the workpiece after the secondary dip-coating of the chromium-free zinc-aluminum coating liquid.

Technical Field

The invention relates to a metal surface treatment method, in particular to a pollution-free composite zinc impregnation process of a super corrosion-resistant metal component.

Background

At present, sewage (such as spent electrolyte) generated by the conventional electroplating process is an important source of water pollution, electroplating wastewater belongs to heavy metal wastewater, sludge generated by electroplating treatment belongs to dangerous waste, and the sewage and the sludge both belong to risk sources of pollution accidents mainly monitored by environmental protection departments; meanwhile, the existing metal surface anticorrosion treatment process includes electroplating, electrophoresis, blackening, metal surface coloring, shot blasting, sand blasting, shot blasting, phosphating, passivation and the like, only aims at the surface of a single metal component to be subjected to anticorrosion treatment, and often, a plurality of metal components are assembled and molded after the surface treatment is finished, the surface anticorrosion layer of the metal component is abraded or even falls off due to the impact of external force, the anticorrosion performance of the part with the abraded or even fallen surface coating is greatly reduced, the corrosion and the intangible surface of the metal component can pollute the environment, although some metal components can be assembled and then subjected to integral surface anticorrosion treatment, the volume of the assembled metal component is larger, the surface structure is more complicated, a plurality of surface processes cannot be met, the limitation is large, and dead corners with incomplete surface treatment exist. At present, the common surface treatment methods in the fastener comprise hot-dip galvanizing, chemical plating of Ni-P alloy coating, vacuum galvanizing, zinc-chromium coating and the like. The hot-dip galvanizing corrosion resistance is general, and the bolt and the nut are difficult to assemble due to the thicker plating layer; the vacuum zincizing layer has certain corrosion resistance, high bonding strength with a matrix and good wear resistance, but the salt spray corrosion resistance is less than or equal to 300h, and the single zincizing treatment has limited corrosion resistance which is not as good as the corrosion resistance of the Dacromet anticorrosive coating; the surface hardness of the fastener which is subjected to the anti-corrosion treatment by the Dacromet coating alone is low, the Dacromet anti-corrosion coating is generally thick and easily affects the thread size specification of the external thread connecting piece, the excessively thick Dacromet coating at the thread part can be peeled off under the extrusion action of the internal thread of the nut in the actual use process, the peeled Dacromet coating can be accumulated in the internal thread of the nut, the Dacromet coating at the thread part of the fastener is further damaged, and the Dacromet coating at the thread part completely loses the anti-corrosion capability. Therefore, the practical use requirements of the fastener on the surface hardness and the corrosion resistance can be fully met by adopting the multilayer composite anticorrosion treatment measure of the surface zincification and the Dacromet coating on the surface of the fastener.

The existing multilayer composite anticorrosion treatment measures, such as a preparation method of a composite coating of a ship fastener disclosed in 2013, 05 and 28 under the patent number CN201310201911.6, relate to a fastener, namely a zincification coating, a zinc-aluminum coating and an epoxy coating which are arranged on the surface of the fastener from inside to outside, and the fastener is sequentially subjected to oil removal and rust removal, then is subjected to zincification treatment, cleaning, dip-coating of a zinc-aluminum coating solution, curing of the zinc-aluminum coating, dip-coating of an epoxy coating solution and curing of the epoxy coating to form a protective layer. However, in the preparation method, the zinc-aluminum coating liquid is prepared from zinc powder, aluminum powder, chromate and surfactant according to the prior art or is purchased as a commercial product, and the chromide can invade human bodies through digestive tracts, respiratory tracts, skins and mucous membranes and is mainly accumulated in livers, kidneys, endocrine systems and lungs. Chromium compounds have carcinogenic effects. Chromium compounds enter human tissues in the form of vapor and dust, are slowly metabolized and removed, and can cause perforation of nasal septum, gastrointestinal disorders, leukopenia, and pulmonary disorders like asthma. The concentration is 3.0 mg/L, so that the freshwater fish can be killed; the concentration is 0.01 mg/L, so that some aquatic organisms can be killed, and the self-purification effect of the water body is inhibited. And the contamination of chromium is mainly caused by industry. Therefore, the chromium content of discharged wastewater, water quality of fishery waters, water quality of farm irrigation, ground water and drinking water is strictly regulated in various countries. Therefore, a preparation method of the composite coating with ultra corrosion resistance and no pollution needs to be developed.

