CN111468705B - Composite treatment process for manufacturing corrosion-resistant protective layer on surface of cast aluminum alloy - Google Patents

Composite treatment process for manufacturing corrosion-resistant protective layer on surface of cast aluminum alloy Download PDF

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CN111468705B
CN111468705B CN202010396598.6A CN202010396598A CN111468705B CN 111468705 B CN111468705 B CN 111468705B CN 202010396598 A CN202010396598 A CN 202010396598A CN 111468705 B CN111468705 B CN 111468705B
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CN111468705A (en
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董寅生
王德洋
余亮
郭超
黄志海
储成林
盛晓波
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Southeast University
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D27/00—Treating the metal in the mould while it is molten or ductile ; Pressure or vacuum casting
    • B22D27/18—Measures for using chemical processes for influencing the surface composition of castings, e.g. for increasing resistance to acid attack
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22C—FOUNDRY MOULDING
    • B22C3/00—Selection of compositions for coating the surfaces of moulds, cores, or patterns
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
    • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
    • C22F1/04—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C22/05—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C22/73—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals characterised by the process

Abstract

The invention discloses a composite treatment process for manufacturing a corrosion-resistant protective layer on the surface of a cast aluminum alloy, which comprises the following steps: firstly, producing an aluminum alloy casting by adopting a type internal oxidation process; and secondly, carrying out heat treatment on the aluminum alloy casting subjected to the internal oxidation treatment obtained in the first step, and then carrying out steam-assisted phosphate coating treatment. The invention aims at the aluminum alloy workpiece manufactured by adopting the type internal oxidation treatment process, and the aluminum alloy workpiece is reinforced by adopting the steam-assisted phosphate coating treatment process after the heat treatment, so that the corrosion resistance of the aluminum alloy casting can be obviously improved, and the service life of the casting is prolonged.

Description

Composite treatment process for manufacturing corrosion-resistant protective layer on surface of cast aluminum alloy
Technical Field
The invention relates to a composite treatment process for manufacturing a corrosion-resistant protective layer on the surface of cast aluminum alloy, belonging to the technical field of cast aluminum alloy surface treatment.
Background
Aluminum alloy is a non-ferrous metal material which is most widely applied in industrial production, and is widely used in the industrial fields of automobiles, aerospace, ships and the like due to the advantages of high specific strength, high specific rigidity, excellent electric and heat conductivity, good processability and the like.
The chemical properties of aluminum and aluminum alloy are active, the standard electrode potential is-1.66V (25 ℃), and the electrode potential is lower and the properties are active in all common metal materials. The aluminum and the aluminum alloy can generate hydrogen evolution corrosion in all pH value ranges, and Al is generated by corrosion in an acid corrosion medium3+Generated in alkaline etching media
Figure RE-GDA0002545107980000011
In a dry neutral environment, because the affinity of aluminum and oxygen is very high, an oxide film can be naturally generated on the surface of the aluminum alloy, and meanwhile, the oxide film has certain self-repairability, can prevent a substrate from contacting with a corrosive medium, and plays a certain role in protection. However, the thickness of the natural oxide film layer on the surface of the aluminum and the aluminum alloy is very thin and is not more than 5nm, and the natural oxide film layer is porous, loose and uneven in surface layer, easy to damage, poor in corrosion resistance and not enough to resist corrosion under severe conditions. Meanwhile, with the aggravation of environmental problems such as acid rain and the like, sulfur and nitride such as salt, sulfur dioxide, nitrogen dioxide and the like in the atmosphere can be settled and enriched on the surface of the casting to generate corrosive electrolyte solution, so that pitting corrosion is easy to occur, and irreversible casting corrosion failure is caused. It is reported that the annual global loss due to corrosion is greater than the sum of all natural disasters, the energy lost due to corrosion and abrasion accounts for about one fifth of the total global energy consumption, and the corrosion protection is increasingly concerned in the engineering field.
The existing aluminum alloy surface treatment processes, such as anodic oxidation, micro-arc oxidation, laser cladding and the like, are generally used in the field of aluminum alloy profile surface treatment, but the cost is high, the environmental problem is serious, and the methods are not suitable for casting aluminum alloy because the cast aluminum alloy has a complex second phase. The in-mold oxidation treatment fully utilizes the heat of molten metal, has the advantages of low cost, simple process, energy conservation, emission reduction and integration with casting surface treatment, and has great practical value. But the casting subjected to the intra-mold oxidation treatment is still obtained by casting a green sand mold, and subsequent heat treatment and proper hole sealing treatment are required.
Disclosure of Invention
The purpose of the invention is as follows: in order to solve the technical problems, the invention provides a composite treatment process for manufacturing a corrosion-resistant protective layer on the surface of a cast aluminum alloy, so as to improve the corrosion resistance of an aluminum alloy casting and prolong the service life of the casting.
The technical scheme is as follows: in order to achieve the purpose, the invention provides the following technical scheme:
a composite treatment process for manufacturing a corrosion-resistant protective layer on the surface of cast aluminum alloy comprises the following steps:
firstly, producing an aluminum alloy casting by adopting a type internal oxidation process;
and secondly, carrying out heat treatment on the aluminum alloy casting subjected to the internal oxidation treatment obtained in the first step, and then carrying out steam-assisted phosphate coating treatment.
Preferably, the method comprises the following steps:
the coating for internal oxidation treatment used in the first step mainly comprises the following components in parts by weight: 60-80 parts of aggregate and 7.4-32.4 parts of Na2Cr2O7·2H2O, 6-9 parts of molten salt, 1.6-3.6 parts of binder and 0.1-0.2 part of suspending agent.
More preferably, the aggregate is Fe2O3、SiO2、Cr2O3、Al2O3、TiO2、SiC、Si3N4、 AlN、ZrO2And one or more of spinel, mullite, zircon and wollastonite, and is powder capable of passing through a 320-mesh sieve.
More preferably, the molten salt is prepared from NaCl, KCl and MgCl2According to a molar weight ratio of 18: 47: 35, the binder is sodium carboxymethyl cellulose (CMC), and the suspending agent is bentonite.
The first step is that the method for producing the aluminum alloy casting by the type internal oxidation process comprises the following steps:
(1) preparing a coating for internal oxidation treatment; (when the above-mentioned type internal oxidation treatment coating material is used, the preparation method is that the aggregate and Na are taken in proportion2Cr2O7·2H2O, fused salt, suspending agent and binder are mixed evenly, and the mixture is added with pure water and ball milled to prepare the coating material)
(2) Coating the coating on the inner cavity of a casting mold or the surface of a sand core, and drying at the temperature of 120-140 ℃; more preferably, the casting mold is a water glass sand mold or a resin sand mold.
(3) And pouring the smelted aluminum liquid into the casting mould, and enabling the aluminum liquid to interact with the coating on the surface of the cavity or the surface of the sand core to obtain the oxide layer on the surface of the casting.
The state of the heat treatment in the second step is T6.
The phosphate coating used in the second step mainly comprises the following components in parts by weight: 10 to 70 portions of Al (H)2PO4)330-80 parts of pure water and Al (H)2PO4)3Al 1.1-1.3 times of the amount required for all chemical reactions2O3And (3) powder. More preferably, the Al is2O3The particle size of the powder is less than 500 meshes.
The process of the steam assisted phosphate coating treatment in the second step is as follows:
(1) preparing a phosphate coating; (when the phosphate coating is used, the raw materials are weighed according to the proportion and are mixed and stirred uniformly to obtain the coating)
(2) Taking the aluminum alloy casting subjected to heat treatment, cleaning and drying;
(3) and (3) coating the coating prepared in the step 1) on the surface of the dried aluminum alloy casting in the step 2), and then placing the aluminum alloy casting in a water vapor environment to obtain the cast aluminum alloy with the aluminum phosphate coating on the surface.