Disclosure of Invention

In conclusion, in order to overcome the defects in the prior art, the invention provides a pollution-free composite zinc impregnation process for a super corrosion-resistant and pollution-free super corrosion-resistant metal component.

In order to achieve the purpose, the invention provides the following technical scheme: a pollution-free composite zinc impregnation process for a super corrosion-resistant metal component comprises the following steps: step S1: removing oil in the curing oven at high temperature; step S2: performing surface oxidation layer treatment by using a shot blasting machine; step S3: zinc impregnation; step S4: shot blasting; step S5: ultrasonic cleaning; step S6: coating a chromium-free zinc-aluminum coating; step S7: and (5) finishing.

By adopting the technical scheme, the chromium-free zinc-aluminum coating is adopted, so that the whole process is environment-friendly and pollution-free, and meanwhile, no peracid is generated, and the strength of the metal member is not influenced; according to the technical scheme, firstly, zinc impregnation treatment is adopted, then a chromium-free zinc-aluminum coating is coated, a multi-layer composite corrosion prevention treatment measure is adopted on the surface of a metal component, the coating is uniform, and the super-strong corrosion resistance is still maintained after assembly.

The invention further provides that: the method comprises the following steps: step S1: the base part is subjected to high-temperature oil removal at the high temperature of 300 ℃ in a curing furnace because oil stain, cooling liquid and the like are left after the base part is machined; s2: the workpiece obtained in the step S1 has the defects that pollutants are remained on the surface of the zincizing workpiece in the processes of machining, forging, welding, heat treatment and the like in the early stage of zincizing, so that the surface of the workpiece is continuously impacted for 20-30 minutes by using a steel shot flow with high-speed motion at 60-110 m/S, the diameter of the steel shot is 0.4-0.8 mm and the shot blasting speed is adjusted according to the surface size of the workpiece, the oxidation layer is removed by 0.10-0.30 mm under the cyclic impact on the surface and the surface layer of the workpiece, and the adhesion force of the subsequent zincizing of the workpiece is improved; s3: before the workpiece obtained in the step S2 is processed by a furnace body, zinc impregnation powder is loaded into a zinc impregnation furnace, the workpiece is put into the furnace body and mixed with the zinc impregnation powder, the furnace body is heated to 350-420 ℃, the temperature is kept for 180-240 min after the workpiece is heated to the required temperature, then the workpiece is cooled to 200-300 ℃ along with the furnace, and the furnace is opened and cooled to room temperature; s4: performing water shot blasting on the workpiece obtained in the step S3 on a high-speed water flow polishing machine, impacting the surface of the workpiece by using polishing liquid sprayed at a high speed, and colliding abrasive particles with the surface of the workpiece at a high speed to enable the stress of a local stress field on the workpiece to be concentrated at a high speed and to change rapidly, so that erosion and shearing are generated to achieve the purpose of removing residual zinc powder, and finally performing high-temperature curing treatment on the part through a curing furnace; s5: the specific process of ultrasonic cleaning comprises the following steps: putting the workpiece subjected to water blasting into an ultrasonic cleaning machine, adding a Ph neutralizing liquid, cleaning impurities on the surface of the workpiece by ultrasonic waves, neutralizing the Ph value of the surface of the workpiece to 7 +/-1, and after the workpiece is detected to be qualified, putting the workpiece into a baking furnace at 100 ℃ for baking and curing for 20-30 min; s6: performing dip coating of the workpiece obtained in S5 with a chromium-free zinc-aluminum coating solution → baking → cooling → secondary dip coating of the chromium-free zinc-aluminum coating solution → baking → cooling, the baking comprising:

pre-baking: the workpiece coated by the chromium-free zinc-aluminum coating through dipping must be pre-dried for 10min-15min at the temperature of 120 +/-20 ℃ as soon as possible, the time is determined by the heat absorption capacity of the workpiece, so that the water of the coating liquid is evaporated, and the coating liquid is leveled uniformly.