Further preferably, the step (3) of coating the surface of the aluminum alloy casting with the coating prepared in the step 1) means that the aluminum alloy casting is immersed in the coating for 2-3 min.
Further preferably, the temperature of the steam environment in the step (3) is 120-180 ℃, and the time for placing the steam environment is 20-90 min.
In-mold oxidation is a surface treatment process that is performed simultaneously during the aluminum alloy casting process. After casting of the casting is completed, heat treatment is usually performed. However, the heat treatment affects the effect of the in-mold oxidation, causing microcracking or partial separation of the previously formed coating. Aiming at the technical problems, the invention combines the steam-assisted phosphate coating treatment process and the in-mold oxidation process, the phosphate coating can permeate into the separation and microcrack positions of the oxide layer after the heat treatment, not only can fill the cracks, but also can play a role in improving the bonding property of the coating, and can strengthen and further thicken the surface coating, the composite treatment can obviously improve the corrosion resistance of the aluminum alloy casting, and the service life of the casting is prolonged.
Has the advantages that:
(1) the invention can prepare an oxide layer with the thickness of 2-4 mu m on the surface of the cast aluminum alloy by applying the internal oxidation treatment technology, is far thicker than an oxide film naturally generated on the surface of the aluminum alloy, has uniform thickness and good combination with a matrix, and the main component of the oxide layer is Al2O3After in-mold treatment, the polarization resistance value of the alloy is improved by more than 5 times, and the corrosion resistance is obviously improved.
(2) The phosphate ceramic coating is prepared by a steam auxiliary method used by the invention to carry out hole sealing reinforcement treatment on the oxide layer, the ceramic coating with the thickness of about 3 mu m is prepared on the surface of the sample, the problem of preparing the ceramic coating on the surface of the cast aluminum alloy at low temperature is solved, the polarization resistance value of the sample after hole sealing treatment is improved by more than 10 times, and the sample has good durability, for example, the polarization resistance of the obtained sample after 120 hours of corrosion still has 5000 omega cm2。
Drawings
FIG. 1 is an SEM image and EDS results of cross-section and surface of a sample treated by type internal oxidation, wherein: (a) the cross-sectional morphology of the sample after the internal oxidation treatment; (b) the surface appearance of the sample after the internal oxidation treatment; (c) scanning and selecting an image by an EDS line; (d) EDS line scanning element distribution curve; (e) distribution of O, Al, Si, and Cr.
FIG. 2 is a comparison of results of electrochemical measurements between a type internal oxidation sample and a non-type internal oxidation-treated sample, wherein the polarization resistance value of the type internal oxidation sample is shown in (a), the self-etching current density is shown in (b), and the fitting circuit used is shown in (c), the same as below;
FIG. 3 shows the cross-sectional micro-topography (fig. a) and the surface micro-topography (fig. b) of the oxide layer after T6 heat treatment, comparing the electrochemical test results between the T6 heat-treated type internal oxidation test sample and the non-heat-treated type internal oxidation test sample (fig. c, d);
FIG. 4 shows the cross-sectional micro-topography (fig. a and b) and the EDS results (fig. c) of the corrosion-resistant composite coating sample, the type internal oxidation sample and the heat-treated type internal oxidation sample, and the electrochemical test results of the three are compared (fig. d and e).
Detailed Description
The invention will be better understood from the following examples; those skilled in the art will readily understand that the specific test results described in the examples are for the cast aluminum alloy EN AC-43000, which is only illustrative of the present invention, and that the surface treatment method is also suitable for other series of cast aluminum alloys, and should not limit the invention described in the claims; the present invention will be further described with reference to the following examples.
The molten salts described in the following examples are all composed of NaCl, KCl and MgCl2According to a molar weight ratio of 18: 47: 35, the binder is sodium carboxymethyl cellulose, and the suspending agent is bentonite. Al (Al)2O3The particle size of the powder is less than 500 meshes.
Example 1
(1) 60 portions of aggregate and 32.4 portions of Na2Cr2O7·2H2O, 6.3 parts of molten salt, 2.2 parts of binder and 0.1 part of suspending agent, and adding pure water and ball-milling for 1 hour to prepare the coating; the aggregate is Fe2O3And is a powder that passes through a 320 mesh screen.