Baking: the pre-baked workpiece must be baked at a high temperature of 320-345 ℃, the baking time is 20-40 min, and the time is determined by the heat absorption capacity of the workpiece.

Cooling: taking out the baked workpiece, placing the workpiece at room temperature, blowing the workpiece to be cold by using a blower, and naturally cooling the workpiece after the temperature is up to 100 ℃.

By adopting the technical scheme, after the workpiece is subjected to oil and rust removal treatment, the workpiece is preheated at the temperature of 100-, the zinc, aluminum and iron matrixes are simultaneously subjected to passive protection to form compounds of Fe, Zn and Al, so that the coating liquid and the workpiece are more effectively combined and better permeate into gaps on the surface of the workpiece to make the surface more compact, because the coating is obtained by direct reaction with the substrate, the anti-corrosion layer is extremely compact, the coating can form numerous galvanic cells in a corrosive environment, namely, the Zn and Al sheets with negative potential are corroded first, so that the base body can be corroded.

The invention further provides that: the step S3 further includes: the zinc infiltration powder comprises the following components in percentage by weight: 15 kg/t-45 kg/t zinc powder, 30% -40% quartz sand accounting for the effective volume of the furnace body, polishing solution is prepared by abrasive micro powder and distilled water, the abrasive powder with 6um granularity can be selected for rough polishing, and the abrasive micro powder with 1um granularity can be selected for fine polishing, and the proportion is as follows: 1:(5-10).

By adopting the technical scheme, the surface of the zinc-infiltrated piece is provided with the zinc-infiltrated layer with uniform thickness and good surface bonding force.

The invention further provides that: according to the manufacturing process, the thickness of the zincized layer of the workpiece after the zincizing in the step S3 is controlled to be 30-50 μm.

By adopting the technical scheme, the surface of the workpiece can reach an enough bottom pre-permeation anticorrosive coating.

The invention further provides that: according to the manufacturing process, the thickness of the zincizing layer left on the workpiece subjected to the shot blasting in the step S4 is controlled to be 10-20 mu m.

By adopting the technical scheme, the surface of the workpiece can reach an effective bottom zinc impregnation anticorrosive layer after shot blasting.

The invention further provides that: the chromium-free zinc-aluminum coating liquid contains metal slurry (zinc sheets, aluminum sheets and silicon adhesives), organic solvent aqueous solution, tackifier and the like, the metal slurry is placed in a deionized water tank with the temperature of 20 +/-1 ℃ to be uniformly stirred, 1/3 organic solvent aqueous solution is added while stirring, after stirring for 1 hour, whether the temperature of the solution reaches 20-25 ℃ is detected, then necessary amount of tackifier and 4-5 times of deionized water are prepared to be stirred, the mixture is poured into a water tank to be uniformly stirred within 0.5-1 minute, the zinc-aluminum coating liquid is stirred for more than 12 hours at the temperature of 20-25 ℃ and then filtered by a 80-mesh filter screen to obtain the required chromium-free zinc-aluminum coating liquid.

By adopting the technical scheme, compared with the existing dacromet coating liquid, the chromium-free zinc-aluminum coating liquid is adopted in the technical scheme of the invention, and the manufactured and molded product does not contain toxic metals (such as nickel, lead, barium, mercury and the like) and hexavalent chromium or trivalent chromium, meets the relevant environmental protection requirement specifications, and has stronger corrosion resistance.

The invention further provides that: according to the manufacturing process, the thickness of the primary chromium-free zinc-aluminum coating obtained by baking and cooling the primary dip-coating chromium-free zinc-aluminum coating liquid in the step S6 is controlled to be 5-7 mu m, and the total thickness of the secondary chromium-free zinc-aluminum coating obtained by baking and cooling the secondary dip-coating chromium-free zinc-aluminum coating is controlled to be 10-12 mu m.

By adopting the technical scheme, the thickness of the coating layer is controlled, on one hand, the salt spray performance and the real matching performance of the product are controlled, the real matching performance of the product can be influenced by the too thick coating layer, the binding force of the too thick coating layer is poor, and the controlled coating layer can achieve the effects of better coating salt spray and binding force.

The invention further provides that: the step S6 further includes: the baking temperature after the primary dip coating of the chromium-free zinc-aluminum coating liquid is 120 +/-20 ℃, the pre-baking time is 10-15 min, the intermediate baking temperature is 320-345 ℃, the baking time is 20-40 min, the baked workpiece is taken out and placed at room temperature, the workpiece is blown by a blower to be cooled, and the workpiece is naturally cooled after the temperature reaches 100 ℃; and (4) repeating the steps of pre-baking, baking and cooling the workpiece after the secondary dip-coating of the chromium-free zinc-aluminum coating liquid.

By adopting the technical scheme, under the conditions of the temperature and the time, the workpiece can be gradually heated, insulated and cooled, and then the binding force of the coating layer is better.

The examples describe specific embodiments of the present invention.

Drawings

FIG. 1 is a process flow diagram of an embodiment of the present invention;

FIG. 2 shows step S6 according to the embodiment of the present invention: a process flow chart of coating the chromium-free zinc-aluminum coating;

FIG. 3 is a table of salt spray test data according to an embodiment of the present invention;

FIG. 4 is a table comparing the performance parameters of conventional electroplating, hot dip galvanizing and composite zincing according to the embodiment of the invention.

Detailed Description

The present embodiment is only for explaining the present invention, and it is not limited to the present invention, and those skilled in the art can make modifications of the present embodiment without inventive contribution as needed after reading the present specification, but all of them are protected by patent law within the scope of the claims of the present invention.

Referring to fig. 1-4, example 1: a pollution-free composite zinc impregnation process for a super corrosion-resistant metal component comprises the following steps: step S1: removing oil in the curing oven at high temperature; step S2: performing surface oxidation layer treatment by using a shot blasting machine; step S3: zinc impregnation; step S4: shot blasting; step S5: ultrasonic cleaning; step S6: coating a chromium-free zinc-aluminum coating; step S7: and (5) finishing.

The embodiment further provides that: s1; the base part is subjected to high-temperature oil removal at the high temperature of 300 ℃ in a curing furnace because oil stain, cooling liquid and the like are left after the base part is machined;

s2: the workpiece obtained in the step S1 has the defects that pollutants are remained on the surface of the zincizing workpiece in the processes of machining, forging, welding, heat treatment and the like in the early stage of zincizing, so that the surface of the workpiece is continuously impacted for 20-30 minutes by using a steel shot flow with high-speed motion at 60-110 m/S, the diameter of the steel shot is 0.4-0.8 mm and the shot blasting speed is adjusted according to the surface size of the workpiece, the oxidation layer is removed by 0.10-0.30 mm under the cyclic impact on the surface and the surface layer of the workpiece, and the adhesion force of the subsequent zincizing of the workpiece is improved;

s3: before the workpiece obtained in the step S2 is processed by a furnace body, zinc impregnation powder is loaded into a zinc impregnation furnace, the workpiece is put into the furnace body and mixed with the zinc impregnation powder, the furnace body is heated to 350-420 ℃, the temperature is kept for 180-240 min after the workpiece is heated to the required temperature, then the workpiece is cooled to 200-300 ℃ along with the furnace, and the furnace is opened and cooled to room temperature;

s4: performing water shot blasting on the workpiece obtained in the step S3 on a high-speed water flow polishing machine, impacting the surface of the workpiece by using polishing liquid sprayed at a high speed, and enabling the stress of a local stress field on the workpiece to be concentrated at a high speed and change rapidly by means of high-speed collision of abrasive particles and the surface of the workpiece, so as to generate erosion and shearing and achieve the purpose of removing residual zinc powder, and finally performing high-temperature curing treatment on the part through a curing furnace

S5: the specific process of ultrasonic cleaning comprises the following steps: putting the workpiece subjected to the water shot blasting into an ultrasonic cleaning machine, adding a Ph neutralizing liquid, cleaning impurities on the surface of the workpiece through ultrasonic waves, neutralizing the Ph value of the surface of the workpiece to 7 +/-1, and after the workpiece is detected to be qualified, putting the workpiece into a baking furnace at 100 ℃ for baking and curing for 20-30 min.