(2) Coating the coating on the inner cavity of a casting mold or the surface of a sand core, and drying at the temperature of 120-140 ℃;
(3) and pouring the smelted aluminum liquid into the casting mould, and enabling the aluminum liquid to interact with the coating on the surface of the cavity or the surface of the sand core to obtain the oxide layer on the surface of the casting.
(4) 10 parts by weight of Al (H)2PO4)380 parts of pure water and Al (H)2PO4)31.1 times the amount of Al required for the entire chemical reaction2O3Weighing raw materials for powder, and uniformly mixing and stirring to obtain a phosphate coating;
(5) heat treatment of cast aluminum alloy samples: placing the molded internal oxidation treatment casting into a heat treatment furnace for heat treatment, wherein the heat treatment state is T6, cleaning the cast internal oxidation treatment casting after the heat treatment, and finally naturally drying the cast internal oxidation treatment casting for later use;
(6) preparing a coating: and (3) coating the paint prepared in the step (4) on the surface of the cast aluminum alloy dried in the step (5) (soaking the aluminum alloy casting in the paint for 2min), and then placing in a water vapor environment with the temperature of 120 ℃ for 50min to obtain the cast aluminum alloy with the aluminum phosphate coating on the surface.
Example 2
(1) 67 parts of aggregate and 25.4 parts of Na2Cr2O7·2H2O, 6 parts of molten salt, 1.6 parts of binder and 0.1 part of suspending agent, and adding pure water and ball-milling for 1 hour to prepare the coating; the aggregate is SiO2And Cr2O3And is a powder that passes through a 320 mesh screen.
(2) Coating the coating on the inner cavity of a casting mold or the surface of a sand core, and drying at the temperature of 120-140 ℃;
(3) and pouring the smelted aluminum liquid into the casting mould, and enabling the aluminum liquid to interact with the coating on the surface of the cavity or the surface of the sand core to obtain the oxide layer on the surface of the casting.
(4) 70 parts by weight of Al (H)2PO4)340 parts of pure water and Al (H)2PO4)31.3 times the amount of Al required for the entire chemical reaction2O3Weighing raw materials for powder, and uniformly mixing and stirring to obtain a phosphate coating;
(5) heat treatment of cast aluminum alloy samples: placing the molded internal oxidation treatment casting into a heat treatment furnace for heat treatment, wherein the heat treatment state is T6, cleaning the cast internal oxidation treatment casting after the heat treatment, and finally naturally drying the cast internal oxidation treatment casting for later use;
(6) preparing a coating: and (3) coating the paint prepared in the step (4) on the surface of the cast aluminum alloy dried in the step (5) (soaking the aluminum alloy casting in the paint for 3min), and then placing in a water vapor environment with the temperature of 180 ℃ for 70min to obtain the cast aluminum alloy with the aluminum phosphate coating on the surface.
Example 3
(1) According to 80 parts of aggregate and 7.4 parts of Na2Cr2O7·2H2O, 9 parts of molten salt, 3.5 parts of binder and 0.1 part of suspending agent, and adding pure water and ball-milling for 1 hour to prepare the coating; the aggregate is spinel and is a powder that can pass through a 320 mesh screen.
(2) Coating the coating on the inner cavity of a casting mold or the surface of a sand core, and drying at the temperature of 120-140 ℃;
(3) and pouring the smelted aluminum liquid into the casting mould, and enabling the aluminum liquid to interact with the coating on the surface of the cavity or the surface of the sand core to obtain the oxide layer on the surface of the casting.
(4) 40 parts by weight of Al (H)2PO4)360 parts of pure water and Al (H)2PO4)31.2 times the amount of Al required for the entire chemical reaction2O3Weighing raw materials for powder, and uniformly mixing and stirring to obtain a phosphate coating;
(5) heat treatment of cast aluminum alloy samples: placing the molded internal oxidation treatment casting into a heat treatment furnace for heat treatment, wherein the heat treatment state is T6, cleaning the cast internal oxidation treatment casting after the heat treatment, and finally naturally drying the cast internal oxidation treatment casting for later use;
(6) preparing a coating: and (3) coating the paint prepared in the step (4) on the surface of the cast aluminum alloy dried in the step (5) (soaking the aluminum alloy casting in the paint for 2.5min), and then placing in a water vapor environment with the temperature of 130 ℃ for 20min to obtain the cast aluminum alloy with the aluminum phosphate coating on the surface.
Example 4
(1) 75 parts of aggregate and 16.1 parts of Na2Cr2O7·2H2O, 5.8 parts of molten salt, 2.9 parts of binder and 0.2 part of suspending agent, and adding pure water and ball-milling for 1 hour to prepare the coating; the aggregate is a mixture of spinel, mullite and zircon, and is a powder capable of passing through a 320 mesh screen.
(2) Coating the coating on the inner cavity of a casting mold or the surface of a sand core, and drying at the temperature of 120-140 ℃;
(3) and pouring the smelted aluminum liquid into the casting mould, and enabling the aluminum liquid to interact with the coating on the surface of the cavity or the surface of the sand core to obtain the oxide layer on the surface of the casting.
(4) 45 parts by weight of Al (H)2PO4)370 parts of pure water and Al (H)2PO4)31.2 times the amount of Al required for the entire chemical reaction2O3Weighing the raw materials in powder, mixing and stirring uniformly to obtain the phosphoric acidSalt coating;
(5) heat treatment of cast aluminum alloy samples: placing the molded internal oxidation treatment casting into a heat treatment furnace for heat treatment, wherein the heat treatment state is T6, cleaning the cast internal oxidation treatment casting after the heat treatment, and finally naturally drying the cast internal oxidation treatment casting for later use;
(6) preparing a coating: and (3) coating the paint prepared in the step (4) on the surface of the cast aluminum alloy dried in the step (5) (soaking the aluminum alloy casting in the paint for 2min), and then placing in a water vapor environment with the temperature of 140 ℃ for 40min to obtain the cast aluminum alloy with the aluminum phosphate coating on the surface.
Example 5
(1) 77.5 parts of aggregate and 10.7 parts of Na2Cr2O7·2H2O, 0.8 part of molten salt, 3.6 parts of binder and 0.2 part of suspending agent, and adding pure water and ball-milling for 1 hour to prepare the coating; the aggregate is Cr2O3、Al2O3And wollastonite, and is a powder that passes through a 320 mesh screen.
(2) Coating the coating on the inner cavity of a casting mold or the surface of a sand core, and drying at the temperature of 120-140 ℃;
(3) and pouring the smelted aluminum liquid into the casting mould, and enabling the aluminum liquid to interact with the coating on the surface of the cavity or the surface of the sand core to obtain the oxide layer on the surface of the casting.
(4) 20 parts by weight of Al (H)2PO4)330 parts of pure water and Al (H)2PO4)31.2 times the amount of Al required for the entire chemical reaction2O3Weighing raw materials for powder, and uniformly mixing and stirring to obtain a phosphate coating;
(5) heat treatment of cast aluminum alloy samples: placing the molded internal oxidation treatment casting into a heat treatment furnace for heat treatment, wherein the heat treatment state is T6, cleaning the cast internal oxidation treatment casting after the heat treatment, and finally naturally drying the cast internal oxidation treatment casting for later use;
(6) preparing a coating: and (3) coating the paint prepared in the step (4) on the surface of the cast aluminum alloy dried in the step (5) (soaking the aluminum alloy casting in the paint for 3min), and then placing in a water vapor environment with the temperature of 150 ℃ for 90min to obtain the cast aluminum alloy with the aluminum phosphate coating on the surface.