S6: dip coating, baking, cooling, secondary dip coating, baking and cooling are carried out on the workpiece obtained in the step S5, and the baking comprises the following steps:

pre-baking: the workpiece coated by the chromium-free zinc-aluminum coating through dipping must be pre-dried for 10min-15min at the temperature of 120 +/-20 ℃ as soon as possible, the time is determined by the heat absorption capacity of the workpiece, so that the water of the coating liquid is evaporated, and the coating liquid is leveled uniformly.

Baking: the pre-baked workpiece must be baked at a high temperature of 320-345 ℃, the baking time is 20-40 min, and the time is determined by the heat absorption capacity of the workpiece.

Cooling: taking out the baked workpiece, placing the workpiece at room temperature, blowing the workpiece to be cold by using a blower, and naturally cooling the workpiece after the temperature is up to 100 ℃.

The embodiment further provides that: the step S3 further includes: the zinc infiltration powder comprises the following components in percentage by weight: 15 kg/t-45 kg/t zinc powder, 30% -40% quartz sand accounting for the effective volume of the furnace body, polishing solution is prepared by abrasive micro powder and distilled water, the abrasive powder with 6um granularity can be selected for rough polishing, and the abrasive micro powder with 1um granularity can be selected for fine polishing, and the proportion is as follows: 1:(5-10).

The embodiment further provides that: according to the manufacturing process, the thickness of the zincized layer of the workpiece after the zincizing in the step S3 is controlled to be 30-50 μm.

The embodiment further provides that: according to the manufacturing process, the thickness of the zincizing layer left on the workpiece subjected to the shot blasting in the step S4 is controlled to be 10-20 mu m.

The embodiment further provides that: the chromium-free zinc-aluminum coating liquid contains metal slurry (zinc sheets, aluminum sheets and silicon adhesives), organic solvent aqueous solution, tackifier and the like, the metal slurry is placed in a deionized water tank with the temperature of 20 +/-1 ℃ to be uniformly stirred, 1/3 organic solvent aqueous solution is added while stirring, after stirring for 1 hour, whether the temperature of the solution reaches 20-25 ℃ is detected, then necessary amount of tackifier and 4-5 times of deionized water are prepared to be stirred, the mixture is poured into a water tank to be uniformly stirred within 0.5-1 minute, the zinc-aluminum coating liquid is stirred for more than 12 hours at the temperature of 20-25 ℃ and then filtered by a 80-mesh filter screen to obtain the required chromium-free zinc-aluminum coating liquid.

The embodiment further provides that: according to the manufacturing process, the thickness of the primary chromium-free zinc-aluminum coating obtained by baking and cooling the primary dip-coating chromium-free zinc-aluminum coating liquid in the step S6 is controlled to be 5-7 mu m, and the total thickness of the secondary chromium-free zinc-aluminum coating obtained by baking and cooling the secondary dip-coating chromium-free zinc-aluminum coating is controlled to be 10-12 mu m.

The embodiment further provides that: the step S6 further includes: the baking temperature after the primary dip coating of the chromium-free zinc-aluminum coating liquid is 120 +/-20 ℃, the pre-baking time is 10-15 min, the intermediate baking temperature is 320-345 ℃, the baking time is 20-40 min, the baked workpiece is taken out and placed at room temperature, the workpiece is blown by a blower to be cooled, and the workpiece is naturally cooled after the temperature reaches 100 ℃; and (4) repeating the steps of pre-baking, baking and cooling the workpiece after the secondary dip-coating of the chromium-free zinc-aluminum coating liquid.

The above is only a preferred embodiment of the present invention, and is not intended to limit the present invention, and various modifications and changes will occur to those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.