Example 6
(1) 74.7 parts of aggregate and 13.8 parts of Na2Cr2O7·2H2Weighing raw materials of 0.8 part of molten salt, 3.3 parts of binder and 0.2 part of suspending agent, adding pure water, and ball-milling for 1 hour to prepare the coating; the aggregate is a mixture of SiC, mullite, zircon and wollastonite, and is powder which can pass through a 320-mesh sieve.
(2) Coating the coating on the inner cavity of a casting mold or the surface of a sand core, and drying at the temperature of 120-140 ℃;
(3) and pouring the smelted aluminum liquid into the casting mould, and enabling the aluminum liquid to interact with the coating on the surface of the cavity or the surface of the sand core to obtain the oxide layer on the surface of the casting.
(4) 60 parts by weight of Al (H)2PO4)380 parts of pure water and Al (H)2PO4)31.3 times the amount of Al required for the entire chemical reaction2O3Weighing raw materials for powder, and uniformly mixing and stirring to obtain a phosphate coating;
(5) heat treatment of cast aluminum alloy samples: placing the molded internal oxidation treatment casting into a heat treatment furnace for heat treatment, wherein the heat treatment state is T6, cleaning the cast internal oxidation treatment casting after the heat treatment, and finally naturally drying the cast internal oxidation treatment casting for later use;
(6) preparing a coating: and (3) coating the paint prepared in the step (4) on the surface of the cast aluminum alloy dried in the step (5) (soaking the aluminum alloy casting in the paint for 2min), and then placing in a water vapor environment with the temperature of 160 ℃ for 60min to obtain the cast aluminum alloy with the aluminum phosphate coating on the surface.
Examples of the experiments
An internal oxidation sample is prepared according to example 3, then the internal oxidation sample is subjected to heat treatment under the condition of T6, the internal oxidation sample after the heat treatment is subjected to steam-assisted method to prepare an aluminum phosphate composite coating, then the cross-sectional morphology of the composite coating is analyzed, and the corrosion resistance of the composite coating is tested by an electrochemical method.
First, the cross-sectional micro-topography (fig. 1a) and the surface micro-topography (fig. 1b) of the sample after internal oxidation treatment were observed using a Sirion field emission Scanning Electron Microscope (SEM).
After the in-mold oxidation treatment, the surface of the cast aluminum alloy matrix is coated with an oxide layer, the thickness of the oxide layer is 2-4 μm (figure 1a), and is much thicker than an oxide film naturally generated on the surface of the aluminum alloy (the thickness of the naturally generated oxide film is about 5nm), and the thickness of the oxide film is 400-800 times of that of the naturally generated oxide film. Meanwhile, as can be seen from FIG. 1(b), the surface of the sample is distributed with Al which grows crosswise2O3Flakes, these lamellar tissue structures being micron-scale.
From the scanning results of FIG. 1(d), it can be seen that the distribution of the aluminum element gradually decreases from the aluminum matrix to the coating boundary, while the distribution of the chromium element and the oxygen element are opposite, and the distribution curves of the O element and the Cr element are very similar, indicating that both the O element and the Cr element in the oxide layer are derived from the decomposition of the oxidizing agent in the type internal oxidation treatment (FIG. 1 c). For the sake of clarity, the distribution of O, Al, Si, Cr is given separately in FIGS. 1 e-h.
In addition, during the intra-type oxidation treatment, when excessive oxygen is generated, pores are generated in the coating layer, as shown in fig. 1 c.
Second, electrochemical testing of the intratype oxidized samples was performed using electrochemical workstation CHI604E
The corrosive liquid is 3.5 percent of wt NaCl solution, and the ratio of the volume of the solution to the total area of the sample is 20ml/100mm2The corrosion solution is replaced every 24 hours, electrochemical alternating current impedance spectroscopy (EIS) and potentiodynamic polarization curve (Tafel) are respectively tested after 1h, 12h, 24h, 48h, 72h, 96h and 120h of corrosion in a water bath heating pot with the temperature of 60 ℃, and then the polarization resistance value and the self-corrosion current density of the sample are obtained according to a fitting circuit (figure 2).
The polarization resistance value and the self-corrosion current density of the contrast type internal oxidation sample and the as-cast sample subjected to the non-type internal oxidation treatment are changed along with the corrosion time, the polarization resistance value of the type internal oxidation sample is far higher than that of the sample subjected to the non-type internal oxidation treatment (figure 2a), the self-corrosion current density is also smaller (figure 2b), the oxide layer subjected to the surface type internal oxidation treatment is compared with the naturally generated oxide layer, the blocking effect on a corrosion medium is stronger, the corrosion electrolyte can be more effectively prevented from permeating into the matrix, and the occurrence of pitting corrosion is avoided. With increasing corrosion time, a significant decrease in the corrosion resistance of the type internal oxidation samples was observed, which then tended to be plateau, but still better than the untreated samples.
And thirdly, the cross-sectional micro-topography and the surface micro-topography of the oxide layer after the T6 heat treatment are respectively shown in the figures 3a and b. In contrast to fig. 1a, b, the oxide layer was slightly detached from the substrate in some areas after the heat treatment, but did not detach. The results show that the oxide layer still adheres to the substrate after the solution treatment and the long-term aging treatment, and the protective effect on the substrate is not failed, which can also be proved by the electrochemical test results.
And (3) comparing and analyzing the polarization resistance value and the self-corrosion current density of the heat-treated type internal oxidation sample and the non-heat-treated type internal oxidation sample after 1h, 12h, 24h, 48h, 72h, 96h and 120h of corrosion. In the early stage of etching, the polarization resistance values of the untreated type internal oxidation samples were larger than those of the T6 heat treatment group (FIG. 3c), but both polarization resistance values tended to decrease with the increase of the etching time, and the values were similar. After the T6 heat treatment, there was a sudden increase in the self-etching current density over time at 48h (FIG. 3d), and the performance appeared unstable due to partial detachment of the coating. The performance of the coating continued to approach that of the untreated sample for extended periods of time due to the small gaps in the detached portions of the coating, which may be filled with corrosion products.
Fourthly, the microscopic shapes of the cross sections of the phosphate composite corrosion-resistant coating prepared by the steam-assisted method are shown in the figures 4a and b.
As can be seen from FIG. 4, the corrosion-resistant coating after the composite treatment is tightly bonded with the cast aluminum alloy substrate, and the coating is uniform and dense. EDS analysis showed that the proportion of P, O, Al in the corrosion resistant coating was high (FIG. 4c), and the oxide layer was successfully strengthened with a phosphate ceramic coating. Compared with the cross-sectional micro-morphology of the type (a) internal oxidation sample of FIG. 1, the coating is denser and the bonding is also tighter.
Comparing electrochemical test results of the composite treatment coating sample, the type internal oxidation sample and the heat-treated type internal oxidation sample, wherein the polarization resistance value of the composite treatment coating is higher than that of the heat-treated type internal oxidation sampleThe polarization resistance value of the sample is improved by more than 10 times (figure 4d), and is also obviously higher than that of the sample oxidized in the form. The self-corrosion current density of the composite treatment coating is also smaller than that of the other two (figure 4e), which shows that the composite corrosion-resistant coating has good corrosion medium barrier effect, can remarkably improve the corrosion resistance of the casting, and the durability of the coating is good along with the prolonging of the corrosion time, and the polarization resistance of the sample after being corroded for 120h is still about 5000 omega cm2Is superior to the type internal oxidized aluminum alloy sample.
In summary, the change in the coating cross section and the change in the corrosion resistance between the type internal oxidation treatment sample, the heat-treated type internal oxidation sample, and the corrosion-resistant treated composite coating were compared. The composite corrosion-resistant coating obtained by the process has the advantages of tight combination with a matrix, no shedding and peeling, larger, uniform and compact coating thickness, obviously improved corrosion resistance of castings and prolonged service life of cast aluminum alloy parts.
Finally, it should be noted that the above examples are only used to illustrate the technical solution and implementation steps of the present invention, and are not limited to the cast aluminum alloy of EN AC-43000, and although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that modifications or similar substitutions can be made to the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention, and all of them should be covered by the claims of the present invention.

Claims (10)

1. A composite treatment process for manufacturing a corrosion-resistant protective layer on the surface of cast aluminum alloy is characterized by comprising the following steps:
firstly, producing an aluminum alloy casting by adopting a type internal oxidation process;
and secondly, carrying out heat treatment on the aluminum alloy casting subjected to the internal oxidation treatment obtained in the first step, and then carrying out steam-assisted phosphate coating treatment.
2. The composite treatment process for producing a corrosion-resistant protective layer on a surface of a cast aluminum alloy as claimed in claim 1, wherein the composite treatment process comprisesThe coating for internal oxidation treatment used in the first step mainly comprises the following components in parts by weight: 60-80 parts of aggregate and 7.4-32.4 parts of Na2Cr2O7·2H2O, 6-9 parts of molten salt, 1.6-3.6 parts of binder and 0.1-0.2 part of suspending agent.
3. The composite treatment process for manufacturing a corrosion-resistant protective layer on the surface of cast aluminum alloy as claimed in claim 2, wherein the aggregate is Fe2O3、SiO2、Cr2O3、Al2O3、TiO2、SiC、Si3N4、AlN、ZrO2And one or more of spinel, mullite, zircon and wollastonite, and is powder capable of passing through a 320-mesh sieve.
4. The composite treatment process for manufacturing a corrosion-resistant protective layer on the surface of cast aluminum alloy according to claim 2, wherein the molten salt is prepared from NaCl, KCl and MgCl2According to a molar weight ratio of 18: 47: 35, the binder is sodium carboxymethyl cellulose (CMC), and the suspending agent is bentonite.
5. The composite treatment process for manufacturing a corrosion-resistant protective layer on the surface of a cast aluminum alloy according to claim 1, wherein the method for producing an aluminum alloy casting by the type-in oxidation process in the first step is as follows:
(1) preparing a coating for internal oxidation treatment;
(2) coating the coating on the inner cavity of a casting mold or the surface of a sand core, and drying at the temperature of 120-140 ℃;
(3) and pouring the smelted aluminum liquid into the casting mould, and enabling the aluminum liquid to interact with the coating on the surface of the cavity or the surface of the sand core to obtain an oxide layer on the surface of the casting.
6. The composite treatment process for producing a corrosion-resistant protective coating on a surface of a cast aluminum alloy as claimed in claim 1, wherein the heat treatment state in the second step is T6.
7. The composite treatment process for manufacturing a corrosion-resistant protective layer on the surface of cast aluminum alloy according to claim 1, wherein the phosphate coating used in the second step is mainly composed of the following components in parts by weight: 10-70 parts of Al (H)2PO4)330-80 parts of pure water and Al (H)2PO4)3Al with 1.1-1.3 times of the amount required by all chemical reactions2O3And (3) powder.
8. The composite treatment process for producing a corrosion-resistant protective coating on a surface of a cast aluminum alloy according to claim 1, wherein the steam assisted phosphate coating treatment in the second step is as follows:
(1) preparing a phosphate coating;
(2) taking the aluminum alloy casting subjected to heat treatment, cleaning and drying;
(3) and (3) coating the coating prepared in the step (1) on the surface of the aluminum alloy casting dried in the step (2), and then placing the aluminum alloy casting in a water vapor environment to obtain a cast aluminum alloy workpiece with an aluminum phosphate coating on the surface.
9. The composite treatment process for manufacturing a corrosion-resistant protective layer on the surface of cast aluminum alloy according to claim 8, wherein the step (3) of coating the surface of the aluminum alloy casting with the coating prepared in the step (1) is to dip the aluminum alloy casting in the coating for 2-3 min.
10. The composite treatment process for manufacturing a corrosion-resistant protective layer on the surface of cast aluminum alloy according to claim 8, wherein the temperature of the water vapor environment in the step (3) is 120-180 ℃, and the time for placing in the water vapor environment is 20-90 min.
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